The bearing capacity of footings on a sand layer overlying soft clay

The bearing capacity of foundations can be improved by superimposing a layer of compacted sand or gravel. Although it is difficult to calculate exactly the bearing capacity of such inhomogeneous soil profiles, approximate solutions have recently been presented for several of the more frequently encountered profiles, in an attempt to provide acceptable design data. This technical note makes recommendations about the practical use of currently available solutions of this problem, which are based on data obtained from recent laboratory model bearing-capacity testing. It first outlines an approximate method for calculating bearing capacity for sand overlying clay, but is mainly concerned with the results of a test programme. The experimental set-up used a strip footing 0.12m wide, and a 2.0m long, 1.4m high 0.3m wide test tank with parallel glass sides, containing Glyben artificial clay overlaid by Leighton Buzzard sand. Stress-settlement relationships were obtained for clay subgrade alone, sand alone, and sand overlying clay. Experimental measurements of bearing capacity were compared with those predicted by the methods of Terzaghi and Peck, Vesic, and Hanna and Meyerhof. Agreement was found to be closer when allowance was made for the local shear failure of the clay.