Figure 6: Correlation of strength index to point load index of weathered sandstone
Empirical Model of Weathered Shale
The indirect method of empirical strength model for weathered shale is an alternative method for predicting its equivalent uniaxial compressive strength.At the same time, the correlation between material properties with respect to its quality of weathering grade can been predicted. From the minimal experimental results of uniaxial compressive strength test and point load test on slightly weathered shale, a semi-log graph was established. The correlation between the two strength indices showed a curvilinear correlation as shown in Fig 7 producing an empirical correlation as in eqn (8),
with R 2 = 0.94 (8)
Hence the reduction in strength of weathered shale is predicted from the point load strength index. The uniaxial compressive strength of the shale is in the range of 6 MPa to 29 MPa for moderately weathered. For slightly weathered shale, the ranges of strengths are slightly higher range from 29 MPa to 35 MPa.