BEARING CAPACITY OF PILES FOUNDED ON A ROCKY BED.

Piles are at times required to be driven through weak layers of soil until the tips meet a hard strata for bearing. If the bearing strata happens to be rock, the piles are to be driven to refusal in order to obtain the maximum carrying capacity from the piles. If the rock is strong at its surface, the pile will refuse further driving at a negligible penetration. In such cases the carrying capacity of the piles is governed by the strength of the pile shaft regarded as a column as shown in Fig. 15.6(a). If the soil mass through which the piles are driven happens to be stiff clays or sands, the piles can be regarded as being supported on all sides from buckling as a strut. In such cases, the carrying capacity of a pile is calculated from the safe load on the material of the pile at the point of minimum cross-section. In practice, it is necessary to limit the safe load on piles regarded as short columns because of the likely deviations from the vertical and the possibility of damage to the pile during driving.

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).