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 (n) 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 (/) of the column (trench) material has been explored for a wide range of cu/(cBf); cu and c imply undrained cohesion and the unit weight of the clay mass, respectively. Factor n increased quite significantly with increases in Bt/Bf and D/Bf. Factor n improved further with (i) increases in / and (ii) decreases in cu/(cBf).

Introduction

A number of investigations, that examined the improvement in the bearing capacity of foundations by the insertion of granular trenches and columns, have been reported in literature. The studies are based on (i) analytical approaches (Madhav and Vitkar, 1978; Bouassida and Hadhri, 1995; Bouassida et al., 1995), (ii) elasto-plastic finite element analyses (Schweiger and Pande, 1986; Mitchell, 1985), and (iii) numerical lower and upper bound finite element limit analyses (Bouassida et al., 2015). Series of small-scale model experiments (Hamed, 1986; Nazir and Azzam, 2010; Bouassida and Porbaha, 2004) and full-scale field tests (Mitchell, 1981, 1985; Stuedlein and Holtz, 2012) have been carried out by a few researchers. These different model and field tests revealed that the bearing capacity of foundations can be increased quite significantly with an increase in the depth of the granular trench. In the present paper, the aim is to determine the bearing capacity of both strip and circular footings, placed on soft to medium soft clays reinforced with a single granular trench and column, respectively, by using the lower bound limit analysis with finite elements and the optimization principle.