The assessment of intact rock strength for penetration

The term “rock mass” involves a complex interplay of two components: the intact rock and spatially distributed rock joints of various orientations and persistence. Besides the degree of rock mass fracturing, the intact uniaxial compressive strength (UCSi) is a crucial parameter for estimating feasible penetration rates for tunnel boring machines (TBMs). Therefore, typical penetration and excavation rate prediction models rely significantly on properly quantifying the (UCSi). However, the failure process of rock specimens subjected to uniaxial compression can be complex due to internal flaws or heterogeneities on the microscopic scale (i.e., not visible before testing by the naked eye), causing premature failure before reaching the true intact rock strength. Internal flaws can substantially contribute to the fracture initiation and propagation processes of the rock, and their impact on strength must be considered when defining intact rock strength properties for penetration prediction. Improper quantification (e.g. an underestimation due to flaws or insufficient specimen preparation) can substantially impact the predicted versus the achieved penetration rates, with a significant impact on cost and time. This paper proposes an engineering approach including a rigorous filtering process of laboratory results obtained according to international standards and regulations. The method allows for a comprehensible determination of the unconfined compressive strength (UCS) for penetration prediction.