Instability and movements of oceanfloor sediments are potentially hazardous to offshore and coastal structures, and a major geologic process of offshore sediment transport. In addition to gravity, other environmental forces associated with wave storm and earthquakes may cause shear failure leading to oceanfloor instability and mass movements. In this paper, the basic aspects of such instability and mass movements are examined, and the current understanding of underlying mechanics and the available methods of analyses are reviewed. In the framework of simplified analyses, the conditions necessary for such instability will be formulated, which may be used to identify potentially unstable sediments in a given offshore region.
In the past few decades, the economic significance of submarine environments, and the need for land reclamation and waste disposal projects have led to proliferation of engineering activities in the ocean. The activities associated with exploitation of offshore oil, gas and mineral resources require installation of bottommounted engineering structures such as pipelines, anchors, gravity structures, and platform structures and communication cables. The use of continental shelves and the deeper-water shelf-slope region is growing and diversifying with technological developments. The instability and movements of oceanfloor sediments are potentially hazardous and constitute important considerations in the planning and design of such facilities (Bjerrum, 1971; Sterling and Strohbeck, 1973). The occurrence of oceanfloor instability and mass movement is a widespread phenomenon (Moore, 1978; Saxov et al., 1982; Lee et al., 1981). Active landsliding and areas of former instability are being identified by sophisticated geophysical survey techniques (Prior and Coleman, 1984). There is evidence of oceanfloor instability in a wide variety of oceanic environment, from shallow water, near-shore zones, continental slopes and beyond to the deep ocean floors. Oceanfloor instability has been responsible for the damage and destruction of offshore structures (Bea, 1983; Christian et al., 1974).