Analysis and Design of Circular Shafts Using Finite Element Method

Abstract

Rapid expansion of urban areas has created a need to expand/rehabilitate the existing buried infrastructure at the same or a greater rate. Technical challenges such as variable soil and rock profiles, high groundwater tables, and limitations imposed by the built environment, combined with present fiscal and schedule limitations, necessitates the implementation of fast and cost effective construction methods. Since drilling procedures and equipment allow for the construction of overlapped pile systems in almost all subsurface conditions, secant pile walls have become a cost-effective shoring option. The secant pile walls constructed in a circular plan layout to form a vertical shaft provide unique advantages such as compression ring behavior. This thesis presents a parametric study to investigate the soil-structure interaction and mechanical response of circular shafts under different loading and boundary conditions using the finite element method. The aspects that were studied included the identification of earth pressure distributions exerted on circular shafts, the impact of excavation of single and multiple holes on the shaft stresses, the stresses in the shaft in cases of sloping bedrock, and the distribution of surcharge pressures on the shaft walls. The results showed that the earth pressures acting on the exterior surfaces of the shaft wall remained between active and at-rest values. The earth pressures can be represented by active distribution at shallow depths and at-rest distribution at larger depths. The results also indicated that the horizontal extent of the spread of surcharge induced lateral pressures around the shaft was significantly influenced by the type of soil as well as the width of the surcharge. It was noticed that the theoretical estimates of pressures underestimated the simulated values at larger depths. The sloping bedrock was also seen to result in significant deviations from the compression ring behavior. A large increase in the maximum compressive stresses and an emergence of some significant tensile stress zones were observed for bedrock inclinations larger than 20 degrees. The results presented in this study address various practical design concerns and should be of interest to those involved in the design and construction of vertical shafts.