Transmission And Substation Foundations - Technical Design Manual (TD06088E)

Saturated Clays φ ’ = 0; c > 0 Test results and analytical studies indicate that the Breakout Factor for saturated clays in undrained loading varies as a function of the Relative Embedment of the plate, i.e., D/B. This is much like the transition of shallow to deep foundation behavior under compression loading. Table 5-3 shows the variation in F C vs. D/B for circular plates. This figure (from Das (1990) shows that F C = 1.2(D/B) ≤ 9, so that at D/B > 7.5, F C = 9 (i.e., the transition from shallow to deep behavior under tension in clays occurs at about D/B > 7.5). In this case, the ultimate uplift capacity is similar to Equation 5-9 and is given as:

Q HU = A H (cF C + γ ’D)

where: Q HU = Ultimate Uplift Capacity c = “cohesion”; for φ ’ = 0 c = undrained shear strength = s u F C = Breakout Factor for φ ’ = 0; F C = 1.2(D/B) ≤ 9 γ ’ = effective unit weight of soil above helical anchor plate D = Depth Note: The term γ ’D is sometimes ignored because it is very small.

In some situations the undrained shear strength of clays under tension loading may be reduced to account for some disturbance effects of the clay above the helical plate but this is a matter of engineering judgment.

TABLE 4-3 VARIATION IN UPLIFT BREAKOUT FACTOR FOR SHALLOW SINGLE-HELIX ANCHORS IN CLAY

DESIGN METHODOLOGY

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