Soil-Wave Tank Studies Of Marine Soil Stability

Abstract

ABSTRACT Soil instability occurred off the Mississippi River Delta during hurricane Camille due to large wave induced bottom pressures. This paper discusses the effect of such waves on soft clayey soils. In particular, the paper is based upon the results acquired from a large model tank which was constructed to study the effect of surface water waves on a clay soil under controlled conditions. The primary motivation for the tank studies was to provide a qualitative means by which an engineer could evaluate the progress of instability under wave loading. Within the scope of the tank study, soil displacement patterns were observed with depth and related to field conditions. Investigations were made into the effects on the soil continuum of wave height and period, soil shear strength, slope variations, and repeated wave loading. Measurements of wave height and period, bottom pressure, and in-situ soil shear strength were obtained to evaluate simple analytical methods for predicting instability. INTRODUCTION The sediments of the Mississippi River have strongly influenced the continental shelf offshore Louisiana. The present Birdfoot delta complex began less than 500 years ago and has been building seaward at rates up to 350 feet per year. 1 The accumulation rate of the sediments has exceeded the rate at which the deposits can dissipate the buildup of excess pore water pressure. As a result, the soils around the Birdfoot complex are very soft and underconso1idated. 2, 3 The weak soils in combination with continuing deposition and environmental forces lead to a dynamic system of delta building. The purpose of this paper is to discuss one of these dynamic processes, namely, wave induced soil instability. This phenomena is initiated by waves of sufficient height and period which produce significant pressures at the seafloor. If the surface waves exert enough pressure, the soil may become unstable. Some areas of the Mississippi Birdfoot complex are susceptible to wave induced soil instability, particularly where weak soils are subject to the effects of hurricane - wind generated waves. 4, 5 This paper is based on the results of soilwave tank studies of wave induced soil instability. The primary motivation for constructing the tank was to obtain firsthand observational experience on the effect of waves on soft soils. Basis for Tank Tests The phenomena of hurricane - wind generated waves creating soil instability was generally unknown prior to research by Shell Oil Company. Henkel,6 for example, published a slip-circle method of analysis after completing his work as a consultant to Shell in 1970. Scott and Zuckerman7 in their 1970 review of slope instability in the ocean floor briefly reported Henkel's work.