Finite element study of 2D equivalence to 3D analysis of a discrete soil nail problem with applications to serviceability design

Abstract

Current trends in design and analysis of soil nailed structures show increasing use of finite element method (FEM) to verify or predict performance of the system. Due to the need to do this computationally efficiently, 2D plane strain idealisations of a discretely placed soil nail have often been used. There are many methods used in the idealisation of a soil nail problem. However there is lack of current consensus on which method best represents the problem and also the limitations of each method. The author has classified these methods broadly into three categories. This thesis seeks through a comparison of 2D analysis using each method with 3D analysis in FE of the soil nail problem to clarify the limitations of each method with recommendations to the limitations and use of each. This is done with both a single row nail comparison as well as a multiple row nail comparison with 3D FE observations as well as behaviour from an instrumented model soil nail experiment. Subsequently, the author attempts to quantify the limitations of 2D analysis by introducing design limits to the use of 2D analysis. It has been observed that the level of mobilization of pullout capacity is also different in 2D and 3D. A method of idealisation utilizing the mobilization factors was also introduced to account for this difference in order to improve 2D simulation of a 3D problem. In addition, intuitively, the influence of the nail decreases with spacing between nails. A numerical pullout simulation is done to investigate this effect and recommendations in the form of a design chart is suggested as a guideline to the design of spacing and also the recommended use of plane strain analysis. The results from the nail spacing design chart was then verified with a parametric analysis from a single row soil nail case with results in good agreement with conclusions from the design chart.