Chance Technical Design Manual

EQUATION 8-45

3’

Helical Anchor Level 1

y = L sin( q )

= 19 sin(15°) = 4.9 ft

5’

where

2

5’

L = Length of Soil Screw anchor q = Installation angle (from horizontal) Average Overburden Depth = 3 + (y/2) = 5.5 ft at Level 1

23’

3

5’

P LEVEL1 = 8 (0.34) 5.5 (120) 14 = 25 kip P LEVEL2 = 8 (0.34) 10.5 (120) 14 = 48 kip P LEVEL3 = 8 (0.34) 15.5 (120) 14 = 71 kip P LEVEL4 = 8 (0.34) 20.5 (120) 14 = 94 kip STEP 8 - DEFINE A TRIAL FACING SYSTEM

4

5’

DESIGN EXAMPLES

HELICAL ANCHOR LEVELS FIGURE 8-13

STEP 7 - ESTIMATE THE TENSION CAPACITY OF THE SOIL SCREW® ANCHORS

Try a 4” thick, 4000 psi shotcrete face with 6 x 6, W2.9 x W2.9 welded wire mesh reinforcing and two #4 vertical rebars at the helical anchor locations. Try a helical anchor spacing of 5 feet vertically and horizontally and an 8” square by 3/4” thick bear ing plate with a steel yield stress of 36 ksi. STEP 9 - DETERMINE THE ALLOWABLE FLEXURAL STRENGTH OF THE FACING For typical helical anchor wall construction practice, the facing is analyzed using vertical strips of width equal to the horizon tal anchor spacing. For facing systems involving horizontal nail spacings that are larger than the vertical spacing or unit hori zontal moment capacities that are less than the vertical unit moment capacities, horizontal strips of width equal to the ver tical anchor spacing should be used. The area of steel (A s ) for a vertical beam of width 5 feet (S H = 5 feet) with the anchor on the beam’s centerline is determined using Equation 8-46. Diameter (d) of the welded fabric wire is 0.192”. Diameter (D) of the rebar is 0.500”. For a 5 foot wide vertical beam centered between the anchors, the rebars are located at the beam edges and should be ignored. A s is calcu lated using Equation 8-47. The corresponding average nominal unit moment resistances are determined using Equation 8-48.

Determine the bearing capacity factor (N q ) for helical anchors for a sand with an effective friction angle, j = 30°. From Figure 8-12, N q = 14. Assumed vertical spacing is 5 feet (see Figure 8-13). Nail pattern is as shown in Figure 8-13. There are eight helices per anchor, as shown in Figure 8-14. The ultimate tension capacity (P) of the helical Soil Screw an chor at Level 1 is determined using Equation 8-43. EQUATION 8-43 8 P = ∑ A i q i N q i = 1 Helical anchors have 8” diameter helixes. The helix area (A) can be calculated using Equation 8-44. EQUATION 8-44 A = p (0.33) 2 = 0.336 ft 2 (use 0.34 ft 2 ) The ultimate tension capacities for the helical anchors at the various levels are determined using Equation 8-45.

HELICAL ANCHOR HELIX SPACING FIGURE 8-14

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