3.1 Site Investigation

The feasibility of using screw anchors to construct a soil nailed wall on a project depends on the existing topography, subsurface conditions, soil/rock properties, and the location and condition of adjacent structures. It is, therefore, necessary to perform a comprehensive site investigation to evaluate site stability, adjacent structure settlement potential, drainage requirements, anchor capacities, underground utilities and groundwater, before designing a soil nailed earth retention system.

Subsurface investigations must explore not only the location of the face of the soil nailed structure, but the region of the anticipated bond length of the nail. Each project must be treated separately, as both the soil conditions and risks may vary widely. A well-planned site investigation should include a review of the regional geology, a field reconnaissance, a subsurface exploration and laboratory testing. The site investigation should provide adequate information to design a stable soil nailed system.

3.1.1 Regional Geology - A review of the regional geology should be performed prior to conducting a field reconnaissance or subsurface exploration to better understand the geology and groundwater conditions of the region. The information acquired in this first phase of the site evaluation will be used to further develop the field reconnaissance and subsurface exploration. Information concerning the regional geology may be obtained from geologic maps, air photographs, surveys and soils reports for adjacent or nearby sites. Sources of information concerning the regional geology may be obtained from the U.S. Geologic Survey, the Soil Conservation Service, the U.S. Department of Agriculture, and local planning boards or county offices.

3.1.2 Field Reconnaissance - Field reconnaissance should be conducted by a geotechnical engineer or by an engineering geologist. A well planned and conducted field reconnaissance should consist of collecting any existing data relating to the subsurface conditions and making a field visit to:

• Select limits and intervals for topographic cross-sections.
  
• Observe surface drainage patterns, seepage and vegetative characteristics to estimate drainage requirements. Corrosion of existing drainage structures should be noted to identify if a corrosive environment may exist for shotcrete and/or steel materials.
  
• Study surface geologic features including rock outcroppings and landforms. Existing cuts or excavations should be used to identify subsurface stratification.
  
• Determine the extent, nature, and situation of any above or below ground utilities, basements and/or substructures of adjacent structures which may impact explorations or construction.
  
• Assess available right-of-way.
  
• Determine areas of potential instability, such as deep deposits of weak cohesive and organic soils, slide debris, high groundwater table, bedrock outcrops, etc.

3.1.3 Subsurface Exploration - The subsurface exploration program may consist of soil borings, test pits, cone penetration tests, soil soundings, etc. The number, type, and location of the subsurface explorations are usually determined by the geotechnical engineer, based on the results of the field reconnaissance. The exploration must be sufficient to evaluate the geologic and subsurface profile in the area of construction. For guidance on the extent and type of required investigation, the 1988 AASHTO "Manual on Foundation Investigations" is recommended. The following minimum guidelines are suggested for the subsurface exploration for a soil nailed wall using screw anchors:

• Soil borings should be performed at intervals of 100 feet along the alignment of the soil nailed wall face and 150 feet along the back of the reinforced soil structure. The width of the soil nailed structure may be assumed as 1.0 times the height of the wall. For sloping ground conditions behind the wall face, the width of the soil nailed structure may be assumed to be 1.5 times the wall height.
  
• The boring depth should be controlled by the general subsurface conditions. In areas of where rock is not encountered, the boring should extend at least to a depth equal to twice the height of the earth structure. Where bedrock is encountered at a reasonable depth, rock cores should be obtained for a length of approximately 10 feet. This coring will be useful in distinguishing between solid rock and boulders.
  
• In each boring, soil samples should be obtained at 5 foot intervals and at changes in strata for visual identification, classification, and laboratory testing. In each boring, careful observation should be made for the prevailing groundwater table, which should be observed at the time of sampling but also at later times to obtain an understanding of the change in groundwater table with time.
  
• Additional information from in-situ testing such as dilatometer, and pressuremeter may be conducted to provide soil modulus values.
  
• Obtain bulk samples of the subsurface soils to be used in the laboratory testing program.
  
• Test-pit explorations should be performed to help assess whether or not the excavated face will stand while temporarily unsupported during the stage of excavation prior to shotcreting the face.

3.1.4 Laboratory Testing - Soil samples should be visually examined and appropriate tests performed for classification according to the Unified Soil Classification System (ASTM D 2488-69). These tests will permit the engineer to decide what further tests will best describe the engineering behavior of the soil at a given project site. Index testing includes determining the moisture content, Atterberg limits, compressive strength and gradation.

Shear strength determination from unconfined compression tests, direct shear tests, or triaxial compression tests will be needed for the stability analysis. Both undrained and drained (effective stress) strength parameters will be needed for cohesive soils to permit evaluation of both long-term and short-term conditions.

Properties to indicate the potential aggressiveness of the in-situ soil within the reinforced zone should be measured. The tests include: pH, electrical resistivity, and salt content (sulfate, sulfides, and chlorides). These test results will provide necessary information for planning degradation potential and protection.

For more information on the development of a site investigation for a soil nailed system or any other mechanically stabilized earth system, the following references are recommended: FHWA-SA-96-069 "Manual for Design & Construction Monitoring of Soil Nail Walls, " and FHWA-SA-96-071 "Mechanically Stabilized Earth Walls and Reinforced Soil Slopes Design and Construction Guidelines."

3.2 Preliminary Feasibility Assessment

Based on the results of the site investigation, a preliminary feasibility evaluation can be made to determine if a successful soil nail design can be implemented with a relatively high degree of confidence. The ground conditions for which soil nailing is well suited and the ground conditions that are problematic are presented in the following sections.

3.2.1 Ground Conditions Best Suited for Soil Nailing with the SOIL SCREW^®Retention Wall System - The economical use of soil nailing using the SOIL SCREW^®Retention Wall System requires that the ground be able to stand unsupported in a vertical or near vertical cut of 4 - 6 feet in height for one or two days. In addition, the screw anchors must be able to penetrate the ground at a rate compatible to the pitch of the helices. The following ground types are considered most favorable for soil nailing using the SOIL SCREW^®Retention Wall System:

• Naturally cemented or dense sand and gravel.
  
• Residual soils and weathered rock (SPT values < 35 blows per foot) without unfavorable oriented joints or low shear strength.
  
• Sands with some "apparent cohesion," due to capillary effects, of at least 100 psf.
  
• Stiff cohesive soils such as clayey or sandy silts and low plasticity clays that are not susceptible to creep.
  
• Soils above the groundwater table.

3.2.2 Ground Conditions Considered Not Favorable for Soil Nailing Using the SOIL SCREW^® Retention Wall System - Soil nailing is not well suited for all soil types and ground conditions. Generally, when the soil type and ground conditions make the installation of the screw anchors difficult, or the standup time of the excavation face is not sufficient enough to allow the application of the shotcrete, soil nailing should not be used. The following ground conditions are considered unfavorable for soil nailing with screw anchors:

• Rock or decomposed rock, with SPT values > 35 bpf. These are materials in which installation of screw anchors is difficult.
  
• Decomposed rock with joints and/or discontinuities that are inclined steeply toward the excavation face.
  
• Loose clean sands with SPT values < 10 bpf. This material will generally not exhibit adequate standup time. These materials may also be susceptible to large volume changes (e.g. densification) due to vibrations from construction equipment.
  
• Poorly-graded, cohesionless soil (coefficient of uniformity < 2) may tend to ravel when exposed, due to lack of apparent cohesion.
  
• Soils that contain pockets of high moisture content or saturated material (e.g., no apparent cohesion) that will slough and create face stability problems when exposed.
  
• Organic soils
  
• Clay soils with a Liquidity Index greater that 0.2 and undrained shear strength less than 1000 psf may continue to creep significantly over the long term.
  
• Moisture sensitive soils (i.e., high frost-susceptible and expansive soils). Moisture changes in these soils can result in a significant increase in nail loading at the face of the wall.

A preliminary assessment of the applicability of soil nailing with the SOIL SCREW^®Retention Wall System can be determined using the parameters outline in Sections 3.2.1 and 3.2.2, together with FHWA-SA-96-069.

3.2.3 Design Charts - The design charts provided in this manual are to assist in determining only the feasibility of using a SOIL SCREW^® Retention Wall System, and should not be used for actual designs. Appendix A contains design charts for soils with effective friction angles that vary from 25° to 35°. The charts are based on an internal factor of safety of 1.5. For a given design height and nail spacing, a preliminary nail length may be determined. The example problem in Appendix A demonstrates how these tables should be used.

3.3 Overview of Design Methodology

The design procedure presented in this manual draws heavily on two FHWA documents: "Mechanically Stabilized Earth Walls and Reinforced Soil Slopes Design and Construction Guidelines" (FHWA-SA-96-071), and "Manual for Design & Construction Monitoring of Soil Nail Walls" (FHWA-SA-96-069). The design is based on a limit equilibrium design approach that combines conventional reinforced slope design requirements with reinforced soil wall design methods.

The design of any soil nail wall must consider internal, external, and global stability, and the ability of the facing system to support the face of the excavation between nails. External stability of the soil nail wall is concerned with the ability of the reinforced soil mass to withstand the earth pressures and surcharge loads exerted on the composite material from the retained soils. The modes of failure for external stability are sliding and bearing capacity (Figure 3.3.1). Global stability; failure surfaces that pass entirely outside the reinforced soil mass; and compound failures, failures that pass partially through the reinforced soil mass and partially outside the reinforced soil mass, must also be considered (Figure 3.3.2). Internal stability considers the ability of the reinforced soil mass to act as a coherent gravity structure. Figure 3.3.3 shows the modes of failure for internal stability. They consist of pullout of the nail from the resisting zone of the composite material, and rupture of the reinforcement. Finally, the facing and the connection between the facing and nail must be evaluated.

3.4 External Stability

3.4.1 Earth Pressures for External Stability - Stability computations for soil nail walls with a vertical face are made by assuming that the reinforced soil mass acts as a rigid body with earth pressures developed on a vertical pressure plane arising from the back end of the nails, as shown in Figures 3.4.1, 3.4.2 and 3.4.3.

The coefficient of active earth pressure is calculated for vertical walls and a horizontal back slope using Rankine earth pressure theory and is determined from the following equation:

Eq. 3.1

The coefficient of active earth pressure for a vertical soil nail wall with a continuous slope above the top of the wall (Figure 3.4.2) is determined using the following equation:

Eq. 3.2

where: ß= angle of slope above the top of the wall

For broken back slope conditions, the angle ß’(Figure 3.4.3) is substituted for the infinite slope angle ß. The angle ß’ is determined by constructing an effective infinite slope line that initiates from the top of the soil nail wall and intersects the ground surface above the wall at a distance of 2H from the wall face.

For an inclined front face, the coefficient of active earth pressure can be calculated from the general Coulomb Equation:

Eq. 3.3

where:
q =  wall face batter measured from vertical
ß =   the angle of the slope above the top of the wall measured from horizontal
d =   the wall friction angle assumed to be equal to zero

3.4.2 Sliding Stability - To determine the preliminary length of the soil nails, a check of the reinforced soil mass' resistance to sliding at the base of the soil nail wall should be performed. The factor of safety against sliding is determined as follows:

Eq. 3.4

where the resisting force is the lesser of the shear resistance along the base of the wall, or of a weak layer near the base of the soil nail wall and the driving force is the horizontal component of the thrust on the vertical plane at the back of the nails (Figures 3.4.1, 3.4.2 and 3.4.3.).

The calculation steps for a SOIL SCREW^®Retention Wall System with a level surcharge (Figure 3.4.1) are:

1) Calculate the lateral earth force F₁:

F₁ = ¹/2 K_a g H² Eq. 3.5

2) Calculate the horizontal surcharge force F₂:

F₂ = K_aq H Eq. 3.6

3) Determine the resistance to sliding at the base of the wall:

R_sliding = W₁ tan Ø Eq. 3.7

where: W₁ = y H L
f = the friction angle of the foundation soil

The factor of safety against sliding should be greater than or equal to 1.5. The required nail length for sliding is determined as follows:

Eq. 3.8

3.4.3 Bearing Capacity - A rule of thumb that has been developed for grouted soil nail walls and is likely to work equally well for walls using screw anchors is that the nail length should be from 0.5 to 0.8 times the wall height for a vertical wall and horizontal ground surface. The required bearing capacity for a soil nail wall must be determined and checked with respect to the allowable bearing capacity. The stress distribution below a soil nail wall is assumed to be the same as for other types of reinforced soil walls (i.e., mechanically stabilized earth) and is modeled using a Meyerhof stress distribution. Figure 3.4.4 shows the classical Meyerhof stress distribution. The contact area at the bottom of the soil nail wall is reduced in the Meyerhof bearing capacity analysis to a width of L - 2e. e is determined by summing moments about the centroid of the reinforced soil mass. For a horizontal backslope the eccentricity is determined as follows:

Eq. 3.9

e must be less than L/6. If e is greater than this value, the length of the nail should be increased. The effect of the surcharge q would be to decrease e. Therefore, q is ignored for this calculation to be conservative. The vertical stress at the bottom of the soil nail wall (for a horizontal backslope) is determined as follows:

Eq. 3.10

The required bearing capacity is determined using classical soil mechanics. The factor of safety for bearing capacity for these relatively flexible reinforced soil walls is typically 2. The factor of safety for bearing capacity is defined as the ultimate bearing capacity (q_ult) divided by the required bearing capacity (s_v).

Eq. 3.11

3.5 Internal Stability

The ability of the soil nail wall to act as a coherent gravity mass is a function of the vertical and horizontal spacing of the nails, the long-term allowable strength of the nails, the stress transfer between the reinforced soil and the nail, the connection strength between the nail and the facing, and the flexural strength of the facing. These parameters and how they are determined are covered in the following sections.

3.5.1 Allowable Nail Strength - The allowable nail strength is a function of the service life of the structure, the grade of steel, the minimum cross sectional area of the nail, and the strength of the connection between the lead section, and extension pieces of the screw anchor.

Figure 3.5.1 shows the typical dimensions of a Chance "SS" type anchor. The allowable tensile force in the screw anchor is determined by multiplying the cross-sectional area of the reinforcement at the end of the service life (considering corrosion of the steel) by the allowable tensile stress of the steel. The allowable tensile force in the screw anchors, T_a, is obtained as follows:

T_a = A_c (RF) F_y Eq. 3.12

where:

A_c = the design cross-sectional area of the steel (defined as the original cross-sectional area minus corrosion losses anticipated during the design life of the wall.)

F_y = the yield stress of the steel.

RF = the global reduction factor applied to the strength of the nail to account for uncertainties in structure geometry, soil properties, external applied loads, the potential for local overstress due to load non-uniformities, and uncertainties in the long-term nail strength, and is typically taken as 0.65.

Corrosion of anchors is a major consideration in permanent reinforced soil structures. For permanent applications, it is therefore, recommended that galvanized anchors be used to reduce the effects of corrosion on the anchor. The Federal Highway Administration (FHWA-SA-96-072) has established, from an extensive series of field tests on metal pipes and sheet steel buried by the National Bureau of Standards, maximum design corrosion rates for buried steel in soils exhibiting the electrochemical index properties listed in Table 3.5.1.

Table 3.5.1
Recommended Electrochemical Properties for Soils when using
the SOIL SCREW^® Retention Wall System

Property	Criteria	Test Method
Resistivity	>3000 ohm-cm	AASHTO T-288-91
pH	>5<10	AASHTO T-289-91
Chlorides	100 PPM	AASHTO T-291-91
Sulfates	200 PPM	AASHTO T-290-91
Organic Content	1% max.	AASHTO T-267-86

The corrosion rates presented below are suitable for designs for screw anchors. These rates of corrosion assume a mildly corrosive in-situ soil environment having the electrochemical property limits that are listed above. The design corrosion rates, per FHWA-SA-96-072, are:

For Zinc

15 mm/year (first 2 years)
4 mm/year (thereafter)

For carbon Steel

12mm/year (thereafter)

The strength of the coupling that connects sections of the anchor together must also be determined (Figure 3.5.1). Double shear of the bolt that connects the sections of the anchor together, controls the connection strength. Chance SS5 anchors use A320 Grade L7 bolts. The design shear strength (V) of the bolt in double shear is determined as follows:

V = 2 A_b (RF) F_v Eq. 3.13

where:

A_b = the cross-sectional area of the bolt at the end of the service life of the nail (considering corrosion).

F_v= ultimate shear stress of the steel

RF = the global reduction factor applied to the strength of the nail to account for uncertainties in structure geometry, soil properties, external applied loads, the potential for local overstress due to load non-uniformities, and uncertainties in the long-term nail strength, and is typically taken as 0.65.

The allowable strength of the soil nail is the lesser of T_a and V. Table 3.5.2 lists the allowable strength of the galvanized SS5 screw anchor for a design life of 75 years, in a soil environment that meets the electrochemical properties listed in table 3.5.1.

Table 3.5.2
Allowable Design Strength of Chance SS5 Screw Anchor
for a Service Life of 75 Years

Ta 75 yrs (kips)	V 75 yrs (kips)	Allowable Design Strength (Temporary Structures) (kips)	Allowable Design Strength (75 years) (kips)
50	37	45	37

3.5.2 Pullout Capacity of Nail - Extensive laboratory and field research has been conducted to evaluate the pullout capacity of screw anchors (Yilmaz and Hanna (1971), Meyerhof (1973), Hoshiya and Mandal (1984), Das (1985), Mitsch and Clemence (1985), Mooney, et al (1985), Rapoport and Young (1985), Stewart (1985), and Hoyt and Clemence (1989). This research has taken the form of small-scale model testing and large-scale field pullout tests. Several proposed analytical models have been developed to estimate the pullout capacity of screw anchors. The method of estimating the pullout capacity of an anchor presented in this manual is as presented in Clemence, Crouch and Stephenson (1994). This approach was selected for two reasons; 1) the results predict, for the cases tested, the actual field results very closely; 2) the analytical approach is relatively straightforward and easy to apply.

The typical configuration of the screw anchors recommended for soil nailing applications is shown in Figure 3.5.1. The center-to-center spacing of the helices along the length of the anchor is nominally 2.5 feet. All helices are 8 inches in diameter.

3.5.2.1 Pullout of Screw Anchors in Sands and Silts - The observed failure mode from both field and laboratory pullout tests of screw anchors in sand when the helices are spaced a minimum of three (3) helix diameters apart, as is the case for Chance SS5 anchors, is individual helix bearing capacity.

The pullout capacity of a screw anchor is a function of the friction angle of the soil the anchor is embedded in, the number of helices behind the critical slip surface, and the effective overburden stress (Figure 3.5.2). The pullout capacity, P, is given by the following equation:

Eq. 3.14

where:

P = ultimate pullout capacity
A_i = area of helix i
q_i = effective overburden pressure at helix i
q_i = g' z_i
g' = effective unit weight of the soil
z_i = depth from the ground surface to helix i
N_qi = the bearing capacity factor at helix i
(Figure 3.5.3)
n = number of helices

3.5.3 Facing Design - The facing of a soil nail wall has several functions: it provides lateral confinement of the soil at the face of the excavation; it prevents or minimizes the deterioration of the soil's shear strength associated with exposure to the elements; and it may support external loads (e.g. facing panels used for decorative purposes). The primary function of the facing is to prevent sloughing between nails. The spacing of the nails has a large influence on the design requirements for the facing. If, for example, the nails were spaced very close together, a facing would not be required.

The distribution of earth pressure on the facing between nails also affects the facing design. The earth pressure distribution on the face of a soil nail wall is non-uniform, (Figure 3.5.4) and is a function of the stiffness of the facing and distance between nails. Soil arching develops both vertically and horizontally between nails, resulting in stress concentrations around the nail face connection.

The design of the facing system (facing and the connection between the facing and the nail) must consider flexural failure of the facing between nails, and punching shear failure of the facing at the connection between the nail and facing. Flexure of the connection bearing plate and shear of the connection bearing plate are typically not analyzed in the design, if the bearing plate meets the following minimum criteria (FHWA-SA-96-069):

Bearing Plate:

Minimum Yield Stress 36 ksi
Minimum Plate Width 8 inches
Minimum Plate Thickness 0.75 inches

If, however, the plate's strength, width or thickness is less than the values listed above, calculations should be performed to verify the adequacy of the connection plate.

3.5.3.1 Flexural Strength of the Facing - The flexural strength of the proposed facing system for a soil nail wall must be analyzed to assure that the loads generated by the non-uniform earth pressure between the nails can be resisted without flexural failure of the facing. A typical facing system is shown in Figure 3.5.5. Horizontal and vertical reinforcing steel is added to the facing at each nail location. The design of the facing is based on a beam spanning vertically from nail to nail. The structural capacity of the facing can, therefore, be determined using standard reinforced concrete design procedures for singly-reinforced, rectangular concrete beams. The maximum moment that a unit width of the facing can withstand is determined as follows:

Eq. 3.15

where:

A_s = the area of vertical steel over the nails (negative moment) and at the midspan (positive moment)

F_y = yield strength of the reinforcing steel

b = width of the beam (horizontal spacing between nails)

d = distance from the extreme face of the shotcrete to the centroid of the reinforcement

f'_c = compressive strength of the concrete

Once the maximum allowable moment of the proposed facing system for a soil nail wall is determined, the maximum nail head load (critical nominal nail head strength associated with flexural failure of the facing, T_FNflexure) that the facing can carry is evaluated. T_FNflexure can be determined as follows:

T_FNflexure = C_F (m_v,neg, + m_v,pos) 8S_H/S_V Eq. 3.16

where:

S_H = horizontal nail spacing
S_V = vertical nail spacing
C_F = is a dimensionless factor to account for facing flexibility.

C_Fmay be estimated using Table 3.5.3. The values listed in this table are based on back analysis of instrumented case histories, calibrated full-scale laboratory tests, calibrated finite-element modeling, experience and judgment. For additional information concerning the development of equation 3.16 or C_F, see FHWA 1996 Synthesis Report on Soil Nail Wall Facing Design.

The above equations have been developed for the typical soil nail project, where the vertical nail spacing is greater than the horizontal nail spacing. If this is not the case, then the facing should be analyzed with respect to the moments in the horizontal direction.

Table 3.5.3 Facing Pressure Factor (FHWA Synthesis Report)

	Temporary Facing	Permanent Facing
Nominal Facing Thickness (inches)	CF	CF
4	2.0	1.0
6	1.5	1.0
8	1.0	1.0

3.5.3.2 Punching Shear Strength of the Facing - The typical connection between the SOIL SCREW^TM Retention Wall System and the facing system for a soil nail wall is shown in Figure 3.5.5. For the design of connections using bearing plates with shear studs, see FHWA -SA-96-069 "Manual for Design & Construction Monitoring of Soil Nail Walls."

Punching shear failure of the connection of a soil nail system, as presented in FHWA-SA-96-069, is shown in Figure 3.5.6, and involves punching a cone of shotcrete centered about the nail head through the facing. There are two components of the resistance of the system to punching shear; the resistance provided by the facing (shotcrete and reinforcing steel); and the resistance provided by the soil behind the facing. The analysis procedure presented herein ignores the contribution of the soil in determining the punching shear strength because the soil at the face of the wall is generally disturbed by the installation of the anchor and may provide little or no bearing resistance. It also assumes that the square bearing plate may be represented by a circular plate with a diameter equal to the width of the plate and that welded wire mesh steel reinforcement does not provide any shear capacity reinforcement.

With these simplifying assumptions, the punching shear strength of the facing system may be determined using standard reinforced concrete design procedures. The punching shear strength of the facing, V_N(in kips), is determined as follows:

V_N = 0.125 (f'_c)_1/2 p D'_c h_c Eq. 3.17

where:

D'_c = the effective cone diameter (see Figure 3.5.6) at the center of the facing (D'_c = b_pl + h_c)
h_c = the thickness of the shotcrete facing
f'_c= compressive strength of shotcrete in ksi

Ignoring the resistance to punching shear provided by the soil, the critical nominal nail head strength associated with punching shear failure, T_FNpunching, is equal to V_N.

T_FNpunching= V_N Eq. 3.18

3.5.4 Cantilever Design Check - The cantilever at the top of a soil nail wall must be checked to assure that the facing has adequate moment and shear capacity to withstand the applied earth pressure that is developed as a result of the self weight of the soil and any surcharge load (Figure 3.5.7). The cantilever moment (M_c) that the wall must resist is determined as follows:

M_c = K_a [g (H₁²/2) (H₁/3) + q H₁²/2 ] Eq. 3.19

The maximum cantilever moment that the facing can withstand is determined using equation 3.15 at the midspan between nails. The factor of safety for cantilever bending, FS_Mc , is defined as the maximum allowable moment divided by the required cantilever moment. FS_Mc should be equal to or greater than 1.5.

The shear force, S_c, that the cantilever section of the wall face must resist is determined as follows:

S_c = K_a [g (H₁²/2) + q H₁] Eq. 3.20

This shear force is resisted by the allowable shear strength of the facing at the top nail tier, which is determined as follows:

V_N = 0.125 (f'_c)^1/2 h_c Eq. 3.21

The factor of safety against shear failure of the cantilever at the top of the wall should be greater than or equal to 1.5.

3.5.5 Nail Strength Envelope - The distribution of allowable load along the length of the nail can be determined once the allowable nail strength, pullout capacity, nail head flexural capacity, and nail head punching shear capacity have been determined. Figure 3.5.8 shows the theoretical distribution of nail strength along the length of the nail. At the face of the wall, the nail strength is governed by the flexural and punching shear strength of the facing system and is the lesser of T_FNflexure and T_FNpunching. In zone A, the nail strength increases until either the pullout strength or allowable strength is reached. If the length of the nail is long enough to develop the allowable strength of the nail (i.e., the pullout capacity of the nail exceeds the allowable strength), then zone B develops. Zone C is the termination zone of the nail, where the strength of the nail decreases, stepwise at each helix, to zero.

3.5.6 Internal Stability Limit Equilibrium Analysis - The ability of the reinforced soil mass to behave as a coherent gravity structure must be analyzed. Figure 3.3.3 shows the internal stability failure modes. Traditional slope stability analysis techniques are utilized to evaluate the internal stability of a soil nail wall.

The inclusion of the screw anchors increases the stability of the reinforced soil mass by increasing the normal force on the failure surface, which intersects the anchors, resulting in an increase in shear strength of frictional soils, and increasing the resisting forces. Figure 3.5.9 shows a free body diagram for the internal stability of a soil nail wall, with the multiple nails idealized as one nail, for a wedge-shaped failure surface. The factor of safety with respect to internal stability is determined as follows:

Eq. 3.22

where:

c = cohesion
L = the length of the failure surface
W = weight of the soil wedge
q = the angle from horizontal of the failure surface
b = the angle of the screw anchor from horizontal
f = the friction angle of the soil
T = the tensile force provided by the screw anchor

The contribution of any anchor to the stability of the reinforced soil mass is a function of the tensile strength of the anchor; the pullout resistance of the anchor beyond the failure surface; or the anchor head strength and the pullout resistance of the length of the anchor between the slip surface and the face of the wall (Figure 3.5.8).

There are several commercially available soil nailing computer programs that have been developed specifically to determine the stability of soil nail walls (i.e., SNAIL and GOLDNAIL). In addition to these programs, commercially available slope stability programs may also be modified to perform this analysis.

3.6 Global Stability

The global stability of all soil nail structures should be evaluated. Figure 3.3.2 shows a global stability failure surface. The typical factor of safety for global stability is 1.3. Limit equilibrium analysis, as with internal stability analysis, is used to check global stability.

3.7 Summarized Design Steps

Feasibility Assessment

External Stability

Internal Stability

3.8 Special Design Considerations

3.8.1 Tiered Walls - Tiered walls, walls with a stepped or benched facing (Figure 3.8.1.), may be used on a project for aesthetic reasons. The setback areas are often used for planting vegetation. Where the horizontal setbacks are less than 0.35 times the height of the tier, the structure will tend to act as a single equivalent wall with a battered face. For walls with larger steps, the walls should be designed individually, with a surcharge load equivalent to the tiers above the top of the individual wall being designed. A check of the global stability of the total system is critical in the design of a tiered soil nail structure, to assure that the length of the nails is long enough to provide stability for the full height of the wall, including all tiers.

3.8.2 Surcharge Loads - The surcharge loads that may be applied to a soil nail structure may range from relatively light (e.g., nominal live loads to account for traffic loads) to relatively heavy loads in relation to the weight of the retained soil (e.g., surcharge corresponding to an bridge abutment spread footing located on top of the soil nail wall). For surcharge loads that act over the reinforced soil (internal stability), the surcharge load should be included in the internal stability analysis. The minimum facing connection system requirement should be determined in accordance with section 3.5, taking into account the loads applied by both the self weight of the soil and the surcharge loads.

If the surcharge load continues behind the reinforced soil mass, its effect on external stability should also be considered. For large surcharge loads (e.g., footing loads) acting behind the reinforced soil mass, Figure 3.4.1 and 3.4.2 should be used to determine the effect of the surcharge on external stability of the structure.