Numerical Modeling Evaluation of Drilling Mud Loss Risk for a HDD Borehole in a Low-Strength Competent Bedrock

Horizontal directional drilling (HDD) plays an important role in pipeline construction. Drilling mud loss, or the maximum allowable drilling mud pressure, in soil and sand layers during HDD has been continuously studied since the 1980s. However, because HDD has been used more often in bedrock, it was realized that there is a lack of studies on this topic for rock HDD projects. In this study, the drilling mud loss risk of a HDD pipeline construction project in competent soft bedrock is studied by numerical modeling. The numerical modeling result shows that, in bedrock, the Delft equation can overestimate the maximum allowable drilling mud pressure when the lateral earth pressure coefficient is less than unity. Through numerical modeling, it is found that tension failure, instead of shear failure, is the main failure mode of bedrock surrounding the borehole. The failure mode in this project looks more like fracturing after expansion, instead of cylindrical expansion. A parametric study is conducted to investigate the required minimum rock mass overburden thickness, effect of overburden stress, and effect of geological strength index (GSI) value.