FINITE-ELEMENT ANALYSIS OF THE NERLERK UNDERWATER BERM FAILURES

Finite element analyses have been carried out to investigate the possi ble causes of failure, during construction, of the Nerlerk underwater berm-a hydraulically placed sand slope, designed to form part of a bot tom-founded offshore oil exploration platform at Nerlerk in the Canadi an Beaufort Sea. The double-hardening Monot constitutive law has been used for modelling the fill materials, while an elastic-perfectly plas tic von Mises idealization has been used for modelling the weak underl ying clay layer, on which the structure was founded. The computed. res ults demonstrate that, for conditions of rapid loading, there are thre e possible failure mechanisms, depending on the shear strength of the clay. All mechanisms result in liquefaction of the Nerlerk fill and ar e hence consistent with the flat post-failure slopes observed on site. However, the most likely cause of failure seems to be that of limited movements in the clay layer triggering liquefaction in the Nerlerk fi ll near to the berm crest. This investigation represents a major step forward over previously published back-analyses, in that no prior assu mptions regarding failure mechanism have been involved. It is also a s ignificant improvement over conventional slope stability calculations, which, for liquefiable soils, tend to greatly overestimate onsite fac tors of safety.