Figure 8: Uniaxial compressive strength w.r.t. elastic modulus and stiffness index of weathered
shale.
Empirical Strength Model of Composite Rock
The empirical strength model of each type of composite rock was determined by the same approach of analysing individual strength of sandstone and shale. Each composite model was developed base on the combination of each group of experimental data for weathered sandstone and shale. It was found that the strength boundary of weathered materials changes from soil-like material to rock-like material at σ2 = 35 MPa or equivalent Isp50 = 0.3 MPa. The strength correlation for the respective weathered state was plotted by using normal scale graph as shown in Fig 9 and Fig 10. The respective empirical strength correlations are simplified below:
Rock-like composite with R 2 = 0.94 (11) or with R 2 = 0.97 (12)
Soil-like composite with R 2 = 0.90 (13)
or with R 2 = 0.94 (14)
General composite material with R 2 = 0.85 (15)
Subsequently the correlation between the uniaxial compressive strength with elastic modulus, E50 , and stiffness Ii of composite is as shown in Fig. 11. The graphical plot produced two empirical correlations. The elastic modulus and stiffness index of the composite can be predicted by the following equations,
with R 2 = 0.7 (16)
with R 2 = 0.72 (17)
Equation (16) for composite material is close to that weathered shale (Eqn. 9) and the stiffness index (Eqn. 17) is close to Eqn. 10. Hence it can be deduced that the presence of shale has greatly influenced the strength of Kenny Hill rock mass.