Those concerned with bottom supported platforms and pipelines for the offshore area surrounding the Mississippi River Delta have been confronted with a difficult engineering problem - the design of permanent structures to resist submarine slides. Recent observations of the prodelta sediments (principally very soft clays) and experience with platforms founded in them indicates that during periods of unusually severe wave action (such as accompanies hurricanes) the bottom sediments may become unstable and move down the Delta slopes. Such soil movements subject the platforms to very large forces. If such forces are not anticipated in the design of the platforms, failures may result.
This paper is concerned with two aspects of designing slide resistant platforms:
Estimating the magnitude and depth of lateral soil movements induced by wave action.
Estimating the maximum lateral forces developed against foundation members by the moving soils.
The first step in developing such estimates is to determine the in-place soil properties. Conventiona1 soil test data and insitu vane shear test data from borings in Shell Oil Company's South Pass Block 70 lease (See Figure 1) are used to illustrate the results of such work. The soil properties are translated to terms of lateral movements through application of a two-dimensional, nonlinear, static, finite element mathematical model. The in-place soil strength data and estimates of the soil and foundation element movements are combined with results from research on laterally loaded piles to develop an estimate of the amount of lateral force developed by the moving soils.
The problems of unstable terrestrial slopes are well known to engineers. The Canadian (1) and Scandinavian (2) flow slides are one example. The slides that accompany earthquakes (3) are another example.
Submarine slope slides are a natural extension of the terrestrial slides. However, in the sea there are additional slide triggering mechanisms and the phenomenon is much more difficult to observe. The reader is referred to a good survey by Morgenstern (4) of past work on submarine slides.
Recently, engineering literature has contained notice of one unique type of submarine slide - the wave pressure induced slide in soft clays. An earlier paper by Bea (5) discussed one example of experience with this phenomenon. Henkel (6) developed an analytical attack to predict the conditions under which wave induced slides might be expected. This work was followed by that of Wright and Dunham (7). They proposed the use of a two dimensional, nonlinear, static, finite element analysis. Such an analysis is not only capable of analyzing failure conditions, but also the stresses, strains and movements in the soil that lead to failure (something Henkel's slope stability analysis cannot do).