Resistance of piles to compressive loads 205Percentage of applied load Q carried in side-wall shear020QQQBQL = BL = 2BL = 3BL = 4BRock, EdConcrete Ec40608010020Percentage of
full socket
lengthv = 0.25Ed/Ec=5
Ed/Ec= 0.540 60 80 100Figure 4.32Distribution of side-wall shear stress in relation to socket length and modulus ratio (after
Osterberg and Gill(4.49)).
settlement of the pile are summarized as the following:
(1) The length to diameter ratio of the socket
(2) The strength and elastic modulus of the rock around and beneath the socket
(3) The condition of the side walls, that is, roughness and the presence of drill cuttings or
bentonite slurry
(4) Condition of the base of the drilled hole with respect to removal of drill cuttings and
other loose debris
(5) Layering of the rock with seams of differing strength and moduli
(6) Settlement of the pile in relation to the elastic limit of the side-wall strength and
(7) Creep of the material at the rock/concrete interface resulting in increasing settlement
with time.
The effect of the length/diameter ratio of the socket is shown in Figure 4.32 for the
condition of the rock having a higher elastic modulus than the concrete. It will be seen that
if it is desired to utilize base resistance as well as socket friction the socket length should be
less than four pile diameters. The high interface stress over the upper part of the socket will
be noted.
The condition of the side walls is an important factor. In a weak rock such as chalk, clayey
shale, or clayey weathered marl, the action of the drilling tools is to cause softening and