Foundation loads due to lateral spreading at the Landing Road Bridge, Whakatane

Technical Abstract

This report investigated liquefaction-induced spreading loads with a case study of the Landing Road Bridge sire in Whakatane, where about 1.5m of horizontal surface displacement occurred following the 1987 Edgecombe Earthquake. Soil mounded behind the piers on the true left bank of the Whakatane River in an apparent passive failure. Preliminary estimates based on CPT data show a passive load of about 500kN per pier, which is of the same order as the collapse load of the raked-pile foundation. Thus, extensive site investigation, laboratory testing and analysis was undertaken to make an improved estimate of the lateral loads.

Trenching at Piers C and E showed clear evidence of failure surfaces consistent with passive failure and observation of the absence of cracking at the top of the piles confirms the bounds on soil loads obtained from analysis of the structural strength of the foundation. Strength testing of intact block samples of the backfill used around the piers could not be achieved due to the presence of large woodchips and other rubbish. Thus, testing of reconstituted specimens was undertaken to estimate the strength of the backfill and total passive load exerted by the lateral spreading soil. Calculations showed a wide range in possible passive soils loads and appeared to be no better than initial estimates.

Simple approaches to analyse lateral spreading loads are presented with possible design methods to minimise potential structural damage.