Foundation punch-through in clay with sand: centrifuge modelling

This paper is concerned with the vertical penetration resistance of conical spudcan and flat footings in layered soils. Centrifuge tests are reported for a clay bed with strength increasing with depth interbedded with dense and medium dense sand. Both non-visualising (full-model) and visualising (half-model) tests were conducted with high-quality digital images captured and analysed using the particle image velocimetry technique for the latter. The load-displacement curves often show a reduction in resistance on passing through the sand layers, which creates a risk of punch-through failure for the foundations when supporting a jack-up drilling unit. For a given foundation, the peak punch-through capacity (qpeak) is dependent on the thickness of both the overlying clay and the sand layer. The failure mechanism associated with the peak resistance in the sand layer involves entrapment of a thin band of top clay above the sand layer that subsequently shears along an inclined failure surface before being pushed into the underlying clay. The top clay height when normalised by the foundation diameter affects the soil failure pattern in this layer and along with the sand layer thickness controls the severity of the punch-through failure (i.e. the additional penetration before the resistance returns to the peak value). Comparisons are made with current industry guidelines for predicting qpeak and the risk of punch-through failure for sand overlying clay. These methods are shown to be conservative in their prediction of qpeak but inconsistent in predicting punch-through.