Characterizing rock transverse anisotropic spatial variations using digital drilling

Transverse anisotropy plays an important role in the engineering behavior of rock masses. This study examines the transverse anisotropic spatial variations and evaluates them using the coefficient of variation (CV) and anisotropy index (AI) through digital drilling tests. Four types of rocks, shale (SH), mudstone (MU), sandstone (SA), and schist (SC), are studied in four directions, revealing significant transverse anisotropy at the cutting point. The results demonstrate that the effect of the depth per rotation on the difference is weaker compared to the scaled weight and torque. The ranking of transverse anisotropy in the tested rocks is: MU > SA > SH > SC. Transverse anisotropy changes along the drilling depth vary across rock types. With the increase of drilling depth, SH experiences a decrease, rebound, and another decrease. MU shows an initial increase followed by a decrease. For SA, the degree of transverse anisotropy decreases and is sustained at a lower extent. SC exhibits slight fluctuations with the exception of the abrupt change. The time when the drilling bit is in perfect contact with the rock and the cutting efficiency in different drilling directions are the main factors causing the transverse anisotropy. The intricate evolution of transverse anisotropy should be worth investigating on a wider scale.