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Proposed Specifications for LRFD Soil-Nailing Design and Construction (2011)

Chapter: Chapter 4 - Conclusions and Suggested Research

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Page 63
Suggested Citation:"Chapter 4 - Conclusions and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2011. Proposed Specifications for LRFD Soil-Nailing Design and Construction. Washington, DC: The National Academies Press. doi: 10.17226/13327.
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Page 63
Page 64
Suggested Citation:"Chapter 4 - Conclusions and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2011. Proposed Specifications for LRFD Soil-Nailing Design and Construction. Washington, DC: The National Academies Press. doi: 10.17226/13327.
×
Page 64
Page 65
Suggested Citation:"Chapter 4 - Conclusions and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2011. Proposed Specifications for LRFD Soil-Nailing Design and Construction. Washington, DC: The National Academies Press. doi: 10.17226/13327.
×
Page 65
Page 66
Suggested Citation:"Chapter 4 - Conclusions and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2011. Proposed Specifications for LRFD Soil-Nailing Design and Construction. Washington, DC: The National Academies Press. doi: 10.17226/13327.
×
Page 66

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63 4.1 Conclusions This study was conducted in the following main steps: (i) review of guidance procedures and specifications for the design and construction of SNWs; (ii) compilation of soil nail load-testing data for developing pullout resistance informa- tion, and load data from instrumented walls for developing load statistics; (iii) development of databases for pullout resistance and loads in SNWs; (iv) development of resistance factors based on reliability methods and on the aforementioned databases; and (v) comparison of designs using the LRFD and ASD methods. The review of existing procedures for the design and con- struction of SNWs was focused on U.S. practice, although the review also included international references. LRFD factors developed for comparable types of retaining structures were also reviewed, including interim editions and the latest edition of the AASHTO LRFD Bridge Design Specifications (AASHTO, 2007). Because the design of SNWs as conducted in the United States is based on limit-equilibrium methods (i.e., related to limit states of overall stability), the load combination selected to design SNWs was the service limit state, consistent with the approach currently adopted in AASHTO (2007) for the limit states of overall stability. A significant amount of soil nail load-test data was collected from several sources. After several results were eliminated due to lack of information or inconsistencies, a database of nail pullout resistance was compiled to support the calibration of pullout resistance factors. The volume of pullout resistance data was sufficient to create data subsets for three subsurface conditions, namely predominantly sandy soils, clayey soils, and weathered rock. More data points were available from projects of SNWs constructed in sandy soils than in clayey soils and weathered rock. To reduce potential scatter in the database due to variable levels of workmanship and equipment among dif- ferent contractors, data points were selected, as much as pos- sible, from the same contractor using the same equipment at the same project. The information available that accompanied the soil nail load-test data was in general insufficient to study other aspects (e.g., construction methods) that may affect the variability of soil nail pullout resistance. In addition, a database of soil nail loads based on instrumented SNWs was created. Resistance factors for elements that are common to other retaining systems (e.g., factor for the nominal tensile resist- ance of steel bars) were adopted from the AASHTO LRFD Bridge Design Specifications (AASHTO, 2007) for consis- tency. Current values were found to be acceptable for the design of SNWS. These resistance factors are presented in Table 4-1. The calibration of the resistance factor for soil nail pullout was conducted using reliability methods and the resistance and load databases mentioned above. The calibration was con- ducted using the procedures suggested for developing load and resistance factors in general geotechnical and structural design (Allen et al., 2005). In this approach, several steps were followed, from selecting a target reliability index that is consistent with the level of structural redundancy of SNWs, to a Monte Carlo simulation to estimate pullout resistance factors. For each soil/rock material considered in the pullout resist- ance database, statistical parameters were obtained for the bias of pullout resistance and loads in SNWs. In addition, the database of soil nail loads allowed an estimation of the statis- tical parameters for the bias of loads. Both load and resistance were considered to be random variables having lognormal distributions. The target reliability index was selected based on a compar- ison of SNWs with other substructures that have a compa- rable level of structural redundancy and for which target reliability indices have been proposed. The reliability selected for SNWs was 2.33, which is consistent with the value used for the calibration of resistance parameters for pullout in MSE walls (Allen et al., 2005). SNWs and MSE walls have compa- rable reinforcement densities (i.e., number of reinforcement elements per unit of wall area), comparable reinforcement C H A P T E R 4 Conclusions and Suggested Research

64 Limit State Resistance Condition Resistance Factor Value Sliding All φ τ 0.90 Soil Failure Basal Heave All φ b 0.70 Slope does not support a structure φ s 0.75 (1 ) Slope supports a structure φ s 0.6 5 (2) (3) Overall Stability NA Seis mi c φ s 0.9 (4 ) Mild steel bars—Grades 60 and 75 (ASTM A 615) φ T 0.56 (5 ) Static High-resistance—Grade 150 (ASTM A 722) φ T 0.50 (5 ) Mild steel bars—Grades 60 and 75 (ASTM A 615) φ T 0.7 4 (5) Nail in Tension Seis mi c High-resistance—Grade 150 (ASTM A 722) φ T 0.6 7 (5) Facing Flexure Tem porary and final facing reinforced shotcrete or concrete φ FF 0.67 (5 ) Facing Punching-Shear Tem porary and final facing reinforced shotcrete or concrete φ FP 0.67 (5 ) A307 Steel Bolt (ASTM A 307) φ FH 0.50 (5 ) Facing Headed-Stud Tensile A325 Steel Bolt (ASTM A 325) φ FH 0.59 (5 ) Sand φ PO 0.4 7 (6) Clay φ PO 0.5 1 (6 ) Weathered Rock φ PO 0.4 5 (6 ) Structural Pullout Soil/Rock Ty pe All φ PO 0.4 9 (6 ) Notes: (1) AASHTO (2007) also considers this value when geotechnical param eters are well defined. (2) AASHTO (2007) also considers this value when geotechnical param eters are based on limited information. (3) For tem porary SNWs, use φ s = 0.75. (4) Per AASHTO (2007) but subject to m odification after new Standard is in place. A value φ s = 1.00 may be acceptable, as long as perm anent deform ations are calculated (see Anderson et al., 2008) and are found not to be excessive. Currently, there is no differentiation for temporary or non - critical structures under seismic loading; therefore, use φ s = 1.00. (5) Calibrated from safety factors. (6) From reliability-based calibration. Values shown correspond to a load factor γ = 1.00. Table 4-1. Summary of resistance factors for SNWs. length/wall height ratios, and thereby comparable and rela- tively high structural redundancies. The calibration proceeded using an iterative scheme in a Monte Carlo simulation. Based on the statistical parameters for load and resistances selected earlier, up to 10,000 random simulations were conducted for each soil type in order to generate a complete distribution of load and resistance. Although the load factor should be selected as 1.0 for serv- ice limit states (per current AASHTO LRFD practice, as men- tioned previously), a series of pullout resistance factors was obtained for a range of load factors other than 1.0 to show the effect of load factors on the pullout resistance factor for each of the soil/rock types considered. The load factors selected were λQ = 1.0, 1.35, 1.5, 1.6, and 1.75. This range represents the val- ues that can be commonly used for retaining structures that are part of bridge substructures. The calibrated pullout resist- ance factors based on this range of load factors is presented in Table 4-2. Calibration resistance factors were subsequently used to perform comparative designs for SNWs for a wide variety of conditions. The objective of the comparative designs was to evaluate differences of the required soil nail length, as obtained using computer programs with the ASD method or the LRFD method. Over 30 design cases were considered to assess the effect of several key factors in the design. These factors included wall height, soil friction angle, bond resistance, and surcharge loads. Results of the comparative designs indicate that the required soil nail length calculated using the LRFD method and the proposed resistance factors are comparable with those obtained with the ASD method. For all cases considered, the length difference is, on average, approximately 4% larger in the LRFD method. None of the factors appear to have a

greater influence than others, possibly with the exception of surcharge loads. The largest difference obtained in the compar- ative analysis was approximately 8%. Discussions on the use of the computer programs GOLD- NAIL and SNAILZ for LRFD-based design of SNWs are also provided in this document. The comparative designs mentioned above have shown that the design of SNWs using the LRFD method would result in quantities comparable to, although slightly higher (i.e., approximately 4% increase of soil nail length on average) than, those obtained with the ASD method. Essentially there are no changes in the requirement of bar diameters, bar lengths, and facing dimensions and quantities. The use of the LRFD method allows for designing SNWs with a reliability level that is compatible with reliability levels of other elements of a bridge superstructure or other comparable retaining systems. Proposed specifications for the design and construction of SNWs were also developed and are provided as appendices to this report. The proposed specifications follow the format of AASHTO (2007). The proposed design specifications include several sections: • Sections 11.12.1 through 11.12.2 provide general descrip- tions, loading conditions, and controlling factors to be used in the design of SNWs. • Section 11.12.3 provides guidance and commentary that aid in conducting evaluations of service limit states for both deformations and overall stability. • Section 11.12.4 addresses safety against soil failure and pro- vides guidance and commentary for conducting evaluations for the limit states of basal heave and sliding stability. • Sections 11.12.5 and 11.12.6 provide guidance and commen- tary for structural limit states—including soil nail pullout and soil nail in tension—and all of the limit states for facings. • Finally, Sections 11.12.7 through 11.12.8 provide guidance and commentary for conducting drainage evaluations and providing corrosion-protection for SNWs. 4.2 Suggested Research The results of this research project have provided a basis for designing SNWs using the LRFD method for various soil conditions. However, some aspects related to SNW construction and design were not addressed in this project but can be expanded through additional research. Some of these aspects and areas of additional research are discussed below: • Addressing limit-equilibrium problems as a service limit in current AASHTO LRFD practice is apparently an unresolved issue and will remain unresolved until additional informa- tion or studies are available. Although this topic is of gen- eral applicability for various bridge substructures, it will affect the design of SNWs if changes are made to the current practice. • The current database of soil nail load tests can be expanded, relying on tests that exhibit clearly a limit state for pullout. This effort should help augment the current data sets not only for the three material types considered but also for other soil types and conditions (e.g., gravelly soils, residual soil, loess, and typical “regional” soils). • The current database of pullout resistance based on soil nail load tests can be expanded and subdivided for certain construction procedures that directly affect pullout capac- ity, including drilling techniques, practice for cleaning the hole, grout characteristics, etc. • The database for loads measured in SNWs can be expanded for other conditions, particularly for larger surcharge loads. • Correlations between soil/rock properties, common field investigation techniques [i.e., SPT as mentioned in this report but also other popular field techniques including cone penetration testing (CPT)], and pullout resistance can be developed as additional predictive tools. • The effect of the number and characteristics of soil nail load testing on the reliability of the design can be explored. It is reasonable to expect that conducting more verification tests, or increasing the test load in verification tests beyond 65 Note: Reliability Index: β = 2.33 Load Factor Pullout Resistance FactorMaterial λQ φPO 1.75 0.82 1.6 0.75 1.5 0.70 1.35 0.63 Sand 1.0 0.47 1.75 0.90 1.6 0.82 1.5 0.77 1.35 0.69 Clay 1.0 0.51 1.75 0.79 1.6 0.72 1.5 0.68 1.35 0.61 Weathered Rock 1.0 0.45 1.75 0.85 1.6 0.78 1.5 0.73 1.35 0.66 All 1.0 0.49 Table 4-2. Summary of pullout resistance factors for various load factors.

200% of the assumed design load, would help establish more precisely the ultimate resistance, would enhance the reliabil- ity of the pullout resistance, and possibly result in more eco- nomical designs. However, it is recognized that this approach may penalize competent contractors who have considerable experience and have the expertise to guarantee the specified bond strength with little testing. • Effects of the spatial variability of subsurface conditions on pullout resistance, which are not commonly taken into account, can be explored in more detail when enough field exploration data is available (i.e., typically much more than what is conventionally produced). While this effect may not be significant for SNWs constructed over small areas, this effect may be significant in the use of SNWs along roadways or as part of the abutments for relatively long bridges. How- ever, it is recognized that a reliable quantification of spatial variability can only be achieved if sufficient field explo- ration data is available. For most project conditions, it is unlikely that enough geotechnical data would be available to quantify spatial variability. • New soil-nailing techniques and new soil nail materials can be considered for possible application for transportation projects. These innovations include self-boring nails, Glass- Fiber Reinforced Polymer (GFRP) bars, and different head nail connections. • Aspects related to the seismic design of substructures that have been recently proposed in interim editions of the AASHTO LRFD Bridge Design Specifications may require evaluation in order to adapt those changes to the design of SNWs. • The current criterion for estimating lateral deformation of SNWs is limited. The quantification of the effects of soil nail layout on the distribution and magnitude of deforma- tions is also suggested as a follow-up research topic. To this end, numerical studies using the finite-element method or comparable techniques are suggested to obtain estimates of constructed and monitored walls. Comparisons of the numerically estimated and measured wall deformations will help calibrate the numerical methods, which can even- tually be used to predict the deformation of future walls. 66

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TRB's National Cooperative Highway Research Program (NCHRP) Report 701: Proposed Specifications for LRFD Soil-Nailing Design and Construction contains proposed specifications for the design and construction of soil-nailed retaining structures.

The American Association of State Highway and Transportation Officials (AASHTO) Standard Bridge Specifications, the AASHTO Load and Resistance Factor Design (LRFD) Bridge Design Specifications, and the AASHTO LRFD Bridge Construction Specifications do not include guidance for soil-nailed structures.

In the absence of AASHTO LRFD specifications, some state departments of transportation will not use soil-nailed retaining structures. Given the potential advantages of soil-nailed structures, there was a need to develop proposed standard design and construction specifications for soil-nailed structures for incorporation into the AASHTO LRFD Bridge Design and Construction Specifications.

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