This paper focuses on the effect of shear modulus on maximum allowable pressures of drilling fluid applied to maintain the stability of a borehole in Horizontal Directional Drilling (HDD). HDD is one of the widely used trenchless methods to install and replace underground pipelines and utility conduits. Despite the popularity and success, borehole instability caused by tensile is still considered as a major challenge in drilling. While there are analytical and numerical models used to design the maximum allowable drilling fluid pressures based on soil profile, the uncertainty in selection of the representative values of soil properties including shear modulus can have an impact on the predicted drilling fluid pressure. Data from field experiments on drilling fluid pressures was used to validate the existing analytical models and the effect of shear modulus is presented. Results indicates that maximum pressures of drilling fluid increase with increasing shear modulus, however this increase depends on the assumption of the radius of plastic zone, which is closely related with the borehole diameter. The results from this study can significantly help in reducing the risks associated with borehole instability, thus lowering the drilling time and cost.
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