Transmission And Substation Foundations - Technical Design Manual (TD06088E)

Sands φ ’ > 0; c’ = 0 In sands the uplift behavior of shallow (generally D/B ≤ 5) single-helix anchors develops a failure zone that looks similar to an inverted truncated cone. The failure is assumed to take place by the perimeter shear acting along this failure surface, which is inclined from the vertical at an angle of about φ ’/2, as shown in Figure 4.2, and also includes the mass of the soil within the truncated cone. The Ultimate Uplift Capacity is calculated from: Q HU = W S + π gK 0 (tan φ ’)(cos 2 φ ’/2) [(BD 2 /2) + (D 3 tan φ ’/2)/3)] Equation 4-13 where: W S = Mass of Soil in Truncated Cone = gV γ = Total (wet) Unit Weight V = Volume of Truncated Cone K 0 = At-Rest Lateral Earth Pressure Coefficient B= helix diameter D = vertical plate depth The volume of the truncated cone is determined from: V = [ π D/3][B 2 + (B + 2Dtan φ ’/2) 2 + (B)(B + 2D tan φ ’/2)] Equation 4-14 Values of the at-rest lateral earth pressure coefficient for sands can reasonably be taken as: K 0 = 1 – sin φ ’

DESIGN METHODOLOGY

Mixed Soils φ ’ > 0; c’ = 0 In mixed soils with both frictional and cohesive components of shear strength, there is an added resisting force in uplift for shallow installations above the value given by Equation 4-13. This added component results from cohesion acting along the surface of the truncated cone failure zone between the helical plate and the ground surface so that an additional term may be added to Equation 5-13 giving: Q HU = W S + π gK 0 (tan φ ’)(cos 2 φ ’/2) Equation 4-15 [(BD 2 /2) + (D 3 tan φ ’/2)/3)] + (c)(A C ) where: A C = Surface Area of Truncated Cone The surface area of a truncated cone can be obtained from:

Figure 4-2 Proposed Failure Mechanism for Shallow Single-Helix Anchors in Dense Sand.

A C = π [(R 2 + r 2 ) + [(R 2 – r 2 ) + (D(R + r)) 2 ] 0.5 ]

Equation 5-16

where: r = Radius of Helical Plate = B/2 R = Radius of Cone Failure Surface at the Ground Surface = B/2 + (D)tan( φ ’/2) The additional component of uplift resulting from soil cohesion, is sometimes ignored since soil cohesion is often lost from water infiltration or rising water table.

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