Analysis and comparison of rock strength properties of calcarenite, acquired by needle penetration, unconfined compression and shear box tests

Abstract

This study focuses on three different, often used methods to determine rock strength: needle penetration, unconfined compression and shear box tests. The investigations were done on calcarenite that was taken directly from the construction site of the new A2 highway tunnel in Maastricht, the Netherlands. Besides determining the strength, it was also intended to obtain a better understanding of the differences, advantages and disadvantages of each method with respect to the others. Additionally, empirical and theoretical relationships for estimating the unconfined compressive strength were evaluated. Needle penetration tests gave questionable results, especially when trying to estimate the UCS by using an empirical relationship: estimations differed by a factor of 10 from actually measured UCS. Unconfined compression test results proved to be strongly affected by anisotropy due to sedimentary bedding, ranging from 0.12 MPa (parallel to bedding) to 0.45 MPa (perpendicular to bedding). Shear box tests yielded relatively high cohesions for this particular material, also depending strongly on the orientation of sedimentary bedding, ranging from 42 kPa (parallel) to 128 kPa (perpendicular). The estimation of UCS from cohesions and friction angles obtained by shear box tests, gave fair but not highly accurate approximations within ±25% of the actually measured UCS.