Numerical Modeling of Spudcan Deep Penetration in Three-Layer Clays

This paper reports the results from numerical modeling of spudcan deep penetration through three-layer clays. Two typical seabed strength profiles commonly encountered in the field, and identified as critical for potential punch-through failure, were considered: (1) uniform stiff-soft-stiff clay and (2) nonuniform clay with an interbedded stiff clay layer. Three-dimensional large deformation finite-element (LDFE) analyses were carried out with and without simulating strain softening and strain rate dependency of the shear strength. The results were compared with previously published LDFE results and centrifuge test data. A detailed parametric study was undertaken, varying the relevant range of layer thicknesses (relative to the spudcan diameter), strength ratios, normalized strength, and strength nonhomogeneity. Punch-through and rapid leg penetration (for stiff-over-soft) and squeezing (for the reverse) were demonstrated by the penetration resistance profiles and associated soil failure mechanisms. The results emphasized that the resistance profile from a spudcan continuous penetration in a three-layer sediment (e.g., stiff-soft-stiff) may be significantly different from the combined profiles resulting from two separate analyses on first-second layers and second-third layers, even if an appropriate depth factor is accounted for. For uniform stiff-soft-stiff clays, the presence of the bottom stiff clay layer within a short distance of one diameter affected spudcan bearing response in the top layer and suppressed the potential for punch-through failure (apart from rapid leg run for t1/D=0.75 and su1s/su2=3). A trapped soil plug beneath the advancing spudcan, following penetration of the top stiff layer, caused the bottom stiff layer to be sensed earlier and led to enhancement of the limiting squeezing depth. For nonuniform clay with an interbedded stiff layer, spudcan bearing response was dominated by squeezing (with limiting squeezing depth of approximately 0.18D) in the top soft layer and potential punch-through and rapid leg run (for t2/D≥0.5 and su2/su3s=2.5) in the middle and bottom layers. In contrast to ISO recommendations, the soft soils in between the advancing spudcan base and the stronger layer did not squeeze out completely. Instead, some of the trapped material was forced into the underlying stronger layer. Punch-through distance was found to reduce significantly with increasing positive strength gradient of the bottom layer.