| While varying the soil parameters from those of soft clay to those of very stiff clay, the elastic and elasto-plastic response of purely cohesive soil to the Horizontal Directional Drilling process is examined using finite element analyses. When there was an elastic response of the host soil, elasticity theory predicted the annular tangential stress in the host soil and could be used to predict the maximum allowable drilling slurry pressure to prevent tensile fracturing. In cases where shear failure of the soil occurred, plasticity theory predicted the annular tangential stress in the yielded areas. Following shear failure, the annular tangential stress increased with an increase in slurry pressure, and tensile fracturing was therefore no longer a threat. By combining the elasticity and plasticity theories, the minimum annular tangential stress could be predicted for a given set of drilling conditions. Design formulae that reflect soil parameters, slurry parameters, and the tensile strength of the soil are presented and compared to another published design equation that quantifies the development of unconfined plastic flow.; Finite element analyses are also used to study the elasto-plastic response of purely frictional soil that included the annulus of cohesive filtercake that forms where the drilling slurry penetrates out into the surrounding soil. (Abstract shortened by UMI.)... |
