Upper bound analytic mechanics model for rock cutting and its application in field testing

The rock cohesion c and internal friction angle φ are essential parameters (combined as the c-φ parameter) used to characterise rock strength. Accurate measurement of these parameters is necessary for surrounding rock stability analysis and supporting scheme design in underground engineering. The currently used indoor test procedure is time-consuming and difficult to quantitatively evaluate the mechanical properties of fragmented rocks because these rocks cannot be effectively cored. Most field test methods can measure rock tensile or compressive strength parameters; however, it is difficult to determine the c-φ parameter. “Digital drilling rig” test technology provides a new way to solve the aforementioned problem. The key to implement this technology is to create a quantitative relation between the drilling parameters and the rock c-φ parameter. In this study, based on the characteristics of the rock cutting failure, an upper bound analytic mechanics model is developed for rock cutting. In this model, the ultimate rock cutting force is derived and the relation between the drilling parameters and the c-φ parameter is obtained. A comparative analysis of indoor tests and theoretical calculations shows that the average difference between the drilling parameters from the digital drilling test versus parameters from the theoretical calculation for the limestone tests is 9.33%; for the sandstone tests, the average difference is 5.85%. This validates the rock cutting mechanical model and the formula for the relation between the drilling parameters and the c-φ parameter. Based on these results, a digital drilling measurement method for the surrounding rock c-φ parameter in the field is proposed. The feasibility and effectiveness of the proposed method is verified via indoor testing. This method is convenient to implement in the field and can effectively measure the c-φ parameter of both intact and relatively fragmented rock mass in the field.