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Soil nail analysis program snap download free download
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This facing is placed on the unsupported excavation prior to advancement of the excavation grades. The effect of the wall height was not significant, and thereby is not longer considered.
Although the average force per nail is quite similar in the two nail patterns, the difference in maximum nail forces between Cases 2 and 4 is significant. Bearing plates are then fixed to the heads of the soil nails. Grab samples obtained from cuttings in borings, test pits, and test cuts can also be used for soil classification and laboratory determination of index parameters, as long as they are sufficiently representative and the in situ moisture content was preserved during sampling and transportation. Contract or Grant No.
E-mail List Signup - Selection of Corrosion Protection SERVICE LIFE TEMPORARY PERMANENT AGGRESSIVITY NOT KNOWN OR AGGRESSIVE CLASS II AGGRESSIVITY NOT KNOWN OR NON-AGGRESSIVE NONE not applicable in soil nails AGGRESSIVE NON-AGGRESSIVE FAILURE CONSEQUENCES CLASS I SERIOUS NOT SERIOUS COST FOR INCREASING CORROSION PROTECTION CLASS I SMALL SIGNIFICANT CLASS II CLASS I Modified from PTI 1996.
Technical Report Documentation Page 1. Title and Subtitle 5. Report Date March 2003 GEOTECHNICAL ENGINEERING CIRCULAR NO. Performing Organization Code Soil Nail Walls 7. Performing Organization Report No. Performing Organization Name and Address 10. TRAIS GeoSyntec Consultants 10015 Old Columbia Road, Suite A-200 Columbia, Maryland 21046 11. Contract or Grant No. Sponsoring Agency Name and Address 13. Department of Transportation 400 Seventh Street, S. Sponsoring Agency Code 15. Supplementary Notes FHWA Technical Consultant: J. HIBT-20 Contracting Officer Technical Representative COTR : Chien-Tan Chang HTA-22 16. Abstract This document presents information on the analysis, design, and construction of soil nail walls in highway applications. The main objective is to provide practitioners in this field with sound and simple methods and guidelines that will allow them to analyze, design, and construct safe and economical structures. This document updates information contained in FHWA-SA-96-069R Byrne et al. The focus is on soil nailing techniques that are commonly used in the U. The contents of this document include: an introduction, a chapter on applications and feasibility, descriptions and guidelines for field and laboratory testing in soil nailing applications, descriptions of the common U. Distribution Statement soil nailing, soil nail walls, soil nail testing, shotcrete, soil nail wall design, soil nailing specifications No restrictions. Security Classification of this report Unclassified Form DOT F 1700. Security Classification of this page Unclassified 21. Price Reproduction of completed page authorized SI CONVERSION FACTORS APPROXIMATE CONVERSIONS FROM SI UNITS Symbol When You Know Multiply By To Find Symbol LENGTH mm millimeters 0. Department of Transportation Federal Highway Administration FHWA , Office of Technology Applications, for providing valuable technical assistance and review during preparation of the document. The authors would also like to thank Mr. Chien-Tan Chang FHWA for providing technical assistance. Robert Bachus for senior reviewing, Mr. Douglas Mandeville for performing stability analyses, Ms. Shana Opdyke for proofreading the manuscript, Ms. Lynn Johnson for word processing, editing, and assisting in the layout of the document, and Mr. The intended audience for this document is geotechnical, structural, and highway design and construction specialists involved in the design, construction, and inspection of soil nail wall systems. This GEC aims to serve as the FHWA reference document for highway projects involving soil nail walls. The technique of soil nailing has been increasing its popularity among contractors because it offers an effective and cost-effective retaining system for a variety of ground conditions. The technique has been used extensively in Europe for the last 30 years. In the Unites States, the past 10 years have seen a continued interest in its applications. FWHA-SA-93-068, Federal Highway Administration, Washington, D. Appendix B — Charts for Preliminary Designs. Appendix C — Corrosion Protection. Appendix D — Design Example. Appendix E — Construction Specifications for Soil Nail Walls. Common Geotechnical Field Procedures and Tests. Cohesionless Soil Density Description Based on SPT N-Values. Fine-Grained Soil Consistency Description Based on SPT N-Values. Common Procedures and Laboratory Tests for Soils. Correlations Between SPT and CPT Results and Friction Angle of Cohesionless Soils. Correlations Between SPT and CPT Results and Undrained Strength of Fine-Grained Soils. Correlations with Index Parameters and Preconsolidation History for Clays. Common Rock Procedures and Laboratory Tests. Criteria for Assessing Soil Corrosion Potential. Estimated Bond Strength of Soil Nails in Soil and Rock. Drilling Methods and Procedures. Tensile Resistance Factors CF. Minimum Recommended Factors of Safety for the Design of Soil Nail Walls with ASD Method. Coefficients γ and β for the Allowable Stress Method. Design Steps For Soil Nail Walls. Facing Resistance for Various Facing Failure Modes x TABLE OF CONTENTS continued LIST OF FIGURES Figure 2. Typical Soil Nail Wall Construction Sequence. Soil Nail Walls for Temporary and Permanent Cut Slopes. Road Widening Under Existing Bridge. Comparison of Construction Cost Data for Various Systems. Construction Cost Data for Soil Nail Walls. Preliminary Geotechnical Boring Layout for Soil Nail Walls. Correlation of Effective Friction Angle as a Function of Soil Classification, Relative Density, and Unit Weight. Total Active Pressure Coefficients: a Horizontal Backslope, and b Correction for Non-Horizontal Backslope. Single Nail Stress-Transfer Mode. Soil Nail Stress-Transfer Mechanism. Simplified Distribution of Nail Tensile Force. Schematic Location of Soil Nail Maximum Tensile Forces. Summary of Maximum Nail Tensile Forces Measured in Walls. Summary of Facing Tensile Forces Measured in Walls. Progressive Flexural Failure in Wall Facings. Geometry used in Flexural Failure Mode. Soil Pressure Distribution Behind Facing. Punching Shear Failure Modes. Geometry of a Headed-Stud. Deformation of Soil Nail Walls. Deformation of Soil Nail Walls. Stability Analysis Comparison of Results Between SNAIL and GOLDNAIL. Drainage of Soil Nail Walls. Typical Drain Pipe Details to Provide Groundwater Control in Soil Nail Walls. Example of Stepped Soil Nail Wall. Soil Nail Patterns on Wall Face. Effect of Different Nail-Length Patterns. Recommended Facing Resistance for Various Facing Failure Modes. Soil Nail Load Testing. Hydraulic Jack Used for Soil Nail Load Testing Setup. Typical Data Sheet for Soil Nail Load Testing. Example of Data Reduction from Soil Nail Load Testing. Example of Data Reduction from Soil Nail Creep Testing. Correction factor for normalized nail lengths for DDH other than 100 mm 4 in. The intended audience for this document includes geotechnical, structural, and highway design and construction specialists involved in soil nail wall systems. The primary goal of this document is to provide the practitioner sufficient information to facilitate the safe and cost-effective use of permanent soil nail walls for a variety of transportation-related projects. This document presents historical background information and a description of soil nail wall systems, step-by-step design procedures, simplified design charts, and general construction specifications. The document concludes with a detailed soil nail wall design example. This document provides sufficient information to confidently design soil nail walls in a wide range of ground conditions. Limitations related to the use of these systems in marginal ground conditions are also provided. Information provided herein is not intended to represent a prescriptive methodology; rather the information should be used in conjunction with good engineering judgment for specific projects. This tunneling method consists of the installation of passive i. This concept of combining passive steel reinforcement and shotcrete has also been applied to the stabilization of rock slopes since the early 1960s e. This ground-support technique relies on the mobilization of the tensile strength of the steel reinforcement at relatively small deformations in the surrounding ground. This support is enhanced by the continuity of the shotcrete. The combination of passive reinforcement and shotcrete when applied to soil, in lieu of rock, is termed soil nailing. One of the first applications of soil nailing was in 1972 for a railroad widening project near Versailles, France, where an 18-m 59-ft high cut-slope in sand was stabilized using soil nails Rabejac and Toudic, 1974. Because the method was cost-effective and the construction faster than other conventional support methods, an increase in the use of soil nailing took place in France and other areas in Europe. In Germany, the first use of a soil nail wall was in 1975 Stocker et al. The first major research program on soil nail walls was undertaken in Germany from 1975 through 1981 by the University of Karlsruhe and the construction company Bauer. This investigation program involved full-scale testing of experimental walls with a variety of configurations and the development of analysis procedures to be used in design Gässler and Gudehus, 1981; Schlosser and Unterreiner, 1991. In France, the Clouterre research program, involving private and public participants, was initiated in 1986. This research effort consisted of 1 full-scale testing, monitoring of in-service structures, and numerical simulations Schlosser, 1983; Clouterre, 1991. One of the first published applications of soil nailing in the United States was the support of the 13. The construction of the retaining system was reportedly conducted in nearly half the time and at about 85 percent of the cost of conventional excavation support systems. In 1984, a prototype soil nail wall 12-m 40-ft high was built near Cumberland Gap, Kentucky, as part of a demonstration project funded by the U. Department of Transportation Federal Highway Administration FHWA Nicholson, 1986. In 1989, the Oregon Department of Transportation built an 8-m 24-ft high wall as the first application of a soil nail wall used in a bridge abutment cut wall end-slope removal. In 1988, a 12. Each wall tier was 6. Other examples of early uses of soil nail walls include those built along Interstate 10 in San Bernadino, California; Interstate 90 near Seattle, Washington; and along George Washington Parkway Interstate 495 in Virginia Byrne et al. The use of soil nail walls has substantially increased in the United States during the last decade because it has been demonstrated that soil nail walls are technically feasible and, in many cases, a cost-effective alternative to conventional retaining walls used in top-to-bottom excavations in temporary and permanent applications. Design engineers are becoming increasingly familiar with soil nailing technology. Most soil nail walls constructed in the United States are still used for temporary retaining structures, however, the use of soil nail walls as a permanent structure has increased substantially in the last five years. The more widespread use of soil nail walls today is due in large part to the efforts of FHWA through the Office of Research and Development. The objective of this first document was to disseminate information to U. In 1992, FHWA sponsored two-week long technical tours by various U. The objectives of the tour were to: 1 learn the then current European state-of-the-practice in soil nail wall technology; 2 update the available information on the mechanisms of soil nail wall performance, design approach, and computer programs; and 3 gather up-to-date construction specifications, corrosion protection details, and information and appropriate contracting practices. These efforts provided the basis for establishing subsequent research and development activities in the United States. In 1993, FHWA sponsored an English translation of the French practice summary on soil nailing FHWA, 1993b. Also in 1994, FHWA launched Demonstration Project 103 Demo 103 to disseminate further the use of soil nail walls among state highway agencies. Documents developed for Demo 103 served as a preliminary design guide and subsequently evolved as a design manual Byrne et al. FHWA also funded various research projects in academic institutions. The design principles presented in this document are based on the ASD procedure. This document also presents new simplified charts that can be used in the preliminary design phase of a project, and discusses advantages and limitations of two computer programs, SNAIL and GOLDNAIL, developed for the analysis and design of soil nail walls. This chapter also presents examples of applications and discusses favorable and unfavorable ground conditions for soil nailing, as well as criteria for feasibility evaluations. In addition, this chapter presents recommendations of soil properties to be used in the analysis and design of soil nail walls. The features of the computer programs GOLDNAIL and SNAIL are also presented. A simplified step-by-step design example is included. As construction proceeds from the top to bottom, shotcrete or concrete is also applied on the excavation face to provide continuity. Soil nailing is typically used to stabilize existing slopes or excavations where top-to-bottom construction is advantageous compared to other retaining wall systems. For certain conditions, soil nailing offers a viable alternative from the viewpoint of technical feasibility, construction costs, and construction duration when compared to ground anchor walls, which is another popular top-to bottom retaining system. This chapter introduces some basic aspects of soil nailing, presents typical highway applications, and discusses criteria to be used in evaluating the feasibility of soil nail walls. This document addresses soil nails that are installed with a near horizontal orientation i. Such soil nail systems are used to stabilize natural slopes and excavations. An alternative application of passive reinforcement in soil is sometimes used to stabilize landslides. In this alternative application, nails are also passive, installed in a closely spaced pattern approximately perpendicular to the nearly horizontal sliding surface, and subjected predominantly to shear forces arising from the landslide movement. However, this application of soil nails as a means to stabilize landslides is not often used in the current U. This document discusses the use of soil nail walls for both temporary and permanent structures. A structure can be characterized as temporary or permanent by its service life or intended duration of use. A structure with a service life of 18 months or less qualifies as temporary; a structure with a longer service life qualifies as permanent. If a structure is initially intended as temporary e. Steel reinforcing bars — The solid steel reinforcing bars are the main component of the soil nail wall system. These elements are placed in pre-drilled drillholes and grouted in place. Tensile stress is applied passively to the nails in response to the deformation of the retained materials during subsequent excavation activities. Grout — Grout is placed in the pre-drilled borehole after the nail is placed. The grout serves the primary function of transferring stress from the ground to the nail. The grout also provides a level of corrosion protection to the soil nail. Nail head — The nail head is the threaded end of the soil nail that protrudes from the wall facing. Hex nut, washer, and bearing plate — These components attach to the nail head and are used to connect the soil nail to the facing. Temporary and permanent facing — The facing provides structural connectivity. The temporary facing serves as the bearing surface for the bearing plate and support the exposed soil. This facing is placed on the unsupported excavation prior to advancement of the excavation grades. The permanent facing is placed over the temporary facing after the soil nails are installed and the hex nut has been tightened. Geocomposite strip drainage — The geocomposite strip drainage systemmedia is placed prior to application of the temporary facing to allow collection and transmission of seepage water that may migrate to the temporary facing. Additional corrosion protection not shown in Figure 2. Complete descriptions of the soil nail wall components commonly used in the typical U. Methods to provide corrosion protection are discussed in Chapter 4 and Appendix C. Initial excavation is carried out to a depth for which the face of the excavation has the ability to remain unsupported for a short period of time, typically on the order of 24 to 48 hours. The depth of the excavation lift is usually between 1 and 2 m 3 and 6 ft and reaches slightly below the elevation where nails will be installed. The width of the excavated platform or bench must be sufficient to provide access to the installation equipment. Drillholes are drilled to a specified length, diameter, inclination, and horizontal spacing from this excavated platform. Nail Installation and Grouting. Nail bars are placed in the pre-drilled hole. The bars are most commonly solid, although hollow steel nails can be also usedhave seen increased usage. Centralizers are placed around the nails prior to insertion to help maintain alignment within the hole and allow sufficient protective grout coverage over the nail bar. A grout pipe tremie is also inserted in the drillhole at this time. When corrosion protection requirements are high, corrugated plastic sheathing can also be 7 used to provide an additional level of corrosion protection. The drillhole is then filled with cement grout through the tremie pipe. The grout is commonly placed under gravity or low pressure. If hollow self-drilling bars are used only as temporary structures , the drilling and grouting take place in one operation. Prior to Step 4 facing placement , geocomposite drainage strips are installed on the excavation face approximately midway between each set of adjacent nails. The drainage strips are then unrolled to the next wall lift. The drainage strips extend to the bottom of the excavation where collected water is conveyed via a toe drain away from the soil nail wall. Construction of Temporary Shotcrete Facing. A temporary facing system is then constructed to support the open-cut soil section before the next lift of soil is excavated. The most typical temporary facing consists of a lightly reinforced shotcrete layer commonly 100 mm 4 in. The reinforcement typically consists of welded wire mesh WWM , which is placed at approximately the middle of the facing thickness see lower part of Figure 2. The length of the WWM must be such that it allows at least one full mesh cell to overlap with subsequent WWM panels. Following appropriate curing time for the temporary facing, a steel bearing plate is placed over the nail head protruding from the drillhole. The bar is then lightly pressed into the first layer of fresh shotcrete. A hex nut and washers are subsequently installed to secure the nail head against the bearing plate. The hex nut is tightened to a required minimum torque after the temporary facing has sufficiently cured. This usually requires a minimum of 24 hours. If required, testing of the installed nails to measure deflections for comparison to a pre-specified criterion and proof load capacities may be performed prior to proceeding with the next excavation lift. Construction of Subsequent Levels. Steps 1 through 4 are repeated for the remaining excavation lifts. At each excavation lift, the vertical drainage strip is unrolled downward to the subsequent lift. A new panel of WWM is then placed overlapping at least one full mesh cell. The temporary shotcrete is continued with a cold joint with the previous shotcrete lift. At the bottom of the excavation, the drainage strip is tied to a collecting toe drain. Construction of a Final, Permanent Facing. After the bottom of the excavation is reached and nails are installed and load tested, a final facing may be constructed. Final facing may consist of cast-in-place CIP reinforced concrete, reinforced shotcrete, or prefabricated panels. The reinforcement of permanent facing is conventional concrete bars or WWM. When CIP concrete and shotcrete are used for the permanent facing, horizontal joints between excavation lifts are avoided to the maximum extent possible. EXCAVATE SMALL CUT GEOCOMPOSITE STRIP DRAIN STEP 2. DRILL NAIL HOLE TEMPORARY FACING NAIL BAR DRAINAGE STRIPS GROUT STEP 3. INSTALL AND GROUT NAIL INCLUDES STRIP DRAIN INSTALLATION STEP 4. PLACE TEMPORARY FACING INCLUDES SHOTCRETE, REINFORCEMENT, BEARING PLATE, HEX NUT, AND WASHERS INSTALLATION FINAL FACING 1 2 3 TOE DRAIN FINAL GRADE 4 STEP 5. CONSTRUCTION OF SUBSEQUENT LEVELS STEP 6. PLACE FINAL FACING ON PERMANENT WALLS INCLUDES BUILDING OF TOE DRAIN Modified after Porterfield et al. Variations of the steps described above may be necessary to accommodate additional preparation tasks or supplementary activities for specific project conditions. For example, shotcrete may be applied at each lift immediately after excavation and prior to nail hole drilling and installation, 9 particularly where face stability is a concern. A detailed description of the major activities related to the installation of soil nails is presented in Chapter 4. They have been used successfully in highway cuts; end slope removal under existing bridge abutments during underpass widening; for the repair, stabilization, and reconstruction of existing retaining structures; and tunnel portals. A discussion of important advantages and considerations related to the use of soil nail walls in some of the applications listed above is presented in the following three sections. The relatively wide range of available facing systems allows for various aesthetic requirements to be addressed. In this application, soil nailing is attractive because it tends to minimize excavation, requires reasonable right-of-way ROW and clearing limits, and hence, minimizes environmental impacts within the transportation corridor. Soil nail walls are particularly applicable for uphill widening projects that must be constructed either within an existing ROW or in steep terrain. Soil nail walls can be installed at comparable costs; however, the installation of soil nail walls does not require that bridge traffic be interrupted. If a ground anchor supported wall is used, soldier beams would have to be installed through the bridge deck because of limited overhead space under the bridge prior to excavating the end slope abutment. This operation results in the disruption of overpass traffic and accrues additional costs associated with lane closures and the procurement of large steel beams. Conversely, steel reinforcing bars used as soil nails are readily available. One disadvantage of the use of soil nail walls for end slope removal projects is that because the first level of soil nails is typically placed within 1 to 2 m 3 to 6 ft from the top of the slope and because the nails are sloped downward, it is possible that the bridge girders will interfere with soil nail installation. This problem can usually be avoided by positioning the soil nails horizontally to be within the clear space between bridge girders. EXISTING BRIDGE EXISTING ABUTMENT CAP PERMANENT SOIL NAIL WALL SOIL EMBANKMENT TO BE REMOVED GEOCOMPOSITE STRIP DRAIN EXISTING ROADWAY PERMANENT SOIL NAIL TYP FUTURE ROADWAY FOOT DRAIN EXISTING ABUTMENT PILE Source: Porterfield et al. The soil nails are installed directly through the retaining structure. In these applications, which represent a departure from the original concept of soil nailing, the ground deformations required to mobilize the reinforcing resistance are not derived from removal of lateral support during excavation but from ongoing movements associated with the distressed structure. The following sections present a discussion of these aspects of the feasibility evaluation. Project experience has shown that certain favorable ground conditions make soil nailing cost effective over other techniques. Conversely, certain soil conditions can be considered marginal for soil nailing applications and may make the use of soil nails too costly when compared with other techniques. Soil nail walls can generally be constructed without complications in a mixed stratigraphy, as long as the individual layers of the soil profile consist of suitable materials. The following two sections present the soil conditions that are considered most and least suitable for soil nail walls. Intermediate soil conditions, for which the feasibility of soil nailing is not readily apparent, are also described. Construction difficulties and long-term complications can generally be avoided when specific favorable soil conditions prevail. Although not an absolute requirement, it is advantageous that the ground conditions allow drillholes to be advanced without the use of drill casings and for the drillhole to be unsupported for a few hours until the nail bars are installed and the drillhole is grouted. Alternatively, soil nails have been installed with success using the hollow-stem drilling method in fully and temporarily cased drillholes. It is important to note that the selection of the drilling method is typically left to the discretion of the soil nail installation contractor. Soil conditions are presumed to be favorable for the construction of soil nail walls when results from field tests indicate competent soils. The Standard Penetration Test SPT, see next chapter provides the SPT value, N, which can be used to preliminarily identify favorable soil conditions. Based on the general criteria for favorable conditions noted above, the following ground types are generally considered well suited for soil nailing applications. Fine-grained or cohesive soils may include stiff to hard clays, clayey silts, silty clays, sandy clays, sandy silts, and combinations thereof. However, the consistency characterization of fine-grained soils should not rely solely on SPT N-values. Source: Terzaghi et al. A corrected value, N60, 27 is obtained for an energy efficiency of 60 percent typical in the U. The corrected value, N60, is commonly normalized to a reference effective overburden pressure to account for the increase of N-values with increasing overburden in a homogeneous material. Some correlations based on SPT-values provide estimates of shear strength parameters for both cohesionless and fine-grained soils. These correlations are presented in a subsequent section. Sampling Samples obtained with the SPT sampler are disturbed and only adequate for soil classification and some laboratory tests, including particle gradation sieve analysis , fines content, natural moisture content, Atterberg limits, specific gravity of solids, organic contents, and unconfined compressive strength tests. Grab samples obtained from cuttings in borings, test pits, and test cuts can also be used for soil classification and laboratory determination of index parameters, as long as they are sufficiently representative and the in situ moisture content was preserved during sampling and transportation. SPT samples should not be used for strength or compressibility testing. As the shear strength and compressibility of fine-grained soils are heavily affected by sample disturbance, samples obtained with the SPT standard split-spoon sampler are unsuitable for laboratory testing of shear strength and compressibility of fine-grained soils. Undisturbed thin-walled samplers, including the Shelby tube sampler with an outer diameter OD of 76 mm 3 in. More appropriate in situ tests should be used for estimating the undrained shear strength Su of fine-grained soils directly from the field. Some of these in situ tests include the cone penetration test CPT , the field vane shear test VST , the pressuremeter test PMT , and the flat plate dilatometer test DMT. The CPT, also known as the piezocone penetration test, has become a common field investigation technique. The CPT is a valuable investigation tool that allows rapid and cost-effective development of subsurface soil profiles. As the CPT-based soil profiles are continuous, this technique permits the identification of thin soil layers that would be otherwise difficult to detect within a relatively homogeneous soil mass. This capability may prove particularly useful when investigating the presence of thin layers of weak soil that may prompt instability behind the proposed soil nail wall. In general, CPT is more cost-effective and faster than conventional SPT. The major disadvantage of this technique is that no sample is recovered. Additionally, CPT sounding cannot be performed in gravelly or bouldery soil. For some large projects, the phased use of CPT and conventional borings is attractive because it provides comparatively more geotechnical information at costs that are comparable than with conventional borings alone. In the first phase, the CPT soundings allow the rapid depiction of the soil stratigraphy and early identification of layers with potential deficiencies e. An initial CPT- based stratigraphy can help determine the location of zones where undisturbed soil samples should be obtained. In the second phase, conventional borings can be used and samples are obtained only at the depths of interest. Using this two-phase investigation strategy, sampling can be optimized and the number of samples can be reduced. The VST is commonly used in conjunction with conventional soil borings to obtain the in situ undrained shear strength of fine-grained soils. The advantage of the VST is that it can provide a direct, in situ estimate of Su in fine-grained soils. The PMT and the DMT are also available, but their use is not as widespread. For more comprehensive discussion, refer to Sabatini et al. Test Pits In soil nail wall applications, the use of test pits in flat areas or test cuts in sloping ground can be particularly beneficial. Test pits are relatively inexpensive and can help assess whether an excavation face will stand unsupported and define the feasibility of soil nailing at a site. Test pits should be approximately 6- to 8-m 20- to 25-ft long and 2- to 2. To evaluate the stand-up time for the excavation, the test pit should be left open for 3 to 4 days. Daily inspection of the excavated test pit is recommended. Developing the site stratigraphy is critical for soil nail walls because the nature, extent, and distribution of the various layers dictates the type of drilling equipment and methods, control the size of the potential sliding soil mass behind the wall, and have an impact on the soil nail lengths. The identification of varying subsurface conditions on plan view is particularly important in long walls, where soil conditions are more likely to vary considerably. Soil stratigraphy is first assessed via visual logging or in situ testing results during the site investigation and subsequently is corroborated or adjusted from laboratory testing results. The location of the soil-bedrock contact must be identified if the soil nails will be partially or totally embedded in underlying rock. The dissimilar subsurface conditions above and below this contact will have an impact on the suitability of the drilling methodology, the drilling equipment, the construction costs, and the nail lengths. Soil nails that are partially or totally embedded in rock are expected to be shorter than those embedded entirely in soil due to the higher bond strengths that tend to develop in rock. These aspects include stability of temporarily unsupported cuts, soil strength and bond strength, corrosion potential, pressure on the facing, drillhole stability, grouting procedures, drainage, and other construction considerations. Groundwater depth should be obtained from borings during drilling and should be monitored for at least 24 hours after drilling. If drilling fluid is used during boring advancement, it may not be possible to locate the groundwater in borings. For soils exhibiting relatively high fines content, the groundwater levels obtained during drilling do not commonly represent stabilized levels of the groundwater table, as the observed levels of water are most likely affected by the relatively low permeability of the surrounding soil. In these soils, it is a good practice to measure the groundwater level a few times over the course of a few hours or days to allow groundwater to reach its equilibrium level. In soils with very low permeability, more extended periods of time, up to several weeks or months, may be necessary for the groundwater level to stabilize. For these cases, it is valuable to convert some of the exploratory borings into piezometers. It is desirable to obtain or estimate the seasonal high and low groundwater levels from piezometers or other sources e. Underestimating grossly the elevation of groundwater during a field investigation can have serious consequences for any earth retaining system. This is particularly true for soil nail walls because these systems are not particularly suited to high groundwater conditions, as discussed in the previous chapter. When the actual groundwater level is higher than previously estimated, field modifications may be required. PROCEDURE Classification Index Parameters Strength Hydraulic Conductivity Compressibility Other STANDARD TEST NAME APPLICABILITY ASTM 1 AASHTO 2 D2488-00 - All soils D2487-00 D422-63 1998 M145 T88 Soil Fraction finer than No. The primary design parameters for soil nail walls are discussed in the next few sections. Atterberg limits fine-grained soils ; and 6. A correct soil classification and typifying is important because the anticipated soil response is commonly associated with typified soil types. As discussed in the previous chapter, although soil nails can be installed in a relatively wide range of soils, soil nail walls are more economically competitive in certain select soil types. Soils should be classified according to the Unified Soil Classification System USCS , which requires that the gradation and Atterberg limits be determined. Sieve analyses in granular soils, along with fines content determinations, can help determine whether soil conditions are favorable e. The natural in situ moisture content determined mostly in fine-grained soils can help detect certain unfavorable conditions. Moisture contents less than about 2 percent in granular soils with little or no fines may indicate the inability of vertical cuts to remain unsupported. Atterberg limits of fine-grained soils must be used to classify fine-grained soils and help assess the potential for creep deformations of the soil behind the proposed wall. These index test results may be used to estimate the shear strength of fine-grained soils, using appropriate correlations as presented in a subsequent section. If the presence of organic materials is suspected from review of existing information or from field observations e. In general, organic soils with high natural soil moisture have a higher corrosion potential than inorganic soils. The unit weight of granular soils and some fine-grained soils can be estimated from soil descriptions in conjunction with descriptions of the relative density, Dr, Figure 3. The unit weight of cohesionless soils can be estimated from correlations with the SPT N-value. Laboratory testing of in situ unit weight of granular soils is not practical because the in situ soil density cannot be easily reproduced in the lab due to sample disturbance. The unit weight of fine-grained soils may be determined in the lab from undisturbed samples e. Navy 1982 , Kulhawy and Mayne 1990. The value of the friction angle is commonly estimated from correlations with results 33 from field tests e. Values of the friction angle as a function of parameters obtained from SPT and CPT are included in Table 3. The bar at the right hand side of Figure 3. Values of the friction angle obtained from uncorrected SPT N-values in Table 3. However, these approximations do not take into account the increase of SPT N-values with increasing overburden that is commonly observed in homogeneous cohesionless soils. Friction angles obtained from Figure 3. Source: Kulhawy and Maine, 1990. Range in column a from Peck, Hanson, and Thornburn 1974. Ranges in column b and for CPT are from Meyerhof 1956. Because in situ compactness of natural cohesionless soils cannot be easily reproduced in the laboratory due to sample disturbance, the friction angle of these soils is not commonly evaluated with lab testing. Therefore, for cohesionless soils, it is common to rely on the STP results and correlations similar to those presented in Figure 3. Fine-grained soils may exhibit drained and undrained shear strengths. The drained strength develops when no excess porewater pressures are generated during loading i. The undrained shear strength of a saturated, fine-grained soil develops when excess porewater pressures are generated during loading i. For normally consolidated, saturated fine-grained soils, an increase of porewater pressure during loading decreases the effective stress in the soil and thus decreases the undrained strength of the soil, whereas a decrease of porewater pressure during loading increases the effective stress in the soil and a corresponding increase in the undrained strength of the soil. In soil nail walls in fine-grained soils, the drained strength should be considered only when analyzing the long-term stability of a soil nail wall under a steady, static loading condition. For this case, the drained strength is mobilized when loads are applied at a slow rate and excess pore pressures are not generated. However, this condition is typically not the most critical in normally consolidated fine-grained soils. The correlation shown in Figure 3. Because of the scatter shown in Figure 3. The consolidated 35 undrained triaxial tests with porewater pressure measurements are the most commonly performed laboratory test to evaluate this parameter. The undrained shear strength must be considered for the short-term stability of slopes and any soil structure constructed in nearly saturated, soft to medium stiff fine-grained soils. It was discussed previously that the construction of soil nail walls in these soils is not typically advantageous. However, if soil nailing is considered, the short-term stability of a soil nail wall must be evaluated using the undrained shear strength, Su. Most commonly, the short-term strength i. Such conditions commonly require a specific site response dynamic analyses in which appropriate soil dynamic properties and representative time histories must be used. Additionally, it may be necessary to perform a Newmark-type of post-seismic deformation analysis, in which the cumulative post-seismic displacement of a potentially unstable mass of soil is calculated in a manner analogous to the sliding of a rigid block on a ramp subjected to cyclic loading. Details of this method can be found elsewhere Kramer, 1996; Kavazanjian et al. Dynamic and deformation analyses may be necessary when the simple pseudo-static method described above is not applicable, specifically in the case of large walls subjected to strong ground motions. However, this level of analytical complexity is commonly not required in the design of most soil nail walls. This force is the combination of the static and dynamic active lateral earth pressures that are induced by the inertial forces. When considering sliding force equilibrium, this increased lateral earth force must be taken into account instead of the static force PA in the sum of driving forces, ΣD, presented in Equation 5. The lateral earth force, including seismic effects, can be evaluated using the Mononobe-Okabe M-O method, which is an extension of the Coulomb theory Mononobe, 1929; Okabe, 1926. Another graphical source that can be used to calculate the total active pressure coefficient with the M-O method is from Lam and Martin 1986. Note that the M-O formulation does not arrive at a solution for certain combinations of the variables, in particular, when the slope of the backslope is greater than 22o. Another limitation of the M-O method is that the seismic coefficient provides a relatively simple approximation and cannot capture the complex deformation response of a soil nail wall system. The M-O procedure does not need to be considered in computer programs that use the seismic coefficients. The recommended design procedures presented in Chapter 6 provide a step-by-step description that includes the evaluation of seismic effects based on the M-O method and developed for MSE walls Elias et al. Soil nails mobilize bond strength between the grout and the surrounding soil as the soil nail wall system deforms during excavation. The bond strength is mobilized progressively along the entire soil nail with a certain distribution that is affected by numerous factors. As the bond strength is mobilized, tensile forces in the nail are developed. Depending on the soil nail tensile strength and length, and the bond strength, bond stress distributions vary and different internal failure modes can be realized. Typical internal failure modes related to the soil nail are Figures 5. Mechanical interlocking provides significant resistance when threaded bars are used and is negligible in smooth bars. The most common and recommended practice is the use of threaded bars, which reduces the potential for slippage between the nail bar and grout. The shear and bending resistances of the soil nails are mobilized only after relatively large displacements have taken place along the slip surface. Some researchers have found that shear and bending nail strengths contribute no more than approximately 10 percent of the overall stability of the wall. Due to this relatively modest contribution, the shear and bending strengths of the soil nails are conservatively disregarded in the guidelines contained in this document. A discussion of a methodology to account for shear and bending contributions is included in Elias and Juran 1991. A discussion of the two most common internal failure modes i. In addition, a section describing the relationship of the pullout resistance and the tensile force distribution in the nail is presented. Considering a single nail segment subjected to a tensile force, To, at one end, and applying equilibrium of forces along the differential length of the nail shown in Figure 5. As a simplification, the mobilized bond strength is often assumed to be constant along the nail, which results in a constant load transfer rate, Q. In the literature, it is common to find references to either qu or Qu. As discussed in Chapter 3, the bond strength depends on various factors, including the soil type, soil conditions, and the nail installation method. Typical values of ultimate bond strength for various soils and drilling methods were previously presented in Table 3. As an alternative to using published typical values, the equations above can be used to calculate apparently uniform, ultimate bond strengths and pullout capacity per unit length from nail pullout tests. These tests are described in Chapter 8. In general, a minimum factor of safety of 2 is recommended against pullout failure. The loads applied to the soil nails originate as reactions to the outward wall movement during excavation of the soil in front of the wall, as discussed earlier. The portion of the nail behind the failure surface i. The tensile forces in the soil nail, T, vary from the anchoring zone to the facing as follows: they start as zero at the end of the nail, increase to a maximum, Tmax, value in the intermediate length, and decrease to a value To at the facing Figure 5. The schematic distribution of the tensile force T along the soil nail is shown in Figure 5. The tensile force the nail increases at a constant slope Qu equal to the pullout capacity per unit length , reaches a maximum value, Tmax, and then decreases at the rate Qu to the value To at the nail head. With reference to Figure 5. The value Tmax is bounded by three limiting conditions: the pullout capacity, RP, the tensile capacity, RT, and the facing capacity, RF. The pullout capacity was defined in section 5. The tensile capacity is defined in section 5. The design nail force Tmax-s is used to verify the tensile capacity failure, which is defined as follows. The tensile capacity provided by the grout is disregarded, due to the difference in stiffness i. In general, a minimum factor of safety of 1. This failure mode should be considered separately for both temporary and permanent facings. This failure mode is only a concern for permanent facings. Appropriate dimensions, strength, and reinforcement of the facing and suitable nail head hardware e. A description of how to calculate maximum nail head tensile force is subsequently presented. Then, a description of each of the failure modes described above is presented in the following three sections, as well as the associated equations used to calculate the capacity for these failure modes. Chapter 6 contains a step-by-step methodology to verify the design capacity of the soil nail wall facing. These values are related to long-term soil nail forces and do not include freezing or other forces at the face. The normalized nail forces at the facing, also referred to as the nail head force, are comparable in distribution to the normalized maximum nail tensile forces shown in Figure 5. By comparing these two figures, the ratio of normalized nail head force to the maximum nail force varies from 0. In the upper half of the wall, the mean, normalized nail head force ranges between 0. These observations are consistent with those made on experimental walls in Germany and in France. In Germany, actual earth pressure measurements, recorded via total stress cells located at the shotcrete-soil interface, indicate that the equivalent earth pressure on the facing between 60 to 70 percent of the Coulomb active earth pressure for most conditions Gässler and Gudehus, 1981. In the French tests, the ratio of the nail head force to the maximum nail force generally varied between 0. In addition, these test results showed that due to the effect of soil arching between nails, a closer spacing of the nails caused a reduction in the measured forces on the wall facing as compared to what would be expected using simple tributary area contributions. Use maximum of SV and SH, the vertical and horizontal nail spacing, respectively, in Equations 5. For a typical nail head spacing of 1. If these moments are excessive, a flexural failure of the shotcrete may occur. Similarities in the loading mechanism between wall facings and continuous concrete slabs supported on columns, suggest that conventional concrete slab analysis and design methods can be applied to the design of soil nail wall facing. After the first yield of the facing section Figure 5. As the lateral pressure increases, fractures grow and deflections δ and nail tensile forces increase. Individual fractures indicate where the flexural capacity is achieved. Eventually, an ultimate stage of the structure is achieved when all fractures connect, act as hinges, and form a mechanism referred to as the critical yield line pattern. Yield line patterns are dependent on various factors including the soil lateral pressures, horizontal and vertical nail spacing, size of bearing plate, facing thickness, reinforcement layout, and concrete strength Seible, 1996 and are associated with a maximum soil pressure. In theory, the soil pressure that causes facing failure i. This force is designated as the facing flexure capacity, RFF, and is related to the flexural capacity per unit length of the facing. The flexural capacity per unit length of the facing is the maximum resisting moment per unit length that can be mobilized in the facing section. The factor CF takes into account the non-uniform soil pressures behind the facing Byrne et al. The soil pressure distribution behind the facing is generally non-uniform. Soil pressure is affected by soil conditions and the facing stiffness, which in turn affects the wall displacement. In the midspan between nails, the displacement of the facing occurs outward and the lateral earth pressure is relatively low. Around the nail heads, the soil pressure is larger than the soil pressure at midspan between nails. The pressure distribution in the facing also depends on the stiffness of the facing. When the facing is relatively thin as with typical temporary facings , the facing stiffness is relatively low, causing the facing to deform in the midspan sections. As a result, the soil pressure tends to be relatively low in the midspan sections. When the facing is relatively thick, the facing stiffness increases and the resulting wall deformations are smaller than would result from a thin wall facing. As a result of the increased wall stiffness, the soil pressure is more uniform throughout. Type of Structure Nominal Facing Thickness mm in. Factor CF 100 4 150 6 200 8 All 2. In practice, the cross section area of reinforcement in the horizontal direction is the same as for the vertical direction i. When the same nail spacing and reinforcement are used in the horizontal and vertical directions, and 420 MPa steel Grade 60 is used, Equations 5. The thickness of the temporary concrete facing is generally conservatively disregarded when evaluating the flexural capacity of the permanent facing as shown in Figure 5. The mesh consists of rebar No. Similar concepts can be applied along the horizontal direction. Given the tensile force at the soil nail head, To, and the facing flexure capacity, the safety factor against facing flexural failure can be defined. In addition, the ratio of the reinforcement in the nail and midspan zones should be less than 2. As the nail head tensile force increases to a critical value, fractures can form a local failure mechanism around the nail head. This results in a conical failure surface, as shown in Figure 5. This failure surface extends behind the bearing plate or headed studs and punches through the facing at an inclination of about 45 degrees, as shown schematically in Figure 5. The size of the cone depends on the facing thickness and the type of the nail-facing connection i. The correction factor CP is used to take into account the effect of the soil pressure behind the facing that acts to stabilize the cone. When the soil reaction is considered, CP can be as high as 1. These equations can be used for both temporary and permanent facing. For the temporary facing, the dimensions of the bearing plate and facing thickness must be considered. For the permanent facing, the dimensions of the headed-studs or anchor bolts must be considered. DH tH LS DS Figure 5. Permanent facing Figure 5. Given the tensile force at the soil nail head, To, Figure 5. Given the tensile force at the soil nail head To Figure 5. The headed studs may also exert excessive compressive stress on the concrete bearing surface. To provide sufficient anchorage, headed-stud connectors should be extended at least to the middle of the section, while maintaining 50 mm 2 in. To provide additional anchorage capacity, the headed studs should be long enough that the head is located behind the reinforcement. When threaded bolts are used in lieu of headed-stud connectors, the effective cross-sectional area of the bolts must be employed in the equations above. The evaluations contained in this section as related to facing failure modes are presented in a stepby-step methodology in Chapter 6. The outward movement is initiated by incremental rotation about the toe of the wall, similar to the movement of a cantilever retaining wall. Most of the movement occurs during or shortly after excavation of the soil in front of the wall. Post construction deformation is related to stress relaxation and creep movement, which are caused by post-construction moderate increases in tensile force in the soil nail described previously. Maximum horizontal displacements occur at the top of the wall and decrease progressively toward the toe of the wall. The size of the zone of influence Figure 5. The movements shown above are considered to be relatively small and comparable to those obtained with braced systems and anchored walls. These estimates of deformations have essentially become recommended design values. The adopted tolerable deformation criterion is projectdependent and should consider not only the magnitude of deformation but also the extent of the area behind the wall that may be affected by wall movements. As a first estimate, horizontal deflections greater than 0. When excessive deformations are considered to be likely with a certain wall configuration, some modifications to the original design can be considered. Soil nail wall deformations can be reduced by using a battered wall, installing longer nails in the top portion of the wall, using a higher safety factor, or even using ground anchors in conjunction with the soil nails. Additionally, some contractors have used soil nails that are grouted partially along their length and then partially tensioned to mobilize some of the nail tensile strength without soil mass deformation near the wall face. In these cases, after the tensioning is complete, the nails are fully grouted and the shotcrete is applied before the next lift is excavated. Post-construction monitoring of soil nail wall displacements indicates that movements tend to continue after wall construction, sometimes up to 6 months, depending on ground type. Typically, the post construction deformation increases up to 15 percent of the deformations observed soon after construction. As a result of this movement, additional tension is developed in the nails. In 106 general, fine-grained soils of high-plasticity i. However, this is only true as long as the strength of the soil behind the wall is not reduced significantly during seismic events. Permanent deformations of soil nail walls due to a seismic event may be estimated using the Newmark 1965 procedure. As introduced previously, this method consists of calculating the displacement of a potential sliding mass of soil in a manner analogous to the sliding of a rigid block on a ramp subjected to cyclic loading. Additional details of this method can be found elsewhere Kramer, 1996; Kavazanjian et al. Also, numerous general slope stability programs have added the capability to model various types of soil reinforcement including the use of soil nails and ground anchors. Although soil nails can be considered in the majority of in-use general slope stability programs, the design of soil nail walls and, in particular, the nail lengths, is not as straightforward in these programs as it is in programs specifically developed for soil nail applications. The two computer programs most commonly used in the United States for the design of soil nail walls are SNAIL and GOLDNAIL. The main features of these programs are described below. The program is based on two-dimensional limit equilibrium that considers force equilibrium only. The failure surface is bi-linear with the failure surface originating at the toe or tri-linear with the failure surface originating at the bottom of the excavation at a point away from the toe. For the case of a tri-linear failure surface, the resisting forces in the lower wedge beneath the wall are calculated assuming passive earth pressure conditions, with the inclination of the passive force fixed at the mobilized friction angle. The methods included in SNAIL only consider force equilibrium. Therefore, in general, although interblock forces are in equilibrium, moment equilibrium is generally not simultaneously achieved with this method. The program allows the user to specify an area in which the program searches for the most critical surface. The search routine is performed at 10 nodes of the search width previously defined and subsequently tries 56 surfaces at each node. While the total number of searched surfaces is 560, SNAIL reports the 10 most critical on the screen and in an output file. SNAIL can model up to seven soil layers. Up to two slope segments can be modeled at the toe. The phreatic or piezometric surface is defined only by up to three points. SNAIL allows up to two uniform vertical surcharge distributions and an internal or external force horizontal or oblique. Therefore, it is possible to model a soil nail wall with an added ground anchor. The soil layers are modeled as lines with end points, whose coordinates are entered by the user. The program has limitations in modeling complex stratigraphy but is adequate to assist analyze and design for a wide range of simple conditions and geometries. For complex wall and ground geometry and loading conditions, the designer may need to make simplifications due to the limitations of the program. The necessary input related to the reinforcement includes the location, diameter, inclination, vertical and horizontal spacing, and tensile strength of the nails. These data can be easily assigned to all nails at once i. The necessary input data for the soil parameters are the unit weight, the ultimate shear strength, the bond strength, and the bond strength reduction factor BSRF , which was developed to easily and selectively reduce values of bond strength for some nails. The BSRF may be interpreted as the inverse of the factor of safety for the bond strength or pullout capacity FSP. When associated to the FSP, BSRF varies typically between 0. Other input parameters considered in the program include the nail bar cross-sectional area, yield strength of the nail bar, and the facing punching shear capacity. Pseudo-static seismic analysis can be performed in SNAIL by entering the horizontal and vertical seismic coefficients discussed earlier in this chapter. The soil strength criterion used in SNAIL is the conventional linear Mohr-Coulomb envelope. Parameters can be entered in either English or Metric units. Bond strength input is associated with the soil input, not with the nail input. Hence, if different bond strengths need to be modeled in an otherwise homogeneous soil profile, a new soil layer must be defined. Although the program has been developed specifically for soil nail walls, it can also calculate safety factors for the stability of unreinforced and MSE walls and slopes, as well as ground anchor walls, as described earlier. Also, because the program allows for failure surfaces to go beneath the wall and daylight in the excavation, a rough estimate of the bearing stability can be performed with this program when failure surface beneath the wall are considered. The designer can change the input parameters interactively and rerun the analysis until the design criteria for nail locations, diameters, and lengths are met. Results are presented graphically on the screen and in output files. The output screen contains minimal information. The output files provide the forces for each nail and for each of the 10 most critical failure surfaces analyzed. A report of the most critical failure modes is provided in the output file. In summary, the program calculates the global factor of safety, FSG and determines the controlling failure mode either global stability, facing, or nail tensile failures if the actual facing capacity i. There is, however, limited technical support available from CALTRANS to support the program. Data can be input with relative ease because they are organized in tables, which facilitates error checking on the screen. The program works in three modes: 1 design, 2 factor of safety, and 3 nail service load modes. In the design mode, a trial run is initiated; subsequently, the program can modify the nail properties i. In the factor of safety mode, the global factor of safety is calculated for specified input parameters. The program can model up to 13 soil layers, complex slopes and subsurface geometries, horizontal and vertical surcharge distributions, groundwater, and pseudo-static horizontal coefficients. Nail and soil parameters are similar to those described in the previous section. A limitation of the program is that all nails must have the same spacing and inclination. Although this scenario is typical for most designs, this assumption may be too restrictive for some cases. In addition, the failure circles can only pass at or above the toe. Therefore, sliding and bearing capacity cannot be assessed with this program, and the analyst must resort to other procedures or computer programs to evaluate sliding and bearing capacity, if deemed necessary. The soil strength criterion is a linear Mohr-Coulomb envelope with the option of using a bi-linear strength envelope. This option is useful because it may result in a closer modeling of typical nonlinear soil behavior as it allows the user to specify a lower friction angle under higher confinement pressure, compared to the values anticipated at lower confining pressures. GOLDNAIL can also be used to analyze unreinforced slopes and walls and ground anchor walls. For the unreinforced case, the program can handle various pressure distributions acting on the wall face. This program satisfies moment and force equilibrium. Similar to conventional slope stability methods, GOLDNAIL divides the potential sliding mass into vertical slices. The program modifies iteratively the normal stresses distribution at the base of the slices until force and moment equilibrium is obtained. Input data can be entered in English, SI, or any compatible unit system. The program allows the consideration of factored reduced strengths when the Load and Resistance Factor Design LRFD method is selected and safety factors when the ASD method is used. Results are presented on the screen and in output files. The graphical information is limited to the minimum calculated factor of safety and its associated critical circle. Output files provide the maximum nail forces and report the controlling type of soil nail failure mode. This strategy provides a wide variation of the calculated factor of safety for the conditions analyzed. In the first case Figure 5. For this case, the computed global factor of safety was 1. In the second case Figure 5. Additional analyses performed using both computer programs and project experience indicate that the factor of safety calculated with these computer programs typically differs by 5 to 10 percent, for the typical range of safety factors between 1. Greater variations, up to about 20 percent, in the calculated FSG occur for factors of safety greater than 1. These results are also comparable to trends that were observed in a more recent study, where the performance of various commercially available software packages for slope stability analysis was evaluated Pockoski and Duncan, 2000. SNAIL does not always give safety factors that are lower than those obtained with GOLDNAIL, although this was not the case in the comparison shown in Figure 5. One of the reasons attributed to the difference in the calculation results from the two programs is the different assumption of the force distribution along the nail. SNAIL assumes a linear distribution of nail forces, starting from zero at the end of the nail to a maximum value that remains constant from the critical failure surface and the wall facing. In GOLDNAIL, a reduction of the nail force near the wall facing is assumed. However, the factor of safety for global stability is not very sensitive to these distributions as shown by comparative analysis of reinforced soil using different force distributions along the reinforcement Wright and Duncan, 1992. Both computer programs allow a fast and thorough design of typical situations encountered in soil nail wall applications. The major difference in performance resides in the ability of the programs to model increasingly more complex geometry and soil properties and the ease with which data are input and results interpreted. Despite the difference between the bi-linear or tri-linear critical failure surface in SNAIL and the circular failure surfaces considered in GOLDNAIL, the calculated critical surfaces exhibit similar locations as shown in Figure 5. For more complex subsurface conditions e. Neither of these programs can analyze composite failure surfaces, which might be applicable when multiple soil layers with dissimilar strengths exist. Also, neither of these programs can simulate cracks at the ground surface. As mentioned earlier, the great advantage of SNAIL is that it can be readily downloaded from Internet and is free of charge. However, technical assistance is limited. Recommended factors of safety, modified after Byrne et al. The 111 recommended factors of safety are only applicable to the ASD method where loads are unfactored see Section 5. Minimum Recommended Factors of Safety Failure Mode Resisting Component Global Stability long-term External Stability Internal Stability Facing Strength FSG Global Stability excavation FSG Sliding FSSL Temporary Structure Permanent Structure Seismic Loads 2 Temporary and Permanent Structures 1. The larger value may be applied to more critical structures or when more uncertainty exists regarding soil conditions. When using stability analysis programs to evaluate these failures modes, the factors of safety for global stability apply. Typical applied loads are dead loads e. Several load combinations that include two or more load types must be considered in design to assess the most critical loading condition. Typical combination coefficients are also listed in Table 5. For most soil nail wall applications, load groups with static or quasi-static loading i. For seismic loads Load Group VII , AASHTO 1996 allows increasing the allowable stresses 133 percent from the values obtained with factors of safety for static loads as indicated in the last column of Table 5. This approach was followed to develop safety factor for seismic conditions included in Table 5. To minimize these complications, surface water runoff and groundwater must be controlled both during and after construction of the soil nail wall. Additionally, it has been shown that soil nail walls perform significantly better when an effective drainage system is installed to control water levels behind the wall. A brief description of the control systems commonly used in soil nail walls is presented below. A surface water interceptor ditch, excavated along the crest of the excavation and lined with concrete, applied during the shotcreting of the first excavation lift, is a recommended element for controlling surface water flows. Additionally, if the design engineer believes that the groundwater impacts are 114 localized or short-term conditions, wells or well points installed beyond the length of the nails may be used temporarily to lower the groundwater table. However, this approach may result in much higher construction costs and delays. These elements are strips of synthetic material approximately 300 to 400 mm 12 to 16 in. They are placed in vertical strips against the excavation face along the entire depth of the wall Figures 4. The horizontal spacing is generally the same as the nail horizontal spacing. The lower end of the strips discharges into a pipe drain that runs along the base of the wall or through weep holes at the bottom of the wall. For highly irregular excavation faces, the placement of prefabricated drain strips against the excavated face is difficult and often impractical. In some cases, the prefabricated drain strips may be sandwiched between the shotcrete construction facing and the permanent CIP facing, with the drain placed over 50 to 75 mm 2 to 3 in. The design engineer needs to provide explicit construction and inspection guidance for this type application, to assure that the performance of the drainage system is not impacted during installation of the shotcrete. If appropriate performance cannot be guaranteed, the effect of the groundwater table needs to be considered in the analysis. Concrete Ditch Groundwater Table Geodrain Strips Weephole Drains Toe Drain Figure 5. Shallow Drains Weep Holes. These are typically 300- to 400-mm 12- to 16-in. Weep holes are also used as the terminating point of the vertical strip drains to allow any collected water to pass through the wall. Horizontal or slightly inclined drain pipes may be installed where it is necessary to control the groundwater pressures imposed on the retained soil mass. Drain pipes typically consist of 50-mm 2-in. Drain pipes are typically longer than the length of the nails and serve to prevent groundwater from being in contact with the nails or the soil nail wall mass, as shown in Figure 5. The lengths of the drains depend on the application. To provide drainage of shallow or perched groundwater occurring erratically close to the facing, drain pipes with lengths varying from 0. They are installed at a density of approximately one drain per 10 square meters 100 square ft of face. Drain pipes are typically deployed after nail installation to prevent potential intrusion of nail grout into the slotted pipes. The pipes typically exit through the face of the wall. The PVC pipe should be slotted, as shown in Figure 5. Although drain pipes are typically installed after nails are in place and the shotcrete is applied to avoid either grout or shotcrete from entering the drain, they can be applied prior to shotcrete application. In this case, a plug of dry-pack and temporary PVC caps must be used to prevent the shotcrete from coming into the drain hole and obstructing the drain slots or perforations. Permanent Surface Water Control. Permanent surface water control measures include installing an interception ditch behind the wall to prevent surface water runoff from infiltrating behind the wall or flowing over the wall edge. A vegetative protective cap may be also be used to reduce or retard water infiltration into the soil. Analysis of soil nail walls for long term conditions may need to take into consideration the potential for clogging. This phenomenon may result in damage to the facing Byrne et al. In situations where the facing is designed to resist frost damage, the nail or to the connection between the nail and the facing can still be impacted by frost. Increases in nail and facing loads should be anticipated in areas where frost durations are generally greater than one 116 week, where frost susceptible soils are encountered near the face, and where the face is in close proximity to a source of water. Soils susceptible to frost action are those exhibiting the following characteristics: 1 more than 3 percent of the solids fraction is smaller than 0. Cu is the uniformity coefficient, which can be obtained from grain size gradation tests see Table 3. END CAP ~ PERMANENT SHOTCRETE 50 mm 2 in. SCH 40 PVC PIPE 10° - 15° PROTECTIVE PVC CAP NOTE 1 TE SLOT N CTIO D SE 2 ft 0. PROTECTIVE CAP NEEDS TO BE REMOVED AFTER FINAL SHOTCRETE IS APPLIED 2. SPACING OF DRAINS IS TYPICALLY 3. This can be done by placing porous backfill e. Because a 25-mm 1-in. CAST-IN-PLACE STRUCTURAL CONCRETE WALL 15° - 25° SHOCRETE TEMPORARY FACING 4-INCH 4:1 PVC SLEEVE WEEPHOLES AT 5' O. For relatively light loading conditions, the external loads can be used to define additional shear forces and flexural moments in the section of the wall above the first row of nails. These loads are then added to the calculated facing loads for subsequent analysis. For more significant loads e. The magnitude and distribution of the load transferred to the wall depends on the distance of the load to the wall and the type of load foundation shallow or deep. The magnitude of these loads can be significantly increased if the structure is subject to seismic forces. This is particularly important for the facing of the initial excavation lifts that becomes unsupported when the next excavation lift is performed. For typical construction facings consisting of 100-mm 4-in. For thicker applied shotcrete facings, support for the shotcrete facing weight by considering the shear capacity of the nails and the bearing capacity of the soils beneath the nails should be formally evaluated. The maximum thickness of shotcrete facing that can be supported in this manner is dependent on the strength of the soils. In competent ground, shotcrete facings up to 200- to 250-mm 8- to 10-in. If necessary, support of the shotcrete facing weight may be achieved by the installation of additional short, steeply inclined reinforcing elements acting as compression struts. The soil nail wall has a conventional headed-stud connection to the facing. The strut nail has a bearing plate and washer connection system, as shown. Note that shear resistance along the facing-soil interface is disregarded. The structure acts as an equivalent battered face wall when the horizontal setback is small in relation to the height of the individual benches. When the horizontal setback is larger than the height of the lower wall H2 , each individual wall will act independently and each wall must be analyzed and designed as two independent soil nail wall structures. If the horizontal setback is smaller than the height of the lower wall, the lower wall must be analyzed considering the upper wall as a surcharge. For instance, a composite system may consist of soil nails in conjunction with ground anchors Figure 5. Both nails and ground anchors are installed as excavation proceeds from top-down. The main objective of using ground anchors is to contribute significantly to global stability. In addition, wall deformation can be greatly reduced, particularly if the ground anchors are installed in the top portion of the wall Figure 5. Ground anchors can be also installed along the full height of the wall by means of precast concrete posts Figure 5. High walls, up to 25 m 82 ft , have been constructed with composite systems. The design methodology depends on the configuration of the support system, particularly on the relative contribution and intended function of the nails and ground anchors. For instance, for the system shown in Figure 5. The soil nailed zone is then considered a rigid block. The ground anchors must provide stability against deeper failure surfaces. Therefore, the length of the ground anchors will be controlled by the stability requirements of the soil nailed block. The reader is referred to GEC No. The five major steps and their substeps in this design method are outlined in Table 6. In the remainder of this chapter, each of these steps is presented. INITIAL SOIL NAIL WALL DESIGN CONSIDERATIONS A. Soil nail vertical and horizontal spacing C. Soil nail pattern on wall face e. Soil nail inclination E. Soil nail length and distribution F. Soil nail material type e. Selection of relevant ground properties for design e. Other initial considerations Step 2. PRELIMINARY DESIGN USING SIMPLIFIED CHARTS These charts are used to preliminarily evaluate nail length and maximum nail force. External Failure Modes 1 Global stability 2 Sliding stability 3 Bearing capacity B. Internal Failure Modes 1 Nail pullout resistance 2 Nail tensile resistance D. Facing Design 1 Nail head load 2 Wall facing type and thickness 3 Facing materials 4 Flexural resistance 5 Facing punching shear resistance 6 Facing head stud resistance 7 Other design facing considerations Step 4. ESTIMATE MAXIMUM WALL DEFORMATIONS Step 5. OTHER DESIGN CONSIDERATIONS A. Support for facing dead load e. After completing the design, the design engineer will prepare soil nail wall specifications see Chapter 7 and recommendations for construction monitoring see Chapter 8. STEP 1: INITIAL SOIL NAIL WALL DESIGN CONSIDERATIONS Wall Layout Establish the layout of the soil nail wall, including: 1 wall height; 2 length of the wall; and 3 wall face batter inclination typically ranges from 0º to 10º. The evaluation of the wall layout also includes developing the wall longitudinal profile, locating wall appurtenances e. Battered wall face can be selected to improve temporary face stability, as a battered face exerts smaller forces on the wall, thus requiring shorter soil nails. The material savings resulting from the use of shorter nails may offset the increased cost of soil excavation incurred to create the batter. A mild batter i. A batter greater than 10 degrees can enhance stability. Soil Nail Vertical and Horizontal Spacing Horizontal nail spacing, SH, is typically the same as vertical nail spacing, SV Figure 6. Nail spacing ranges from 1. This reduced spacing for driven nails is required because driven soil nails develop bond strengths that are lower than those for drilled and grouted nails. A soil-nail spacing of 1. Soil nail spacing may be affected by the presence of existing underground structures. The design engineer should specify a minimum horizontal soil nail spacing of about 1. Design forces from global stability analysis and facing design are affected by soil nail spacing. In general, the larger the spacing, the greater the design forces. The purpose of the minimum nail spacing is to reasonably ensure that group effects between adjacent soil nails are minimized due to potential nail intersection as a result of drilling deviations. Group effects reduce the load-carrying capacity of individual soil nails. The maximum soil nail spacing should also be specified. Soil Nail Pattern on Face The soil nail pattern is commonly one of the following see Figure 6. A square pattern results in a column of aligned soil nails, and facilitates easier construction of vertical joints in the shotcrete facing or easier installation of precast concrete panels. Also, a square pattern enables a continuous vertical installation of geocomposite drain strips behind the facing to be easily constructed. In practice, a square pattern is commonly adopted. GEOCOMPOSITE DRAINAGE STRIPS SH SVO 15° 2 MANHOLE 100 mm 4 in. It is noted that the tensile strength and punching failure mechanisms are explicitly considered subsequently in this chapter. E or calculated in the preliminary design and perform global stability analysis using SNAIL. Sliding Stability Figure 5. Bearing Capacity Figure 5. Seismic Considerations Consider seismic loads to ensure that nail lengths calculated in the previous step for static loading condition provide adequate factors of safety for seismic loading conditions. Determine the seismic zone of the project site. Use national seismic maps e. Alternative sources such as NEHRP 1997 and IBC 2000 can be used. Read maximum ground acceleration coefficient, AI, from maps. Establish soil profile type at the site this step requires results from the site investigation. Determine site coefficient S from Table 6. Obtain site coefficient S from Table 6. Soil Profile Description S I 1. Stiff soil conditions where the soil depth is less than 60 m 200 ft and the soil types overlying bedrock are stable deposits of sands, gravels, or stiff clays. The lower values correspond to stiffer soils. This analysis may be performed using SNAIL. This 50 percent reduction of Am is based on results from seismic deformation analyses on translational slope failures. These results indicate that cumulative permanent seismic deformation is relatively small e. Seismic sliding stability C. In a SNAIL analysis, the calculated nail lengths corresponding to an acceptable global factor of safety for a given critical failure surface are based on pullout capacity values, which have already been reduced by the factor of safety with respect to pullout, FSP. Steel reinforcement: Grade fy : 420 MPa Grade 60 , 520 MPa Grade 75. WWM features refer to Appendix A, Table A. Rebar features refer to Appendix A, Table A. Select headed-stud characteristics Table A. Select bearing plate geometry: min. Typically, the amount of reinforcement at the nail head is the same as the amount of reinforcement at the mid-span i. For temporary facing, if waler bars are used at the nail head in addition to the WWM, recalculate the total area of reinforcement at the nail head in the vertical direction see Equation 6. Calculate the reinforcement ratio ρ at the nail head and the mid span as see Section 5. Verify that the reinforcement ratio of the temporary and permanent facing at the midspan and the nail head are greater than the minimum reinforcement ratio i. Verify that the reinforcement ratio of the temporary and permanent facing at the midspan and the nail head are smaller than the maximum reinforcement ratio i. Using the recommended factor of safety for facing flexure FSFF listed in Table 5. Alternatively, use the equations presented in Sections 5. Using the recommended factor of safety for punching shear FSFP listed in Table 5. Calculate the maximum tensile resistance due to headed-stud tensile failure RHT using Table 6. Provide sufficient anchorage to headed-stud connectors and extended them at least to the middle of the facing section and preferably behind the mesh reinforcement in final facing. Provide a minimum 50 mm 2 in. If capacity is not enough, adopt larger elements or higher strengths and recalculate. Additional are included in section 5. If the plate and other hardware elements are not within the ranges recommended, a formal calculation of capacities should be performed. Note that some proprietary systems employ spherical seat nuts that do not require washers. One potential scenario for these types of analyses is when soft sites and there exist a potential for site amplification Kramer, 1996; Kavazanjian et al. If soil is susceptible to frost action, provide a facing thickness greater than the frost penetration depth. Use insulators to reduce thickness requirements see Figure 5. To take into consideration the effect of storage and heavy construction equipment. Also, consider a load of 15 kPa about 100 psf for temporary conditions. EXAMPLE PROBLEM The following section presents a step-by-step example problem that illustrates the recommended design procedures described previously in this chapter using only the simplified design charts of Appendix B. A complete and more detailed design example is presented in Appendix. The project consists of a wall near the access of a non-critical, lightly trafficked road. The site is rural. No buildings are located near the proposed location of the soil nail wall. The area is flat and the elevation of the groundwater table is significantly below the bottom of the proposed excavation. Site access: easy site access. Utilities: no disturbance of utilities. Adjacent structures: none; extent of deformed zone behind wall is not an issue. Other geometric constraints: no headroom limitations; nails can be installed without difficulties behind wall. STEP 1: INITIAL CONSIDERATIONS AND PARAMETERS A. Soil nail pattern on wall face: uniform D. Soil nail length distribution: uniform F. Calculate normalized bond strength: q u D DH 18 psi × 144 in. Other design considerations 1 Drilling method: For the existing ground conditions, conventional rotary drilling is possible Chapter 3. Per Appendix C guidelines, for temporary structures and ground with unknown aggressivity, provide Class II corrosion protection as a minimum. Provide a minimum grout thickness of 25 mm 1 in. This value is on the low side. Bar installation: This bar can be installed with no difficulty in the drillhole. Available cover is at least 6 - 1. STEP 3: FINAL DESIGN A. External Failure Modes Only the preliminary design developed in Step 2 is considered in this example. Seismic Considerations No seismic considerations are necessary in this example. Internal Failure Modes Only the preliminary design developed in Step 2 is considered in this example. DSH tSH LS DSC A-5 APPENDIX B CHARTS FOR PRELIMINARY DESIGN A series of design charts were developed as a design aid to provide preliminary nail length and maximum design tensile forces. The charts were developed using the computer program SNAIL see Section 5 and Appendix F. These charts are only strictly applicable for the conditions they were developed for. The charts should only be used to obtain preliminary design values and should not be used in lieu of comprehensive analyses. Two types of charts were created. The first type of chart Figures B. A total of six charts were created, one for each combination of α and β values shown in Table B. For intermediate values of α and β values, it is acceptable to interpolate between charts. As discussed in Chapter 5, FSG is set at 1. After Tmax-s is determined and a steel tensile strength is selected, the necessary cross sectional area of the nail can be calculated. Other design parameters listed in Table B. Subsequently, these parameters were varied see ranges in Table B. Nail inclination was kept constant at 15 degrees. The results indicate that the normalized length and maximum forces in the nail also depend on the global factor of safety, drillhole diameter, and the soil cohesion. The effect of the wall height was not significant, and thereby is not longer considered. Included Not Included Parameter Units Fixed Value used for Design Chart Pullout Factor of Safety, FSP - 1. Two sets of correction factors have been developed. Values of C1L are shown graphically as a function of the drillhole diameter, DDH in B-2 Figure B. Values of C2L and C3L are expressed as mathematical formulas also included in Figure B. The second set of correction factors, C1F and C2F, were developed to correct tmax-s for drillhole diameter and soil cohesion. No correction for global safety factor is necessary for tmax-s. Values of C1F are shown graphically as a function of the drillhole diameter, DDH, in Figure B. Values of C2F are expressed as a mathematical formula included in Figure B. Design Force, tmax-s a 00 0. Design Force, tmax-s a 00 0. Design Nail Force, tmax-s a 00 0. Design Force, tmax-s 00 0. Design Force, tmax-s a 00 0. The level of corrosion protection that soil nails require depends largely on the nature of the project; specifically, the expected life, the perceived importance of the structure, and consequences of failure. In general, corrosion protection of soil nails is required for all permanent soil nail walls. Corrosion protection of soil nails can be achieved by physical and chemical protection measures or a combination of both. The selection of the type of corrosion protection depends on the design life of the structure i. In this appendix, basic information on corrosion is presented along with a description of corrosion protection systems used in soiling applications and the criteria used to evaluate the necessary level of corrosion protection for a soil nail system. Most refined metals revert naturally and irreversibly from a less stable state to their native, more stable state, if conditions promoting corrosion are present and prevail over factors inhibiting corrosion. The basic mechanism of metallic corrosion consists of the movement of ions in an electric circuit: from a metal surface anode , through a nonmetallic conductor in solution electrolyte , and onto another surface cathode due to a voltage difference, differences in oxygen concentrations, or differences in other environmental conditions. With time, the anode is consumed by the loss of metal into the electrolyte. In grouted soil nails, flow of ions can occur between bare reinforcing steel and a nearby metal object or between points on the metal surface not covered by grout. Numerous factors affect the rate of corrosion, including the characteristics of the metallic surface e. For grouted nails, the main corrosion mechanism is pitting or localized corrosion. Other mechanisms such as stress corrosion, hydrogen embrittlement and fatigue corrosion are usually not significant for soil nail applications. This section provides a general overview of general and localized C-1 corrosion. Detailed information on the effects of various corrosion mechanisms on steel is provided in Weatherby 1982. General Corrosion: General corrosion occurs as a thin layer of rust uniformly distributed on the surface of steel bars. This type of corrosion can develop when unprotected steel is exposed to the environment during shipping and improper on-site storage. Under certain conditions, the thin rust layer becomes a protective film against further corrosion, a mechanism referred to as passivation. Metal loss is typically not significant with general corrosion, as long as the exposure time is limited and detrimental conditions promoting corrosion see below are absent. It is good practice to inspect nail bars for any surface corrosion and to remove thin layers of rust by wiping before the bars are installed in the drillholes. Lightly rusted bars may be inserted into the drillhole without rust removal in temporary applications. Pitting or Localized Corrosion: Localized corrosion develops as pitting or crevices and is confined to specific locations along the steel bar. In very corrosive or aggressive ground conditions, grouted bars will pit after only a few weeks of exposure. In general, localized corrosion is triggered by the removal of the protective passivation coating. Pitting and crevices promote non-homogenous concentrations of ions, an increase in electrochemical potential and further localized corrosion. As the pit propagates deeper, the bar cross-section is reduced and this may eventually lead to a brittle, sudden failure. Pits or cracks on the surface of the steel bar are adequate reason for rejection of the bar. The factors above collectively define ground corrosion potential or aggressivity of the ground. In general, the ground is classified with a strong corrosion potential or aggressive if any one of the conditions listed in the first column of Table C. In addition, buried structures immediately adjacent to the project having suffered from corrosion or direct chemical attack might be an indication of strong corrosion potential. If all the conditions listed in the first column of Table C. Tests from a nearby site can be used to evaluate the corrosion potential of the site if the designer can establish that the ground conditions are similar. Otherwise, if tests are C-3 not performed, then the ground should be assumed to be aggressive. Classification of ground aggressivity should consider the possibility of changes during the service life of the soil nail wall, which may cause the ground to become aggressive e. Only experienced personnel must perform tests on soil resistivity, soil chemistry, and presence of stray currents. Physical protection involves placing a continuous barrier between the nail bar, other metallic parts, and the corrosion sources. Chemical protection consists of the use of a sacrificial material or a dialectric material, which will preclude the flow of electric current. Some of the corrosion protection systems currently in use utilize a combination of these mechanisms. In addition, when stray currents are of concern, electrical isolation of the nail assemblies should be used. The most common systems used to provide corrosion protection for soil nails are described below. After the bar is centered in the drillhole, neat grout is injected and fills up the annular space around the steel bar. Grout encapsulation provides both physical and chemical corrosion protection. When a minimum grout cover is in place, components such as carbonates and chlorides in the soil, and oxygen and humidity in the air are prevented or delayed in reaching the bar due to passivation. Additionally, the grout must have low permeability to ensure the effectiveness of the encapsulation. The grout provides an alkaline environment that reduces the corrosion potential. A minimum grout cover of 25 mm 1 in. Cement grout is placed around all epoxy-coated nail bars. The minimum required thickness of epoxy coatings is 0. The epoxy coating provides physical and chemical protection, as epoxy is a dielectric material. In transporting and handling bars, the epoxy coating may be damaged before nail installation. Therefore, it is not uncommon to spray epoxy coating in the field on chipped or nicked surfaces. Applicable standards for epoxy coating are found in ASTM A-775. The process is performed by hot-dipping bars and other metallic C-4 pieces with zinc. The protection provided by galvanized coating is both physical and chemical, as this process forms a protective layer of zinc oxide. Cement grout is placed around all galvanized nail bars. This practice is common in soil nail applications used for permanent soil nail walls built in an aggressive or unknown environment. The sheathing is corrugated to transfer the effect of anchorage to the surrounding grout. Grout must completely fill the annular spaces inside and outside the sheathing. The minimum grout cover between the sheathing and the nail bar is 10 mm 0. This distance allows the injected grout to flow without difficulty and provides sufficient physical protection. Outside the sheathing, the minimum grout cover between the sheathing and the drillhole wall must be 20 mm 0. PERMANENT FACING TEMPORARY FACING BEARING PLATE HEADED-STUD TYP. MINIMUM COVER 25 mm 1 in. MINIMUM COVER NAIL GROUT 150 mm 6 in. In some systems, the inner annular space is grouted in the shop and the whole assembly transported to the project site. The sheathing must be sufficiently strong to resist transportation, handling, and installation. Additionally, sheathing must be non-reactive with concrete, chemically stable, ultraviolet-light resistant, and impermeable. The minimum sheathing wall thickness is typically 0. Certain sheathing techniques may be proprietary. Epoxy coating can be applied on bearing plates and nuts. PERMANENT FACING TEMPORARY FACING GEOCOMPOSITE STRIP DRAIN BEARING PLATE HEADED STUD TYP UNCOATED NAIL BAR INNER GROUT NUT AND BEVELED WASHER PVC SHEATHING OUTER GROUT CENTRALIZER 50 mm 2 in. MINIMUM COVER END CAP ASSEMBLY ~ 20 mm 0. MINIMUM COVER 150 mm 6 in. MINIMUM COVER PVC SHEATHING FACING REINFORCEMENT NOT SHOWN OUTER GROUT Modified from Byrne et al. The method consists of interrupting the current passage between the electric source and the nail bar. The isolation can occur along the path or at the nail. PVC sheathings can be used for corrosion protection to provide isolation. When the sheathing is not present, the bearing plate and the nail head should be isolated from steel in the facing and all other nails. Effectiveness of electrical isolation must be field tested and verified by personnel qualified for this task after nail installation and before grouting. The use of grout in conjunction with PVC sheathing is known as Double Corrosion Protection and is used in aggressive or unknown conditions. When using corrosion protection levels Class I or Class II as defined here, it is not necessary to incorporate a sacrificial thickness into the design. Sacrificial thickness is never used as the sole protection method in permanent applications. In temporary applications, unprotected, bare bars can be driven, as long as the soil corrosion potential is mild or insignificant. A preliminary and safe for most conditions estimate of the required sacrificial total thickness in unprotected bars is 2 mm 0. Although the detrimental effects of corrosion may be less in passive nail bars as compared to post-tensioned ground anchors, the use of the PTI guidelines in soil nail walls is helpful because the protection methods in both applications are similar. However, some elements of corrosion protection present in ground anchor applications i. The PTI guidelines for ground anchors indicate that the selection of the level of corrosion protection is dictated by the following factors. Service Life: Service life is based on the permanency of the structure i. A service life of 18 months or less qualifies the structure as temporary. If the service life of the temporary structure is likely to be extended due to construction delays, an evaluation C-7 should be made to determine if additional corrosion protection, particularly in corrosive ground conditions, is necessary. Ground Corrosion Potential: Nails in environments with high corrosion potential require the highest class of corrosion protection listed for each service life. Class II corrosion protection for temporary soil nail walls and Class I corrosion protection for permanent soil nail walls. Failure Consequences: Serious consequences include loss of life, collapse of the wall, damage to nearby utilities and structures, structural repairs, and impact to traffic. These risks are expected in urban areas, walls alongside heavily traveled highways, and areas with problematic soil conditions where slope movements have been experienced. The PTI guidelines specify a Class I protection when the potential failure consequences are serious, regardless of soil corrosion potential. The owner of the project should consider whether the increased costs of providing the higher level of corrosion protection are justifiable. Selection of Corrosion Protection SERVICE LIFE TEMPORARY PERMANENT AGGRESSIVITY NOT KNOWN OR AGGRESSIVE CLASS II AGGRESSIVITY NOT KNOWN OR NON-AGGRESSIVE NONE not applicable in soil nails AGGRESSIVE NON-AGGRESSIVE FAILURE CONSEQUENCES CLASS I SERIOUS NOT SERIOUS COST FOR INCREASING CORROSION PROTECTION CLASS I SMALL SIGNIFICANT CLASS II CLASS I Modified from PTI 1996. The similitude of project conditions will be helpful in comparing soil nail and ground anchor technologies. INITIAL CONSIDERATIONS A 10-m 33-ft high soil nail wall is to be constructed as part of a roadway project. The road where the wall is to be constructed has a low to medium volume of traffic and thereby can be considered to be non-critical. Because the wall will be a permanent structure, aesthetic requirements call for a CIP concrete permanent facing. The wall is to be constructed in medium dense silty sand with clay seams, as shown in Figure D. Underground utilities will be installed in the future. Although the final location of the utilities is not known, the utilities are expected to be installed in the area of influence of the nails, as indicated in Figure D. Additionally, some light structures will be built in the future approximately 10 m 33 ft behind the wall. There is no source of corrosion potential at the site. The site is in a seismic zone and the horizontal seismic coefficient to consider in the analyses was estimated to be 0. SUBSURFACE CHARACTERIZATION General Geotechnical borings drilled in front of, alongside, and behind the proposed wall alignment indicate that the subsurface stratigraphy is relatively uniform. The profile shown in Figure D. Predesign considerations Some of the Predesign considerations the design Engineer must consider i. STEP 1: Initial Soil Nail Wall Considerations Section 6. Wall height H , wall length Le , and face batter α are as follows: a. Soil Nail Vertical and Horizontal Spacing, SH and SV 1. Soil Nail Pattern on Wall Face 1. Select a rectangular pattern, as the ground conditions are not so poor so as to justify a staggered pattern, in which a more uniform distribution of soil nail forces behind the wall is created. Soil Nail Inclination, i 1. D-3 GEOSYNTEC CONSULTANTS Written by: CAL Client: FHWA E. Two nail length patterns are considered: one with uniform nail length and one with non-uniform nail length Figure D. The uniform pattern was selected to evaluate a widely used soil nail configuration that can be directly designed using design charts, is less prone to avoid cause poor performance in relation to sliding stability, and is easier to construct; and 3. The non-uniform pattern was selected with the objective of installing shorter nails in the last rows so that they would not penetrate in the lower, dense stratum, and thereby avoiding potential difficult drilling in this layer. This configuration would ideally be more desirable from the viewpoint of construction and post-construction deformation, as smaller wall lateral deflections are expected. The major limitation of this configuration is that it is more prone to sliding stability, which must be thoroughly verified. It can be noticed that the candidate nail distributions selected in this example meet the criteria established in Section 6. Shorter nails at the bottom should be not smaller than 0. Soil Nail Materials 1. Select threaded solid bars; 2. In addition, a profile of the soil penetration resistance, which is represented by corrected and normalized SPT N1 values, is also shown in Figure D. Soil Unit Weight a. If this information is not available, the unit weight can be estimated from the SPT N-values provided in Figure D. Soil Shear Strength Parameters a. If this information were not available, estimate the soil ultimate shear strength parameters following procedures described in Section 3. The shear strength of the silty sand behind the wall is mainly frictional, and the internal friction angle can be derived from correlations with the soil penetration resistance e. For the temporary unsupported open face only, a nominal apparent cohesion of 2. Bond Strength The selection of the ultimate bond strength is deferred until the drilling technique is discussed in item H, part 4. Groundwater Conditions Groundwater was not encountered in any of the borings. These observations and supplementary review of groundwater data in the general project area indicate that groundwater levels at the site occur below elevations 93 m 305 ft. Summary For the conditions shown in Figure D. Other Initial Considerations 1. Soil testing indicates that the ground at the site has a resistivity greater than 5,000 ohm-cm and a pH between 6. Analyses also indicate that sulfides, sulfites, chlorides, and other substances known to promote corrosion are either absent or occur in insignificantly low concentrations. Additionally, stray currents are not present near the project site. Although the assessment of corrosion protection requirements can be deferred to after stability analysis, it is advantageous to perform this assessment first to minimize design iterations. To accommodate the sheathing, a relatively large drillhole may be required. As a larger drillhole increases the nail pullout capacity, it is important to identify early in the design process the need to deploy sheathing. Now the necessary level of corrosion protection is determined based following the flow chart of Figure C. SERVICE LIFE TEMPORARY PERMANENT AGGRESSIVITY AGGRESSIVITY NOT KNOWN OR AGGRESSIVE NON-AGGRESSIVE CLASS II PROTECTION NOT KNOWN OR AGGRESSIVE NON-AGGRESSIVE CLASS I NONE FAILURE CONSEQUENCES PROTECTION SERIOUS NOT SERIOUS COST FOR INCREASING CORROSION PROTECTION LEVEL CLASS I PROTECTION SMALL CLASS I CLASS II PROTECTION PROTECTION Figure D. The flow chart in Figure D. The minimum grout cover is 25 mm 1 in. Drilling Methods It is anticipated that either auger first option or driven casing methods will be used for the ground conditions at the site. The drillhole diameter is estimated as the minimum compatible with the predominant soil conditions, drilling and nail installation method, and corrosion protection requirements. Verify the available cover for the selected drillhole diameter. It is assumed conservatively that a threaded nail bar size No. This bar size has a maximum diameter of the threads of 36 mm 1. Bond Strength Ultimate bond strengths were estimated from Table 3. Safety Factors The safety factors adopted for the project conditions are adopted from Table 5. Resisting Component Symbol Minimum Factor of Safety Static Loads Seismic Loads Global Stability long-term condition FSG 1. The combination of loads for the project conditions is adopted from AASHTO 1996 recommendations. Loads due to wind, ice, rib shortening, shrinkage, and temperature are not present. Only two load groups are considered: basic loads and seismic loads. However, some of the loads usually considered in these load groups, including buoyancy, centrifugal force, and live impact load, are absent. In addition, a separate case of temporary load during excavation, with only permanent loads D, is considered. This additional load is considered in the global stability analysis. STEP 2: Preliminary Design Using Preliminary Charts Section 6. A preliminary design of the nail length and bar diameter can be performed using the series of simplified charts included in Appendix B. These charts are useful to obtain, in a simple way, initial estimates of the bar length and diameter without going through a full design. Although it is not necessary to use these charts in conjunction with a full design, this design example will present both approaches to illustrate the methodologies. The nail length for the uniform nail length pattern can directly read from the charts in Appendix B. However, the nail length for the non-uniform pattern will be estimated considering that the total nail length for non-uniform patterns is approximately 10 to 15 percent larger than that for uniform nail length patterns under similar conditions; C. Note that in using the simplified charts, a number of simplifications must be made as follows. D-10 GEOSYNTEC CONSULTANTS Written by: CAL Client: FHWA D. For the conditions defined previously, and with pullout factor of safety, FSPP, from Table D. To account for the added surcharge from live load, increase wall height by 0. Non-Uniform Nail Length Pattern 1. The nail length for the non uniform pattern, LTOT NU, is estimated to be 10 percent larger than the total length of the uniform pattern Section 6. Nail Maximum Tensile Force 1. Read normalized maximum design nail force, tmax-s, from charts in Appendix B reproduced as Figure D. A summary of parameters obtained from the simplified charts is presented below. This value is considered tentatively the same for the uniform and non-uniform patterns. External Failure Modes 1. Evaluation of Global Stability. The global stability is evaluated with the computer program SNAIL. The following geometric and load conditions considered in this example are included in Table D. Load Case Nail-Length Pattern Loads Number of Nails Excavation Depth H, in m ft Failure Surface Figure 1 2 3 4 5 No nails Uniform Non-uniform Non-uniform Non-uniform D DL + L D DL + L D + EQ 0 7 4 7 7 First Lift 0. The soil shear strength parameters and bond strengths were entered. The same soil-strength parameters were considered for both static and seismic loads. The potential for liquefaction is considered negligible. The nail lengths and maximum design nail forces are the iteratively calculated with SNAIL for the FSG included in Table D. To ensure that pullout failure controls over tensile or punching shear failure, artificially large values of nail diameter and facing capacity are entered in SNAIL. Factors of Safety: The results of the SNAIL analyses are presented in Figures D. SNAIL also reports nine other failure surfaces in output files. The computed factors of safety for each of these cases are summarized below. Case Description Calculated FSG 1 2 3 4 5 No nails Uniform Pattern — Static Half Excavation — Static Non-uniform Pattern — Static Non-uniform Pattern — Seismic 1. Case 1 illustrates the beneficial effect of the cohesion on the stability of the first lift. Case 2 shows adequate stability margin. This case proved to be the most critical for the maximum design nail forces, Tmax-s, for the temporary facing. The nail length and nail forces were determined based on Case 4 because this case is the one that can be least represented with the simplified charts. The nail length determined for Case 4 was also verified for seismic loads Case 4. These analyses gave a much higher FSG than the minimum recommended factors of safety. Sliding Stability Due to space limitation a sliding stability is not performed in this example. A discussion on the ground conditions is warranted. Based on similarities in ground conditions and project characteristics, it can considered that the dense silty sand at the bottom of the excavation is sufficiently competent so that a sliding stability failure is not likely for the uniform nail distribution. However, these favorable conditions may not hold true for the non-uniform nail distribution. Therefore, it is recommended that sliding stability must be given full consideration in any design of soil nail wall and design engineers must consider very cautiously non uniform nail distributions. The difference between the SNAIL-base nail lengths and those calculated with the design charts is within 10 percent. This comparison indicates that the design charts can provide reasonably close agreements with nail lengths computed with software. Maximum Design Nail Forces The maximum design nail forces are calculated for the most critical failure surface following the procedure described in Section 6. As such, this maximum design nail force takes into account the soil strength full mobilization. The calculation of the maximum design nail forces corresponding to the most critical failure surfaces was carried out for Cases 2 and 4, as indicated in Table D. Although the average force per nail is quite similar in the two nail patterns, the difference in maximum nail forces between Cases 2 and 4 is significant. This departure can be explained by the dissimilar nail-force distribution with depth in both cases. In Case 2 with uniform lengths, the nail length beyond the critical failure surface increases significantly near the bottom of excavation Figure D. As a result of these large nail lengths behind the failure surfaces, the manner in which the maximum design nail forces are mobilized are significantly different from nail to nail. On the other hand, in Case 4 with non-uniform length, the nail lengths beyond the critical failure surface Figure D. This value is approximately 8 percent larger than the value estimated using the design charts for a uniform length pattern. This favorable comparison again indicates that the design charts provide a valuable tool to obtain reasonably good estimates of Tmax-s to size the nail bar. Nail Force in kN kip Nail Symbol Case 2: Uniform Pattern 1 T1 0. Seismic Considerations Section 6. Define seismic loads Already given. Horizontal seismic coefficient, kh Given. Seismic global stability analysis Results of global safety factors are included in Table D. Sliding Stability Sliding stability analysis is not performed in this example. Internal Failure Modes Section 6. Nail Pullout Resistance Intrinsically accounted for in SNAIL. D-21 Case 4: Non-Uniform Pattern Static 87. Select Bar Size Threaded bars with a cross-sectional area of 510 mm2 0. The safety factors correspond to the potential failure modes of the nail-facing connection including the flexural and punching shear failures. Because a two-phase facing construction is used in this project, flexural and shear-punching failure modes must be evaluated separately for the temporary and the permanent facing. Additionally, for the final facing, a tensile failure of the headed studs is considered. Using the largest calculated force at the wall facing i. The existing facing capacities are established once the facing design, which is presented in the following section, is adopted. Facing Thickness The facing system features are shown in Figure D. Facing Materials See Table D. Element General Reinforcement Description Temporary Facing Permanent Facing Thickness h 100 mm 4 in. Facing Type Shotcrete CIP Concrete Comp.
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