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D-1
APPENDIX D
Comparison of ASD- and LRFD-Based
Designs of Soil Nail Walls
Program Capabilities
D.1 Introduction
SNAILZ can model only two-dimensional wall geome-
This appendix presents a comparison of SNW designs
tries. It is based on the limit-equilibrium method and only
based on the ASD and LRFD approaches. The comparison was
achieves force equilibrium. Moment equilibrium is gener-
between designs of identical cases and conditions of SNWs for
ally not achieved in this program; therefore, results from
both the ASD and LRFD approaches. Designs were performed
SNAILZ are only approximate but are considered accept-
using the computer programs SNAILZ (Caltrans, 2007) and
able for design purposes.
GOLDNAIL (Golder, 1993), the two most commonly used
SNAILZ uses two-part planar wedges. It can model slip sur-
computer programs for SNW design in the United States.
faces with one wedge exiting the SNW toe and the other to the
Section D.2 provides a brief description of these two computer
ground surface behind the modeled wall [Figure D-1(a)]. This
programs. Section D.3 provides an overview of the comparisons.
is the most common scenario for SNWs. The program can also
Section D.4 contains results of a parametric study conducted to
model approximately a slip surface extending behind and below
assess the design sensitivity to various factors. Section D.5
the wall using a simplified passive earth pressure formulation
presents results of a comparison based on a design example
for the section below the wall toe [Figure D-1(b)]. However,
presented in the FHWA Geotechnical Engineering Circular
this solution approach is only approximate. Therefore, the slid-
(GEC) 7 (Lazarte et al., 2003) of a SNW using the ASD method
ing and basal heave limit states can be modeled only approxi-
and designs for the same wall using the LRFD method.
mately with this program.
SNAILZ can model up to seven soil layers. Up to three
points define the water table location, which for some ground-
D.2 Computer Programs Used in
water conditions may not be sufficient. SNAILZ allows a max-
Comparative Analyses
imum of two uniform surcharge distributions behind the face
D.2.1 SNAILZ of the wall. Therefore, the program may have limited capa-
bilities to model complex stratigraphy and load conditions.
Basic Features
For complex wall geometries, stratigraphy distributions, or
SNAILZ, developed by the California Department of Trans- load conditions, the design engineer may need to simplify
portation (Caltrans, 2007), is an updated version of the pro- actual conditions due to the program limitations. However,
gram SNAIL (Caltrans, 1991) and is currently the most widely for most common conditions encountered in SNW design
used program in the United States for the design of SNWs. practice, this program produces acceptable results, even in
The program is available through the public domain and can relatively complex design situations.
be downloaded free-of-charge from http://www.dot.ca.gov/
hq/esc/geotechlrequest.htm. Technical support is limited.
Input Parameters
SNAIL was originally in a Microsoft® DOS platform. SNAILZ
Parameters selected for input in SNAILZ include those
runs within a Microsoft Windows® environment. SNAILZ is
related to reinforcement, loads, and soil. Reinforcement param-
versatile as it allows the design engineer to consider various
eters include nail head depth on the wall face, nail diameter, nail
design scenarios and the most common elements that partic-
inclination, vertical and horizontal nail spacing, bar cross-
ipate in the design of a SNW. The user can input nail bond
sectional area, and nail tensile resistance. These parameters can
and tensile resistances, as well as the facing resistance.
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D-2
Figure D-1. Slip surfaces used in SNAIL: (a) two wedges through
toe and (b) two wedges under toe.
be assigned either to individual nails or globally to all nails. Up The ASD and “LRFD” modes would be equivalent in
SNAILZ only for γ = 1.0. If load factors different than 1.0 were
to two uniform surcharge distributions can be input. Pseudo-
used in the “LRFD” format in SNAILZ, inconsistent results
static seismic loads can be considered in SNAILZ by entering
between the ASD and LRFD “modes” would be obtained. In
horizontal and vertical seismic coefficients. Soil parameters
addition, affecting soil loads with load factors different than
include soil unit weight, soil cohesion, friction angle, and bond
1.0 is not possible in SNAILZ. For example, an attempt to affect
resistance. Soil nominal resistance is modeled in SNAILZ using
the soil unit weight by earth load factors (in general > 1.0)
the Mohr-Coulomb failure envelope model. Another input
would also affect earth load effects on the resistance side and
parameter that must be included is the facing resistance. Input
would ultimately produce inconsistent results between the ASD
data can be entered in the English or SI unit systems.
and the LRFD-equivalent analyses in SNAILZ.
In summary, the only practical way to use SNAILZ with a
Use of Computer Programs for LRFD Method Analyses
LRFD format is to set all load factors equal to 1.0, which is con-
SNAILZ is ASD based; therefore, SNAILZ strictly provides cal- sistent with a service limit state for overall stability as is currently
culated global factors of safety, FSG, for overall stability. The adopted in the LRFD Bridge Design Specifications (AASHTO,
program cannot be used to perform an analysis using LRFD 2007). For, load factors > 1.0, inconsistent results are obtained.
methodologies unless simplifying assumptions are made and
intermediate calculations are performed. The user may manually D.2.2 GOLDNAIL
input reduced values of nail tensile, pullout, and facing resistances
Basic Features
(i.e., nominal values multiplied by the corresponding resistance
factors) before the program executes any computations. The GOLDNAIL is a Windows-based proprietary program
user must use the “pre-factored” option available in SNAILZ developed by Golder Associates (Golder, 1993). Although
for reduced values of nail tensile, pullout, and facing resist- GOLDNAIL is not as commonly used as SNAILZ, the program
ances. By selecting this option, only soil parameters (cohesion offers more advanced analysis capabilities and options that
and tangent of friction angle) are affected by FSG, while the allow considering a wider range of scenarios and material prop-
other resistances remain constant throughout the analysis. erties than SNAILZ. The program is commercialized and some
External loads (i.e., two uniform loads available in SNAILZ) can technical support can be obtained for a fee.
be entered pre-multiplied by a resistance factor. Earth loads
cannot be entered pre-multiplied by a resistance factor.
Program Capabilities
When the “pre-factored” option is selected and factored val-
ues for resistance are entered, SNAILZ can provide equivalent This program is two-dimensional and satisfies moment and
results in ASD format or in a format resembling LRFD. How- force equilibriums. GOLDNAIL uses circular failure surfaces
ever, this is limited to the condition of load factor γ = 1.0. When and analyzes SNWs as a series of slices instead of wedges. In
SNAILZ is used to perform an LRFD-equivalent analysis for GOLDNAIL, the sliding soil mass is divided into vertical slices,
γ = 1.0, factored values of the nominal resistances must be like is typically done in most slope-stability methods. The pro-
entered. For this step, the nominal resistances of soil cohesion, gram iteratively modifies the normal stresses distribution at
cs, and the friction angle, ϕs, must be affected by multiplying the base of the slices until force and moment equilibriums are
obtained. The program constrains circular slip surfaces to pass
manually these values by soil resistance factors. Note that in
SNAILZ, ϕs, not the tangent of the angle (tan ϕs) is input. There- through or above the SNW toe. Input data can be entered in the
fore, an equivalent reduced friction angle (ϕs red) is computed English or SI unit systems, or any other compatible unit system.
as ϕs red = tan−1 [tan (ϕs) × φ] and entered. With these factored Sliding and basal heave cannot be assessed using this program.
nominal resistances entered, the condition FSG = 1.0 in SNAILZ GOLDNAIL may also be used to analyze unreinforced slopes
and anchored walls.
would represent a limit state for global stability.
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D-4
Table D-1. Soil nail wall input parameters comparable using both the LRFD and ASD methods. For all
(baseline case). cases considered, the calculated nail length is, on average,
approximately 4 percent larger in the LRFD method. No single
Description Quantity
factor appears to have a significantly greater influence on the
Wall height (ft) 30
results. Slightly larger differences were obtained for large loads
Wall batter (deg) 0
and for high nominal pullout or bond resistances. The largest
Number of soil nail levels 6
difference obtained for nail length was approximately 8 percent.
Nail diameter (in.) 1.128
The soil nail loads calculated via the ASD or LRFD modes
Diameter of grouted hole (in.) 6
were similar, with differences on average less than 3 percent.
Nail inclination (deg) 15
As a result, it is expected that the differences in calculating the
Nail vertical spacing (ft) 5
Nail horizontal spacing (ft) 5 necessary nail cross sectional area and required facing resist-
Soil unit weight (pcf) 120
ance would be almost identical using either the ASD or LRFD
Soil friction angle (deg) 35
methods for γ = 1.0.
Bond stress (psi) 15
D.5 Example Design of a SNW
nail yield resistance and facing resistance. However, this is not
a typical manner of analyzing SNWs. D.5.1 Design Conditions
Because the calibrated pullout resistance factors are values
In this design example (Figure D-3), the soil profile behind
that are close to 0.5 (a value that would have been derived
the proposed SNW and the project requirements are similar
through a calibration with safety factors), the results between
to those of the design example presented in Appendix D of
the LRFD and ASD methods are expected to be similar.
FHWA GEC No. 7 (Lazarte et al., 2003). The objective of this
exercise is to compare the results obtained from the two most
D.4.2 Results
common SNW software programs in the design of a wall with
Results for the over 30 analyzed design cases are summarized realistic conditions.
in Table D-2. Results confirm what was expected: using the The wall conditions are as follows. A 10-m (33-ft) high
reliability-calibrated resistance factors of Chapter 3, the calcu- SNW is to be constructed as part of a roadway project. The
road adjacent to the proposed wall is of low-to-medium
lated nails length that are required to satisfy design criteria are
Table D-2. Comparison of required nail length using ASD and LRFD approaches.
Required Length, L (ft) LRFD to ASD Percent
Difference
(w/ respect to baseline case, %)
Variable Compared Case Variable Value ASD LRFD
φPO = 0.49 φPO = 0.47 φPO = 0.49 φPO = 0.47
FSG = 1.5
H = 30 ft, ϕs = 35°
– Baseline 23.43 24.14 24.48 3.03 4.48
qu = 15 psi, Q = 0
1 H = 40 ft 31.24 32.18 32.63 3.01 4.45
Wall Height
2 H = 20 ft 15.62 16.16 16.31 3.46 4.42
ϕs = 28 °
1 27.59 28.43 28.99 3.04 5.07
ϕs = 32 °
Friction Angle 2 25.22 25.99 26.51 3.05 5.11
ϕs = 38 °
3 21.64 22.29 22.74 3.00 5.08
1 qu = 10 psi 26.28 27.59 28.42 4.98 8.14
Bond Resistance 2 qu = 20 psi 18.93 19.39 19.78 2.43 4.49
3 qu = 25 psi 17.14 17.67 17.83 3.09 4.03
1 Q = 250 psf 36.09 37.18 38.69 3.02 7.20
Surcharge
2 Q = 500 psf 40.67 41.91 43.61 3.05 7.23
Note: φ PO were calibrated for a reliability factor of 2.33
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D-7
Table D-4. Resistance factors for overall stability.
Resistance Factor Value
Soil Shear Resistance φs 0.65
Nail Pullout Resistance, φPO 0.49
Nail Tendon Resistance, φT 0.56
Nail Head Resistance (flexure and punching shear), φFF (controls) 0.67
Nail Head Resistance (headed-stud in tension), φFH 0.50
D.5.2 Design Procedures Upper Silty Sand Deposit:
ϕs = 33 degrees
Based on the recommendation of AASHTO (2007), the c′ = 0
overall stability of the SNW is assessed using the load combi- γs = 115 pcf
nation for service limit state.
The resistance factors in Table D-4 are used based on the Lower Silty Sand Deposit:
recommendation of AASHTO (2007). For the pullout ϕs = 39 degrees
resistance factor, the values calibrated in Chapter 3 for sand c′ = 0
are used. γs = 125 pcf
The overall stability of the SNW system is evaluated using 3) Input the following load and resistance factors:
SNW design software. The following are examples of design In the safety factor screen, select LRFD mode. The load
analysis results obtained using the programs SNAILZ and factors for water weight, soil weight surcharge, and seismic
GOLDNAIL. load are all selected to be 1.0. Input the resistance factors
as shown in Table D-4.
4) Compute the necessary nail length and head resistance:
GOLDNAIL
In GOLDNAIL, run analysis using the design analysis
mode. The required nail lengths to achieve a resistance-to-
1) Define geometry of wall. Trial nail lengths are selected as
load ratio greater than 1.0 are calculated.
follows:
Nail Layers Required Nail Length (ft)
Nail Layers Trial Nail Length (ft)
1 through 3 32.7
1 through 3 30
4 and 5 22.9
4 and 5 21
6 and 7 16.3
6 and 7 15
The maximum force occurs in the lowermost nail at
Due to the limitations in the program, the nail inclination
32,070 kip (as obtained from the nail service mode in
of nail layer #1 is selected to be the same as other layers
GOLDNAIL).
(i.e., 15° instead of 20° as shown in Figure D-4).
Figure D-5 shows the calculated critical failure surface.
2) Input the following parameters:
To ensure that pullout failure controls over tensile or
punching-shear failure, artificially large values of nail diam- SNAILZ
eter and nail head resistance can be entered in GOLDNAIL.
In order to perform an analysis that resembles the LRFD for-
For consistency with the example in GEC No. 7, the follow-
mat in SNAILZ, resistances must be modified. Note that in this
ing nail bar and head resistances are selected:
example, the service limit state is analyzed and all load factors
• Threaded bar: No. 8, 25 mm diam., cross-sect. area =
are equal to 1.0. Below is a summary of the modified input
510 mm2 (0.79 in.2)
parameters using a FS = 1.0 (i.e., an equivalent of the LRFD):
• Nail nomin. tensile resist. = 0.79 in.2 × 75 ksi = 59.3 kips
• Nail head nominal resistance (for permanent facing) = Upper Silty Sand Deposit:
ϕs = tan−1(0.65 tan33°) = 22.9°
92 kip, from page D-28 (Lazarte et al., 2003)
c′ = 0 × 0.65 = 0
• Nail pullout nominal resistance (per linear ft):
Upper silty sand: π × 6 in. × 1 ft × 12 in./ft × 14.5 psi = γs = 115 × 1.0= 115 pcf
qu = 14.5 psf (nominal value)
3,280 lbs; and
Lower silty sand: π × 6 in. × 1 ft × 12 in./ft × 21.8 psi = BSF = 0.49 (Bond Stress Factor, equivalent to pullout
4,931 lbs. resistance factor)
q = 14.5 psf × 0.49 = 7.11 psf (factored value)
• Soil Design Parameters:
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D-8
Figure D-5. Critical failure surface calculated using GOLDNAIL.
Lower Silty Sand Deposit: Note that these values are almost identical to those obtained
ϕs = tan−1(0.65 tan39°) = 27.8° using the ASD method according to GEC 7 (and using the pre-
c′ = 0 × 0.65 = 0 factored mode in SNAILZ).
γs = 125 × 1.0 = 125 pcf The comparison indicates that SNAILZ requires nails that
qu = 21.8 psf (nominal value) are approximately 4 percent longer than those obtained
BSF = 0.49 (Bond Stress Factor) using GOLDNAIL. The maximum nail forces in SNAILZ are
q = 21.8 psf × 0.49 = 10.7 psf (factored value) approximately 2 percent larger than with GOLDNAIL.
Nail Head and Nail Tensile Resistances:
D.6 Discussion of Results
Facing resistance = 92 (nominal) × 0.67 = 61.3 (kips); and
Tensile resistance (force) = 59.3 (nominal) × Comparative analyses show that both the LRFD and ASD
0.56 = 32.9 (ksi). method provide comparable design values for soil nail walls
Tensile resistance (stress) = 75 (nominal) × under various conditions. Overall, the comparisons indicate
0.56 = 41.7 (ksi). that the required soil nail length calculated using the LRFD
method and the proposed resistance factors are compara-
Nail lengths need to be computed in SNAILZ iteratively in
ble with those obtained with the ASD method. For all cases
different runs until a target factor of safety of 1.0 (i.e., a con-
considered, the length difference is on average approxi-
dition equivalent to the limit state) is achieved. Figure D-6
mately 4 percent larger in the LRFD method. No factor
shows the critical failure surface calculated by SNAILZ. The
appears to have greater influence than others do. Slightly
required nail lengths as calculated with this procedure are
larger differences were obtained for large loads and for high
listed below.
nominal pullout or bond resistances. The largest difference
obtained in the comparative analysis was approximately
Nail Layers Required Nail Length (ft)
8 percent. In all cases, soil-nail loads calculated using either
1 through 3 34.1
method are comparable, with a difference of less than about
4 and 5 23.9
3 percent.
6 and 7 17
The analyses using the LRFD method with SNAILZ and
The maximum calculated nail force is 32.7 kip (in the low- GOLDNAIL show that the differences and nail loads are very
small, 4 and 2 percent, respectively.
ermost nail).
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D-9
Figure D-6. Critical failure surface calculated using SNAILZ.
D.7 Summary bility. However, it is expected that slightly different results and
design quantities would be obtained for conditions other than
The comparative analyses confirm that the calculated quan- load factors = 1.0.
tities, including soil nail lengths and cross-sectional areas (as a
function of the maximum soil nail force), as obtained using the
References
LRFD and ASD methods are very similar. The reason for these
similar trends, which were already apparent in Chapter 3, stem AASHTO (2007). LRFD Bridge Design Specifications, 4th Edition, Amer-
ican Association of State Highway and Transportation Officials,
from the fact that the calibrated resistance factors for pullout are
Washington, D.C.
very similar to those that could have been obtained directly
Caltrans (1991). “A User’s Manual for the SNAIL Program, Version
from a calibration using factors of safety. The differences were 2.02—Updated PC Version.” Division of New Technology, Material
small between LRFD and ASD methods using the same pro- and Research, Office of Geotechnical Engineering, California
Department of Transportation, Sacramento, California.
gram (i.e., GOLDNAIL) and between different programs using
Caltrans (2007). “A User’s Manual for the SNAILZ Program, Version
LRFD and ASD methods. Therefore, the calibration and com-
2.02—Updated PC Version.” Division of New Technology, Mate-
parison demonstrate that the parameters currently used in rial and Research, Office of Geotechnical Engineering, California
practice should not be altered. Adopting the LRFD method and Department of Transportation, Sacramento, California. http://
the calibrated resistance factors used herein would only result www.dot.ca.gov/hq/esc/geotech
Golder (1993). “GOLDNAIL Soil Nailing Design Program.” Golder
in a change of design format. However, the design would result
Associates, Seattle, Washington.
in essentially the same quantities. A limitation of these compar- Lazarte, C. A., V. Elias, R. D. Espinoza, and P. J. Sabatini (2003). “Soil
isons is that analyses have been performed for load factors equal Nail Walls.” Geotechnical Engineering Circular No. 7, No. FHWA-
to 1.0, per the current AASHTO LRFD practice of overall sta- IF-03-017, Federal Highway Administration, Washington, D.C.
OCR for page 136
Abbreviations and acronyms used without definitions in TRB publications:
AAAE American Association of Airport Executives
AASHO American Association of State Highway Officials
AASHTO American Association of State Highway and Transportation Officials
ACI–NA Airports Council International–North America
ACRP Airport Cooperative Research Program
ADA Americans with Disabilities Act
APTA American Public Transportation Association
ASCE American Society of Civil Engineers
ASME American Society of Mechanical Engineers
ASTM American Society for Testing and Materials
ATA Air Transport Association
ATA American Trucking Associations
CTAA Community Transportation Association of America
CTBSSP Commercial Truck and Bus Safety Synthesis Program
DHS Department of Homeland Security
DOE Department of Energy
EPA Environmental Protection Agency
FAA Federal Aviation Administration
FHWA Federal Highway Administration
FMCSA Federal Motor Carrier Safety Administration
FRA Federal Railroad Administration
FTA Federal Transit Administration
HMCRP Hazardous Materials Cooperative Research Program
IEEE Institute of Electrical and Electronics Engineers
ISTEA Intermodal Surface Transportation Efficiency Act of 1991
ITE Institute of Transportation Engineers
NASA National Aeronautics and Space Administration
NASAO National Association of State Aviation Officials
NCFRP National Cooperative Freight Research Program
NCHRP National Cooperative Highway Research Program
NHTSA National Highway Traffic Safety Administration
NTSB National Transportation Safety Board
PHMSA Pipeline and Hazardous Materials Safety Administration
RITA Research and Innovative Technology Administration
SAE Society of Automotive Engineers
SAFETEA-LU Safe, Accountable, Flexible, Efficient Transportation Equity Act:
A Legacy for Users (2005)
TCRP Transit Cooperative Research Program
TEA-21 Transportation Equity Act for the 21st Century (1998)
TRB Transportation Research Board
TSA Transportation Security Administration
U.S.DOT United States Department of Transportation
