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SUMMARY
Proposed Specifications for LRFD Soil-Nailing
Design and Construction
NCHRP Project 24-21 was conducted to develop procedures based on the load and resist-
ance factor design (LRFD) method for the design of soil nail walls (SNWs) according to the
most common U.S. practice in this technology. The work consisted of several tasks, includ-
ing (i) a review of procedures and specifications for the design and construction of SNWs in
both the LRFD and the allowable stress design (ASD) methods, (ii) compilation of soil nail
load-test data and load data from instrumented walls, (iii) development of databases for
pullout resistance and loads in SNWs, (iv) development of resistance factors based on the
databases using reliability methods, and (v) comparison of designs using the LRFD and ASD
methods and establishment of differences. The review of procedures for the design and con-
struction of SNWs was focused on U.S. practice, although international references were also
consulted. The task also comprised the review of current/interim editions of the American
Association of State Highway and Transportation Officials (AASHTO) LRFD Bridge Design
Specifications (AASHTO, 2007).
A significant volume 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 subsur-
face 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 the scatter due to variable levels of workmanship and
equipment among different contractors, data was selected, as much as possible, from the
same contractor using the same equipment at the same project.
Statistical parameters were obtained for four soil/rock types for the pullout capacity. In
addition, soil nail load data allowed an estimation of the statistical parameters for the bias
of loads. Load and resistance were considered as lognormal random variables. Resistance
factors for elements that are common to other retaining systems (e.g., factor for the nomi-
nal tensile resistance of steel bars) were adopted from the AASHTO LRFD Bridge Design
Specifications (AASHTO, 2007) for consistency. Current values were found to be acceptable
for the design of SNWs. The calibration of the resistance factor for soil nail pullout was con-
ducted using reliability methods as suggested by Allen et al. (2005) for the development of
load and resistance factors in geotechnical and structural design. The target reliability index
was selected based on a comparison of SNWs with other substructures that have compara-
ble levels 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 mechanically stabilized earth (MSE) walls.
The calibration used a Monte Carlo simulation using statistical parameters for load and
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resistances selected earlier and up to 10,000 random simulations for each of the load and
resistance variables.
To be consistent with the AASHTO (2007) specifications, overall stability was adopted to
be a service limit state where limit-equilibrium methods are applied. Although load factors
are 1.0 for service limit states, 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 fac-
tor 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 values that can be commonly used for retaining
structures that are part of bridge substructures. Calibrated pullout resistance factors based
on this range of load factors are presented.
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 eval-
uate 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 fac-
tors were quite close to those obtained with the ASD method. For all cases considered, the bar
lengths are, on average, approximately only 4% longer in the LRFD method. None of the fac-
tors studied in this comparison appear to have a greater influence over other factors on the
calculated nail lengths, possibly with the exception of surcharge loads. The largest difference
obtained in the comparative analysis was approximately 8%. The comparative designs men-
tioned previously have shown that the design of SNWs using the LRFD method would result
in comparable, although not identical (only slightly higher), quantities to those obtained with
the ASD method. There are no essential differences in the requirement of bar diameters, bar
lengths, and facing dimensions and quantities using either method. The use of the LRFD
method allows SNWs to be designed 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).
