This study widely
investigates the ultimate bearing capacity of a rigid footing on the free
surface of sand overlying clay using the rigid plastic finite element method
(RPFEM). Interface elements were introduced with the new constitutive equations
developed by the authors to properly evaluate the interaction between the
footing and the soil, as these elements greatly affect the failure mechanism of
the footing-soil system. Two friction conditions were employed for the footing
surface, namely, the perfectly rough condition and the perfectly smooth
condition. The RPFEM was extended to calculate the distribution of contact
normal stress along the footing base corresponding to changes in the thickness
of the sand layer. Several design charts were developed to directly determine
the ultimate bearing capacity by increasing the internal friction angle, the thickness of the sand layer, and the shear
strength of the clay layer. Two cases were considered for the clay layer below the
sand layer, namely, a weak layer and a stiff layer. The failure mode of two-layered soils was
found to change from the general shear mode to the punching shear mode for both
friction conditions by a reduction in the shear strength of the clay layer. The
sheared area of the ground was limited to the sand layer in the general shear
mode, while the sheared area was distributed throughout the two layers in the
punching shear mode. New bearing capacity formulas during the punching shear
mode were proposed for the two friction conditions in a wide range of strength
and geometric parameters, which were in close agreement with the experimental
studies and are efficient enough to be used in practice.
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