Recommendation of Specimen Geometry in Determining Point Load Strength Index for Anisotropic Rocks

ABSTRACT: The point load strength index (Is(50)) is the critical parameter in evaluating the mechanical response of rock. Due to its ease in specimen preparation, this index test becomes even more important for calculating the strength index of layered anisotropic rocks. However, an accurate estimation of Is(50) in anisotropic rocks remains a challenge due to the effect of weak planes. This challenge can be overcome by understanding the accurate specimen geometry (length/diameter "t/d" ratio). Therefore, in the present study, the axial point load test of foliated phyllite has been carried out on the specimens having t/d = 1 and 0.6 at various foliation angles (β) of 0°, 15°, 30°, 45°, 60°, 75°, and 90°. The results indicate a significant difference in the Is(50) value across the two specimen geometries. The trend of Is(50) value with β angle for t/d = 1 shows an increasing order up to β = 60° followed by a decrease at β = 75° and 90° with an R2 value of 0.76. Whereas the continuously increasing trend was observed for the t/d = 0.6 having an R2 value of 0.96. Specimens with a t/d = 0.6 exhibit less discrete data sets with valid failure modes, whereas those with a t/d = 1 display large discreteness in the data sets along with invalid failure modes at higher β angles. 1. INTRODUCTION Major Engineering applications such as dams, tunnels, underground caverns, bridges, etc., are confined within hard and crystalline rocks and stretched within weak, soft, and anisotropic rocks (i.e., sedimentary and low-grade metamorphic rocks). Dissimilar distribution of stresses due to inherent anisotropy often leads to instability of foundations, yielding of rock masses, and excessive deformation, demanding a detailed investigation of the durability and deformability properties such as "strength" within such rock types. Point Load Test (PLT) is the most practical and reliable index test that has acquired worldwide acceptance as a strength index in rock mechanics and rock engineering practices as per the International Society for Rock Mechanics and Rock Engineering (ISRM) and American Society for Testing Materials (ASTM) standard.