Twin-Shear Unified Strength Theory–Based Solution for Maximum Allowable Mud Pressure in Horizontal Directional Drilling

In horizontal directional drilling (HDD), high mud pressure promotes cutting delivery and overall efficiency at the expense of mud loss and possible borehole wall instability. Accurate calculation of the maximum allowable mud pressure (MAMP) according to the actual HDD project specifications is vital. Engineering formulas based on the Mohr–Coulomb strength criterion hardly apply to complex geological strata, and the results may deviate from actual field records. This study attempts to provide a twin-shear unified strength theory–based solution for the HDD MAMP. It can adapt to complex geological conditions and effectively account for the contribution of the intermediate principal stresses to the overall strength by determining the magnitude of the b-values of different geotechnical materials through plane-strain and conventional triaxial tests according to the actual engineering geological conditions. The MAMP calculations can be adapted to specific geological conditions by adjusting the lateral earth pressure coefficient at rest. The numerical simulations conducted and verified using ABAQUS CAE (version 2017) enable the utilization of the proposed unified solution for the preliminary design of the HDD MAMP. Through practical engineering verification, it is found that the accuracy and applicability of the unified solution are particularly excellent under complex geological conditions, which can effectively avoid engineering instability accidents caused by high mud pressure.