Strength Index, Is of
R2 = 0.9305
Weathered Sandstone
Strength index, Is, is defined as the ratio of uniaxial compressive strength to point load strength and is a dimensionless parameter. It represents the ratio of compressive strength to the tensile strength properties of a rock material. Based on the plotted data as shown in Fig 5 and Fig 6, the strength index of sandstone correlated exponentially to σ2 (Eqn. 6) and Isp50 (Eqn. 7) as follows:
with R 2 = 0.9 (6)
with R 2 = 0.98 (7)
Based on the above correlations, it is observed that at σ2 > 60 MPa , Is < 125, the strength index reduction rate is not significant probably due to the high brittleness of weathered sandstone. On the contrarily the weathered sandstone having σ2 < 60 MPa and Is >125, can be due to reduction in the brittleness indicated by a large increment of strength index at low compressive strength. The profile revealed the boundary between rock-like weathered sandstone and soil-like highly weathered sandstone. The similar behaviour was also observed at the boundary of point loads strength of 1 MPa. The sandstone behaved as a brittle material when the point load strength is more than 1 MPa, otherwise it can be considered as a ductile material.