Static and cyclic undrained response of square embedded foundations

Abstract

Results of a three-dimensional finite-element study for the effect of embedment on the undrained bearing capacity, the elastic stiffness, and the cyclic behaviour of square-in-plan foundations are presented. Uniaxial horizontal (Q) and pure-moment (M) limit loads, as well as the respective elastic stiffnesses (KHH and KMM) are obtained, and simplified models are developed to interpret the observed trends. The substantially different role of embedment in increasing elastic stiffness and in increasing ultimate loads is interpreted in simple soil mechanics terms. Extensive comparisons are made with the two-dimensional results for a strip foundation. Combined (QM) loading capacities are obtained and presented as ‘interaction’ diagrams; the significance of the vertical load (N) is also addressed. The importance of the type of contact between the foundation interfaces (vertical or horizontal) with the surrounding or underlying soil is explored. A substantial reduction in all capacities is shown when the interfaces are tensionless and of limited shear (sliding) resistance (TSI), compared with the capacities for the ideal case of fully bonded contact (FBC). The cyclic moment–rotation (M–θ) response of embedded foundations carrying a simple slender structure is investigated parametrically. It is found that the monotonic loading curves provide approximately the envelope for the cyclic M–θ loops. But the shape of these loops and the ensuing settlement of the foundation are both functions of the factor of safety (FSV) against vertical bearing capacity. In case of surface and shallowly embedded foundations with high values of FSV (i.e. with light loading or on very stiff soil) the loops pass nearly through the centre (M = θ = 0) of the coordinate axes and the residual settlement is negligible; both are a consequence of the predominantly geometric non-linearity in the form of separation of the walls and uplifting of the base from the soil. On the contrary, with low FSV values (heavy loading or soft soil) and deeper relative embedment, broad hysteresis loops and accumulating settlement are the rule – a product of material inelasticity of the soil, under limited soil–foundation detachment.