Effect of strain rate and strain softening on the penetration resistance of spudcan foundations on clay

In practice, natural soils exhibit strain-rate dependency and also soften as they are sheared and remolded. Traditional design approaches, however, have been developed based on idealized rate-independent perfectly plastic soil. This paper presents results for deep penetration of spudcan foundations in strain-softening, rate-dependent, soft clays quantifying the effects relative to results for ideal soil. The analysis was carried out using a large deformation finite-element (LDFE) approach, modifying the simple elastic-perfectly plastic Tresca soil model to allow strain softening, and incorporate strain-rate dependency of the shear strength. Parametric analyses were undertaken varying the strain-rate parameter, the sensitivity and ductility of the soil, the normalized penetration rate, and the soil strength nonhomogeneity. Overall, penetration resistance for rate-dependent, strain-softening clays lay below that for ideal soil. Increased brittleness of the soil led to marked reduction in penetration resistance, only partly compensated by strain-rate dependency. Key results have been presented in the form of design charts, fitted by simple expressions to estimate the limiting cavity depth above the advancing spudcan and the limiting bearing capacity factor at depth. © 2009 ASCE