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:

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)