Civil Engineering Reference
In-Depth Information
Figure 4.32
Variation of reduction factor with pile stiffness ratio.
700 MPa. Assuming a Young's modulus for the concrete of 25 000 MPa, the quantity
that controls the compressibility,
L
, may be calculated as 1.9 (see equation (4.45)).
The initial stiffness may be estimated approximately from equation (4.45) as
P
μ
/
w
=
.
7
3 MN/mm.
The relatively compressible nature of rock sockets, together with high base capacity,
results in partial (or complete) slip along the pile shaft generally occurring under work-
ing conditions. Analytical and numerical solutions for estimating settlement and load
transfer in rock sockets under conditions of partial slip have been presented by Rowe
and Armitage (1987) and Carter and Kulhawy (1988). As slip occurs, the shaft friction
will often decrease, as the cohesive or dilational component of the shaft friction will
reduce to zero (Johnston and Lam, 1989). Since the shaft capacity is mobilized at much
smaller displacements than the base capacity, the overall load-displacement response
of the rock socket may then show a decrease in capacity at intermediate displacements
(due to strain softening of the shaft) prior to increasing again at larger displacements
as the base capacity is mobilized. Figure 4.33 shows schematically the different types
of rock socket response, depending on the rapidity of strain softening and the rate of
mobilization of the base capacity.
4.3 Lateral capacity of piles
Almost all piled foundations are subjected to at least some degree of horizontal loading.
In many cases, the magnitude of the loads in relation to the applied vertical loading
is small, and no additional design calculations are necessary. For example, the piled
