Finite Element Analysis A Basis for Sea-Floor Soil Movement Design Criteria

Abstract

ABSTRACT The application of a recently proposed finite element analysis is presented as a basis for the development of wave induced sea-floor soil movement design criteria. Experience gained in the application of this analysis leads to the following conclusion: Failure to recognize the effects of pressure and cyclic loading on soil strength properties, and the influence of strength profile shape and gravity stresses can lead to unrealistic design criteria. However, used prudently as a basis for soil movement and foundation design criteria, the finite element analysis represents a significant improvement over slope stability analyses which are currently in use. INTRODUCTION Recent experience1,2 in the Mississippi Delta area of the Gulf of Mexico indicates that bottom pressures generated by storm waves can trigger large scale sea-floor soil movements. These movements can produce lateral forces on an offshore platform that exceed the magnitude of design level wind, wave, and current forces combined. The technology required to develop soil force and foundation design criteria can be divided into three elements:Limiting Equilibrium - What conditions are required to be present in order for soil movement to occur?Soil Response - What are the pattern and magnitudes of displacements and the state of stress in the soil during movement?Soil-Structure Interaction - How does the soil movement affect the platform? This paper will concentrate on the conditions of limiting equilibrium and soil response, which are related to soil movements generated locally by wave bottom pressures. The soil-structure interaction problem is treated elsewhere3 The question of probability of occurrence, which includes an evaluation of wave occurrence and the relationships between wave kinematics and bottom pressures, is also of prime importance in the development of design criteria but is beyond the s cope of this paper. A lower bound solution, employing a simplified finite element model, has recently been proposed4 to study the waveinduced soil movement problem. This static two-dimensional model includes the effects of large vertical deformations of the seafloor boundary and the nonlinear stress strain characteristics of the soil. This finite element analysis has the capability of resolving the questions of limiting equilibrium, pattern and magnitude of displacements, and state of stress, which are required in the development of design criteria. The validity of design criteria, which are developed using the results of any analytical model, will depend on at least three factors which are beyond the validity of the analytical model itself:Selection of input data, especially soil properties, which are representative of field conditions.Complete interpretation of the analytical results and their relationship to field response.Incorporation of field parameters, especially gravity forces due to bottom slope, which are not included in the analytical model.