Bearing capacity of foundations on soft clays with granular column and trench

The bearing capacity of foundations on soft clays under undrained conditions has been computed with inclusion of (i) a single vertical granular trench below a strip footing and (ii) a granular column placed below a circular footing. A lower bound plane strain and axisymmetric limit analysis, in conjunction with finite elements and an optimization procedure, has been used. The efficiency factor (ξ) has been determined by varying Bt/Bf; where (i) Bt = diameter of the column (width of the trench) and (ii) Bf = diameter of the circular footing (width of the strip footing). The effect of (i) the depth (D) of the column (trench) and (ii) the angle of internal friction (ϕ) of the column (trench) material has been explored for a wide range of cu/(γBf); cu and γ imply undrained cohesion and the unit weight of the clay mass, respectively. Factor ξ increased quite significantly with increases in Bt/Bf and D/Bf. Factor ξ improved further with (i) increases in ϕ and (ii) decreases in cu/(γBf).