Estimation of shear strength parameters of intact limestones using miniature triaxial test

Abstract

Determination of the strength parameters of intact rock samples is an important requirement in rock engineering. On the other hand, proposing a simple method as an index to estimating the strength parameters is one of the tasks of engineering geology. The conventional triaxial test (CTT) is one approach used to determine shear strength parameters (φ and c) that requires standard core cylindrical samples. As it is not possible to prepare standard core samples or collect rock block samples to prepare standard cores under all conditions, the shear strength parameters should be determined using samples having non-standard geometry and dimensions. For this purpose, a miniature triaxial test (MTT) device is introduced and used as an index test to apply triaxial loading on small cubic rock samples in order to estimate their shear strength parameters. The repeatability of the MTT results was determined by performing 27 MTT on 6 × 6 × 6 mm3 polymethyl methacrylate (PMMA) cubic samples. The results showed that the values measured by the device were repeatable with over 97% similarity. The internal friction angles (φ) of the mortar samples obtained from the CTT on cylindrical samples and MTT on the 6 × 6 × 6 mm3 samples are similar. However, the cohesion (c) value measured from the MTT test is nearly double the value obtained from the CTT test. The shear strength parameters of 12 different intact limestone samples from the Asmari Formation located in the Siyakouh section were obtained by performing CTT on standard NX core samples and MTT on the 6 × 6 × 6 mm3 samples. The correlation between the φ and c from the CTT and MTT had coefficients of determination (R2) of 0.92 and 0.88, respectively. The precision of the empirical relation for the CTT-MTT correlation was evaluated and showed that shear strength values estimated by the MTT had about 90% similarity with the measured shear strength values from the CTT.