5. Liquefaction Induced Downdrag and Dragload from Full-Scale Tests

Frequently, deep foundations extend through potentially liquefiable sand layers near the ground surface and bear on more competent layers at depth. When liquefaction occurs, the skin friction in the liquefied layer would be expected to decrease to some negligible value, but as the liquefiable layer settles, negative skin friction could potentially develop around the pile in this layer as effective stress increases. To investigate the loss of skin friction and the development of negative skin friction, axial load tests were performed on instrumented full-scale piles before and after blast-induced liquefaction at sites in Vancouver, Canada and Christchurch New Zealand. Following blasting, liquefaction developed within sand layers resulting in significant settlement. Skin friction in the liquefied layer initially dropped to essentially zero. However, as the liquefied sand reconsolidated, negative skin friction became equal to about 50% of the pre-blast positive skin friction. Despite significant ground settlement, pile settlement was relatively small. A neutral plane approach for computing pile settlement resulting from negative skin friction provided reasonable agreement with observed behavior.