Liquefaction and Dam Failures

Abstract

This paper is based on a critical state soil mechanics concept that liquefaction occurs when soil is on the dry side of critical states, near zero effective stress, and in the presence of high hydraulic gradients. In this view liquefaction is one of a group of phenomena; including piping, boiling, fluidization; with pipes and channels and hydraulic fractures, internal erosion and void migration. This paper will refer to some aspects of the failures of Fort Peck, Baldwin Hills, and Teton Dams in support of this view. Casagrande held an opposite view that liquefaction occurs by a chain reaction among sand grains on the wet side of critical states. Cam-clay provides a model for ductile stable yielding and deformation of an aggregate of grains wetter than critical states. A layer of such sediment can form folds during deformation. If a soil aggregate is denser than the critical state, it can fail with fault planes on which gouge material dilates and softens, or it can fracture and crack into a clastic debris, or develop pipes and channels. The critical state explanation of rapid failure is rapid transmission of pore water pressure through such opening cracks or channels. The Baldwin Hills and Teton dam failures were failures with cracks and pipes. In the case of the Fort Peck failure we suggest that high pore pressures from the core hydraulic fill were transmitted in the layer beneath the part of the dam that failed; Casagrande's view of the failure as evidence of a "chain reaction" is questioned. Selection and control of fills to ensure ductility and avoid over compaction and measures to ensure stability are discussed.