Liquefaction-Induced Downdrag on Tapered Piles from Full-Scale Blast Liquefaction 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 an instrumented full-scale tapered pile before and after blast-induced liquefaction at a site in Mirabello (Ferrara, Italy) that was affected by liquefaction following the 2012 Emilia earthquakes. The test pile was a 16.5 m long concrete pile with a diameter of 0.52 m at the head tapering to 0.26 m at the toe. Following blasting, liquefaction developed within a 6-m thick sand layer below a clay surface layer 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 ultimate positive skin friction. Negative skin friction in the overlying non-liquefied clay layer was only 80% of the ultimate positive skin friction. This is likely due to the surrounding soil moving slightly away from the tapered pile as the soil settled vertically downward. Despite significant ground settlement, pile settlement was relatively small because of the resistance provided by the toe of the pile.