Limiting Slurry Pressures to Control Hydraulic Fracturing in Directional Drilling Operations in Purely Cohesive Soil

Hydraulic fracturing during horizontal directional drilling can permit drilling mud to travel to the ground surface, to collect under overlying pavements, or to be released into the riverbed during river crossings. These events can decrease drilling efficiency, damage adjacent infrastructure, and cause environmental damage. Current design equations for maximum drilling pressure focus on shear failure around the borehole drilled to house the new pipe. The present research examines, instead, the potential for tensile fracture of cohesive soils surrounding the borehole. Two-dimensional nonlinear finite element modeling is used to study the effectiveness of closed-form solutions for calculation of the drilling slurry pressures that fracture the surrounding soil. Tangential stresses are calculated for a borehole with a 0.2-m diameter in an undrained clay soil with various K0 values, construction depths, and drilling slurry pressures. Elastic plate theory is found to provide effective values of (a) tangential crown and spring line stresses when the soil responds elastically and (b) the decreases in tangential crown stress that occur as drilling slurry pressures increase. Closed-form plasticity solutions provide good values of tangential stresses once the soil yields. Following yield, increases in mud pressure result in increases in tangential stress, so hydraulic fracture from tension is no longer an issue. The study indicates that mud loss in low-strength clays or those with K0 close to unity is most likely the result of unconfined plastic flow (blowout). Mud loss for stiff clays and those with other K0 values is more likely the result of tensile fracture.