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.