Case study of lateral spreading forces on a piled foundation

A case study is presented of the effect of lateral spreading on the piled foundations of the Landing Road Bridge, Whakatane, New Zealand, during the M-L 6.3, 1987 Edgecumbe earthquake. Trenching at two piers on the floodplain of the left bank revealed slip surfaces in the 1.5 m thick non-liquefied crustal layer, consistent with passive failure as the buried piers and raked piles resisted its displacement towards the river channel. The passive force on the buried portion of the slab piers is estimated at 850-1000 kN per pier, compared with roughly 50 kN in drag forces between the underlying liquefied sand and the set of eight, 400 mm square raked piles per pier. The collapse load of the foundation system is estimated to be about 1200 kN. Thus the force imposed by the non-liquefied crust was close to the ultimate capacity of the foundation. Our main conclusion is that the chief threat to piled foundations from lateral spreading comes from loads imposed by the non-liquefied crust, not from the drag forces of the liquefied soil. In this case, clear evidence was found of passive failure as the crust drove against the buried pier, piled through to firm ground.