Numerical Investigation of the Circumferential Stresses around Boreholes during Horizontal Directional Drilling

Borehole stability during horizontal directional drilling is one of the most important issues affecting success of pipe installations using this technique under roads, railways, and other obstructions. Two failure mechanisms, shear failure (blow-out) and tensile fracture (hydrofracture), have previously been identified. Both failure mechanisms can be seen in saturated clay based on total stress analysis, and shear failure dominates in dry sand based on effective stress analysis. Numerical modeling was used to investigate these failure mechanisms, and parametric studies were implemented. It was found that changes of elastic moduli and dilation angles have a minor impact on the numerical results when linear elastic analysis up to the initiation of yielding is undertaken. It was also found that the numerical results at the crown (and invert) can deviate from the theoretical results if yielding occurs at the shoulder (and haunch) of horizontal boreholes. Linear regression analysis was used to approximate these deviations. It was concluded that the ratios of intercepts of linear regression lines and vertical stresses are related to the undrained shear strength of clay and the critical friction angle of sand. However, there are no obvious relationships between the slopes of the linear regression lines and other factors. The ranges for coefficients in the modified solution are given.