Numerical simulation and parametric study on maximum mud pressure associated with horizontal directional drilling

During the pilot hole, pre-reaming and pipe pullback processes in horizontal directional drilling (HDD), drilling fluid is used to cut soil, suspend and transport cuttings out of the borehole, form a filter cake to prevent slurry loss and maintain borehole stability. Excessive drilling fluid pressure; however, could cause fluid loss and large ground displacement, which is a significant issue on HDD projects. Stability of the borehole and drilling fluid pressure are determined by several influencing factors such as depth of cover, borehole diameter, cohesion, friction angle, Poison's ratio and unit weight of soil. With the use of the finite difference method (FLAC3D), a series of numerical simulations were conducted to study how these influencing factors affect the plastic zone development around the borehole and the value of the maximum allowable drilling pressure. Criteria for maximum allowable drilling fluid pressure and development of maximum plastic zone radius in the Delft solution are demonstrated. The research results reveal that soil elastic modulus almost has no influence on maximum allowable drilling fluid pressure. Also, the value of maximum allowable drilling fluid pressure increases with increase in soil weight, friction angle, cohesion, Poison's ratio and depth of cover and decreases with increase of borehole diameter. Those factors have a significant influence on the value of maximum drilling fluid pressure.