Bearing Capacity of Footings over Two-Layer Foundation Soils

The bearing capacity of strip footings over a two-layer foundation soil is considered. The kinematic approach of limit analysis is used to calculate the average limit pressure under footings. The method is applicable to any combination of parameters of the two layers, but the results are presented only for a specific case when a footing is placed on a layer of granular soil resting on clay. The depth of the collapse mechanism is found to be very much dependent on the strength of the clay. Very weak clay can “attract” the mechanism even at great depths. The results are presented as limit pressures rather than traditional bearing-capacity coefficients. The latter are strongly dependent not only on the internal friction angle of the sand, but also on the thickness of the sand layer, cohesion of the clay, and surcharge pressure. Results are presented in the form of dimensionless charts for different internal friction angles of sand. It was found that linear interpolation within 5° increments is acceptable in the range of φ from 30° to 45°.