If piles are driven to weak rocks, working loads as determined by the available stress on the material of the pile shaft may not be possible. In such cases the frictional resistance developed over the penetration into the rock and the end bearing resistance are required to be calculated. Tomlinson (1986) suggests an equation for computing the end bearing resistance of piles resting on rocky strata as
Boring of a hole in rocky strata for constructing bored piles may weaken the bearing strata of
some types of rock. In such cases low values of skin friction should be used and normally may not
be more than 20 kN/m2 (Tomlinson, 1986) when the boring is through friable chalk or mud stone.
In the case of moderately weak to strong rocks where it is possible to obtain core samples for unconfmed compression tests, the end bearing resistance can be calculated by making use of Eq. (15.64).