Initiation of static instability in the submarine Nerlerk berm

Abstract

According to stability postulates by other authors, soils that exhibit nonassociated flow may become unstable when exposed to certain stress paths inside the failure surface. Several series of conventional triaxial tests on fully saturated specimens have been performed to study the regions of stable and unstable behavior. For saturated specimens that tend to compress, undrained conditions lead to effective stress paths directed within the region of potential instability, and instability is observed, provided the yield surface opens up in the outward direction of the hydrostatic axis. Thus, instability occurs inside the failure surface. Instability is not synonymous with failure, although both lead to catastrophic events. The location of the region of potential instability and its determination are discussed. The submarine Nerlerk berm, which suffered six slides during its construction, is analyzed using a newly developed method of instability analysis. It is shown that conventional slope stability methods do not capture the mechanics of instability and subsequent liquefaction. The proposed analysis method is based on the location of the region of potential instability and the stress state in the ground. In addition, a trigger mechanism is required to initiate the instability. These topics are discussed with reference to the slides in the Nerlerk berm. Key words : instability, nonassociated flow, plastic properties, sand, shear strength, submarine slope.