Transmission And Substation Foundations - Technical Design Manual (TD06088E)

Sands φ ’ > 0; c’ = 0 For clean, saturated sands, the “cohesion” is normally taken as zero, reducing the ultimate capacity, as in Equation 5-10, to: Q H = A H (q’N q + 0.5 γ ’BN γ ) Even in moist sands or sand with a small amount of fines that may give some “cohesion”, this is usually ignored. Because the area of the plate is small, the contribution of the “width” term to ultimate capacity is also very small and the width term is often ignored leaving: Q H = A H (q’N q ) Equation 4-18 For deep installations, the bearing capacity factor Nq is usually obtained from values used for determining the end bearing capacity for deep pile foundations, which is different than the values used for shallow foundations. There are a number of recommendations for Nq available in foundation engineering textbooks as shown in Figure 4-6. The difference in Nq values shown in Figure 4-6 is largely related to the assumptions used in the failure mechanism. Figure 4-7 gives a reasonable chart of Nq values as a function of the friction angle of the soil, φ ’, that may be used for screw-piles and helical anchors. The value of Nq in Figure 5-7 is obtained from: Note: In some sands, the unit end bearing capacity of deep foundations may reach a limiting value. The point at which this occurs is generally termed the “critical depth”. Critical depth is defined as the depth at which effective vertical stress, a.k.a. overburden pressure, will not increase with depth. Critical depth is not specifically defined for screw-piles and helical anchors, but engineers often use it with deep installation in saturated sands. Mixed Soils φ ’ > 0; c’ > 0 The ultimate capacity of a deep single-helix screw-pile in mixed-grain soils can be taken from traditional bearing capacity theory using Equation 5-11: Q H = A H (cN c + q’N q + 0.5 γ BN γ ) Note: The term 0.5 γ ’BN γ is typically ignored for helical piles because the helix plate is small. Nq = 0.5 (12 x φ ’) φ ’/54 Equation 4-19 Under tension loading, the ultimate capacity of a single-helix screw-anchor in clay the ultimate capacity is calculated using the same approach given in Section 5.2.2.1.a. In some cases a reduction may be made in the undrained shear strength to account for soil disturbance above the helical plate as a result of installation, depending on the Sensitivity of the clay. Also, as previously noted in Section 5.2.1.2.a, for a deep installation (D/B > 7.5) the Breakout Factor, F C has a default value of 9. The bearing capacity equation becomes: Q HU = A H [(9)s u + γ ’D] Sands φ ’ > 0; c’ = 0 In sands, the tension capacity of a helical anchor is generally assumed to be equal to the compression capacity provided that the soil above the helix is the same as the soil below the helix in a zone of about 3 helix diameters. Again, for clean, saturated sands, the “cohesion” is normally taken as zero, reducing the ultimate capacity to: TENSION LOADING – AXIAL UPLIFT (DEEP SINGLE-HELIX) Saturated Clays φ ’ = 0; c’ > 0

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