Ultimate Bearing Capacity Tests on Sand with Clay Layer

Loading tests on nonuniform sand beds with a clay layer are carried out to make the effect of an interstratified weak layer on the ultimate bearing capacity clear. The tests are done under plane strain conditions using a testing box with an inside size of 0.4 m × 0.3 × 0.06 m, while the friction between soil and walls is minimized. Two results are important in practical application. First, even when the clay layer is underlain five times as deep as the footing width, the presence of the weak layer still plays an important role in determining the ultimate bearing capacity. This suggests that appreciable amount of plastic strain occurs even in a clay layer underlain at that depth, and that the plastic strain tends to lower the stability of the upper sand bed. Second, if the clay layer is thin compared to the footing width, a critical depth exists at which the clay layer exerts the most dangerous effect on the bearing capacity.