The summary of the physical and mechanical properties of sandstone and
shale with respect to their degree of tropical weathering is tabulated
in Table 2, Table 3 and Figure 2. From the results the qualitative and
quantitative methods of assessment have conclusively showed the
gradually decreased of weathered sandstone physical properties and
strength differences from 6% to 33% from BP2 to BP5. Similarly the
complex properties of weathered shale resulted to strength differences
from 17% to 28% from S2 to S5’.
EMPIRICAL STRENGTH MODEL
The principles of empirical strength model of sedimentary rock is based
on the assumptions that the physical properties of sandstone and shale
gradually changed from fresh to highly weathered, hence their respective
strength reduces proportionately with the grade of weathering . It was
assumed that the four empirical models in Figure 1 were based on a
column weathering profile of rock mass. The analyses were carried out in
sequence of the strength of weathered sandstone; weathered shale ;
weathered composite material dominated by sandstone and weathered
composite material dominated by shale . In the composite rock material
models, the structure orientation was assumed to be ideally horizontally
paralleled.
In summary, the research design of the empirical models were based on
the following assumptions:
-
quality of weathered rocks below the ground increases with depth
-
quality of rock weathering is based on strength envelopes.
-
broad range in strength index are due to inherited non-homogeneity of
weathered rock
-
the highest and the lowest strength indices are recorded for each
weathered group of rock
-
the average strength index of rock from each group was used as a
guideline
-
the empirical correlations of material strength was used to predict
the mechanical properties of composite rock material.
Uniaxial Compressive Strength to Point Load Strength of Weathered
Sandstone
From the laboratory results, the uniaxial compressive strength of the
weathered sandstone, (σ, σ1 or
σ2 ) versus the respective point load strength
index, Isp50 , were plotted on semi-log graph as
shown in Fig 3. The strength correlation had two different profiles. The
uniaxial compressive strength of slightly weathered to moderately
weathered sandstone has relatively linear correlation to point load
strength as shown by equation (1), meanwhile moderately weathered to
highly weathered sandstone strength concludes logarithmic correlation as
shown by equation (2). Both equations have significant strength
correlation of more than 90%.
with R 2 = 0.93 (1)
with R 2 = 0.97 (2)
Further data regression from slightly to highly weathered sandstones
produced power regression line as in equation (3), with degree of
significant equal to 0.91.
with R 2 = 0.91 (3)
Strength to Elastic Modulus and Stiffness of Weathered Sandstone
The elastic modulus of weathered sandstone was determined from uniaxial
compressive strength test. Graphical plot of the uniaxial compressive
strength (σ2 ) with respect to elastic modulus
(E50 ) and stiffness index (Ii) is as shown
in Fig. 4. From Eqn. 4 and Eqn. 5 respectively with each
R2 value equal to 0.52 and 0.23 had concluded a low
significant of correlations obviously observed from the scattered data.
with R 2 = 0.52 (4)
with R 2 = 0.23 (5)