Behaviour of circular footing resting on layered foundation: sand overlying clay of varying strengths

摘要

A series of laboratory model tests was performed on rigid circular footing, 150 mm in diameter (D), resting on surfaces of different layered foundations. The experimental programme considered different foundation configurations by varying the thicknesses (H) of dense sand layers, in the range of 0.63-2.19D, overlying clay subgrades of different strengths, ranging from very soft (c_u = 7 kPa) to stiff (c_u = 60 kPa).The responses of homogeneous foundations (configured with the sand and clays) were also obtained in order to compare the performances. The pressure-settlement responses of homogeneous and layered foundations depicted non-linear variations within the range of footing settlement tested (s = 0-24% of D). The layered foundations indicated significant improvements in bearing capacities for the softer clay subgrades (c_u < 30 kPa) as compared to corresponding homogeneous clay beds; while, reduced bearing pressures were noted for stiff clay subgrade having c_u = 60 kPa. In general, foundation responses depicted higher bearing pressures for stiffer subgrades (c_u), thicker sand-layers (H/D) and at greater footing settlement levels (s/D); however, in terms of bearing pressure ratios (BPR), a decreasing trend was observed with increase in stiffness of the clay subgrades (c_u). In the testing programme maximum of about 5.34-fold improvement in bearing pressure was obtained for layered foundations having c_u = 7 kPa; while, it was about 0.9 for the stiff clay subgrade (c_u = 60 kPa) as compared to the corresponding homogeneous clay beds. Responses of the model tests were in significantly good agreement with that of the theoretical analyses reported earlier.