Determine the screw anchor spacing (S_V, S_H), screw anchor length and facing requirements for an excavation support system for a 23 foot deep excavation in a silty sand.

The required design factor of safety for internal stability is 1.5, and for global stability is 1.3.

Step 1 - Define Design Parameters

Given: The unit weight and friction angle of the silty sand is 120 pcf and 30º respectively. The allowable bearing capacity of the silty sand at the bottom of the excavation is 4000 psf.

The electrochemical properties of the silty sand are listed below:

A design live surcharge load of 100 psf is considered to be applied uniformly across the ground surface at the top of the wall. The wall face is vertical. Groundwater is located 60 feet below the ground surface.

Chance Type SS5 screw anchors, for which lead sections and extensions are available in 5' and 7' lengths, are to be used for the screw anchors. The design life of the structure is one year. Design screw anchor lengths will be governed by the lead and extension pieces and thus will be 10', 12', 14', 15', 17', 19', etc.

Step 2 - Check the Preliminary Feasibility of the SOIL SCREW® Retention Wall System

The medium dense, silty sands at this site are well suited for the SOIL SCREW® Retention Wall System (i.e., good stand up time). The water table is well below the bottom of the excavation. The conditions at the site are therefore favorable for the SOIL SCREW® ^{Retention Wall System.}

^{Design charts are used to determine preliminary screw anchor spacing and lengths for the given wall geometry, loading and soil conditions. For the soil conditions, f = 30o, enter the design chart (Figure A-2) along the x-axis at a wall height, H = 23 ft. A typical screw anchor spacing for soils with "good" stand up time is 5 ft. x 5 ft. Therefore, use the S_VS_H = 25 curve to determine the preliminary screw anchor length, L = 16 ft. (see Figure EX-1).}

^{Use equation 3.1 to determine the earth pressure at the back of the reinforced soil mass.}

^{= 0.33}

^{Available screw anchor lengths for Chance SS 5 anchors are 10', 12', 14', 15', 17', 19', etc. The 16 ft. preliminary length determined in Step 2 does not account for surcharge loading, which tends to increase screw anchor lengths. Try 19' screw anchors (length to height ratio of 0.83). For preliminary designs, for walls with the given soil and loading conditions, a length to height ratio of 0.8 to 1.0 is a starting point for the analysis and appears to be conservative.}

^{The horizontal force from the retained soil is determined using equation 3.5.}

^{F₁ = 1/2 K_a gH2
F₁ = 1/2 0.33(120)232 = 10474 lb/lf of wall}

^{The horizontal force from the surcharge load is determined using equation 3.6.}

^{F₂ = K_aq H = 0.33(100)23 = 759 lb/lf of wall}

^{Using 19' screw anchors installed at a 15º angle, the horizontal length, L_x, of the screw anchor is determined:}

^{L_x = Lcos15°
L_x = 19cos15° = 18.4 ft}

^{The factor of safety against sliding is determined as follows:}

^{Given the allowable bearing capacity is 4000 psf.}

^{q_allow = 4000 psf > s_v = 3532 psf}

Step 6 - Determine the Allowable Screw Anchor Strength

The screw anchor wall is a temporary structure with a design life of one year. Using Table 3.5.2, the allowable design strength of the SS5 anchor is 45 kips.

^{Step 7 - Estimate the Pullout Capacity of the Screw Anchors}

^{Determine the bearing capacity factor for helical anchors for a sand with an effective friction angle, f = 30o. From Figure 3.5.3, N_q = 14:}

^{Using equation 3.14, determine the pullout capacity of the screw anchors.}

^{Assumed vertical spacing is 5 feet (Step 2). Nail pattern is as shown.
There are eight helices per anchor, as shown below.}

^{Screw anchors have 8-inch diameter helixes.}

^{For typical soil nail wall construction practice, the facing is analyzed using vertical strips of width equal to the horizontal anchor spacing. For facing systems involving horizontal nail spacings that are larger than the vertical spacing or unit horizontal moment capacities that are less than the vertical unit moment capacities, horizontal strips of width equal to the vertical anchor spacing should be used.}

^{The area of steel for a vertical beam of width 5 feet (S_H = 5 feet) with the anchor on the beam's centerline is determined as follows:}

^{Diameter (d) of the W2.9 x W2.9 welded fabric wire is 0.192 inches
Diameter (D) of the #4 rebar is 0.500 inches}

^{Step 10 - Determine the Maximum Screw Anchor Head Load that will produce the allowable moments determined in Step 9, using equation 3.16:}

^{where:
f'_c = 4,000 psi = 4ksi
h_c = 4 in.
D'_c = 8+4 = 12 in.}

^{GoldNail 3.11, "A Stability Analysis Computer Program for Soil Nail Wall Design," developed by Golder & Associates, was used to perform the internal and compound stability analysis.}

^{The nail strength envelope developed in Step 13 needs to be modified for GoldNail. The increase in pullout capacity along the length of the nail is estimated for GoldNail as straight lines not step functions. An example of this modification for anchor level 2 is shown below:}

^{Within GoldNail there are several analysis options. The option used for this example is "Factor of Safety." Using this option, the internal Factor of Safety, FS_internal = 2.11 for the nail pattern defined in Step 7. See attached computer printout (Attachment EX1). The GoldNail output printout lists "Global Stability" not "Internal Stability." However, the location of the critical failure surface (circle no. 13) indicates an internal mode of failure, as shown on the GoldNail geometry printout}

Step 15 - Check Global Stability

Analysis performed for the given slope geometry by the computer program PCSTABL6H, developed by Purdue University and modified by Harald Van Aller, and the pre-processor STED, developed by Harald Van Aller. The resulting global Factor of Safety, FS_global = 1.93. See attached computer printout (Attachment EX2).

Step 16 - Check Cantilever at Top of Wall

In Step 7 layout of anchor was assumed. The cantilever at the top of the wall from Step 7 is 3 feet. Using equation 3.20 check cantilever moment:

Maximum allowable moment at midspan (Step 9) is 566 lb-ft/ft.