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.