Estimation of rock strength and integrity in TBM tunnel using rock fragment size and shape characteristics

Rock fragments are the direct product of the tunnel boring machine (TBM) breaking rock, and their size and shape contain detailed geological information. Currently, the research on rock fragment identification primarily focuses on identifying rough rock mass grade, which cannot effectively guide TBM tunneling and support optimization. Hence, this study proposes estimating rock strength and rock mass integrity simultaneously using rock fragment size and shape characteristics. Firstly, the size and shape characteristics of rock fragments are quantified, including the distributions of equivalent diameter, area, radius ratio, roundness, and proportion of rock powder. Secondly, Image-Pro software is used to outline rock fragment profiles, and a database containing rock fragment parameters, rock rebound strength, and rock mass intactness index is developed. Finally, correlation analysis is performed to identify relevant rock fragment parameters, and mapping formulas linking these parameters to rock rebound strength and the rock mass intactness index are established via multivariate regression. The results show that the shape parameters (radius ratio and roundness) of rock fragments are closely related to rock rebound strength, and the size parameters (equivalent diameter and area) of rock fragments and the proportion of rock powder are closely related to the rock mass intactness index. Moreover, the mean absolute percentage errors of the estimation formulas for rock rebound strength and rock mass intactness index are 7.63% and 7.37%, respectively. Therefore, the proposed method is feasible and effective and can guide the optimization of TBM tunneling control parameters and support measures.