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MULTI LAYER SOILS - Strong on Top - Weak on Bottom

Foundations are often supported by multilayer soils. Multiple soil layers influence the depth of the failure surface and the calculated bearing capacity. Solutions of bearing capacity for a footing in a strong layer that is overlying a weak clay layer are given below. These solutions are valid for a punching shear failure. The use of more than two soil layers to model the subsurface soils is usually not necessary.

Depth of Analysis
The maximum depth of the soil profile analyzed need not be much greater than the depth to the failure surface, which is approximately 2B for uniform soil. A deeper depth may be required for settlement analyses. If the soil immediately beneath the foundation is weaker than deeper soil, the critical failure surface may be at a depth less than 2B. If the soil is weaker at depths greater than 2B, then the critical failure surface may extend to depths greater than 2B.

Dense Sand Over Soft Clay
The ultimate bearing capacity of a footing in a dense sand over soft clay can be calculated assuming a punching shear failure using a circular slip path.

Wall Footing q_u = q_ub + 2Y_sand H_t²/B (1+2D/H_t) K_psTAN(Φ_sand) - Y_sand H_t < q_ut
Circular Footing q_u = q_ub + 2Y_sand H_t²/B (1+2D/H_t)S_s K_psTAN(Φ_sand) - Y_sand H_t < q_ut

where
q_ub = ultimate bearing capacity on a very thick bed of the bottom soft clay layer, ksf
Y_sand = wet unit weight of the upper dense sand, kips/ft³
H_t = depth below footing base to soft clay, ft
D = depth of footing base below ground surface, ft
K_ps = punching shear coefficient
Φ_sand = angle of internal friction of upper dense sand, degrees
S_s = shape factor
q_ut = ultimate bearing capacity of upper dense sand, ksf

B is the diameter of a circular footing or width of a wall footing. The shape factor S_s , which varies from 1.1 to 1.27, may be assumed unity for conservative design.



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