Bearing Capacity and Stiffness of Embedded Circular Footings on Stiff-Over-Soft Clay

This technical note presents results from a series of undrained load tests performed at model scale on circular embedded footings founded in a two-layered stratigraphy, comprising uniform stiff clay overlying soft clay. The observed foundation stiffness and bearing capacities are compared with available empirical, analytical, and numerical solutions. The dependence of the equivalent linear stiffness at a settlement ratio of 1% ($E_{0.01}$) and the bearing capacity factor, $N_c$, on the normalized depth to the soft clay ($H/B$) and strength ratio of two clay layers ($s_{ut}/s_{ub}$) are presented. A simple expression is developed for $N_c$, which provides a good match to experimental data and is consistent with numerical predictions. It is also shown that the critical $H/B$ ratio, above which the underlying soft layer does not affect the footing response, is significantly smaller for $E_{0.01}$ than for $N_c$ at any given $s_{ut}/s_{ub}$ value. This finding has important practical design implications for footings on stiff-over-soft clay deposits.