Pile Design and Construction Practice, Fifth edition

308 Piles to resist uplift and lateral loading
Tension pile B
Pile cap
Compression
pile A
Figure 6.5 Restraint of horizontal force by raking piles.
6.2 Uplift resistance of piles
R
H
p q
Triangle of
forces
6.2.1 General
The simplest method of restraining piles against uplift is to employ a pile shaft that is
sufficiently long to take the whole of the uplift load in shaft friction. However, where there
is rock beneath a shallow soil overburden it may not be possible to drive the piles deeply
enough to mobilize the required frictional resistance. In such cases the shaft resistance must
be augmented by adding dead weight to the pile to overcome the uplift load, or by anchoring
the pile to the rock.
Adding dead weight to counteract uplift loading is not usually feasible or economical.
The piles may be required to carry alternating uplift and compressive loads, in which case
the added dead weight would result in a large increase in the compressive loading. In the
case of shipbuilding dock floors (Figure 6.1), dead weight in the form of a thick floor would
add considerably to the construction costs, and in piled dolphins (Figure 6.4) the provision
of a massive pile cap could make a substantial addition to the load on the compression
rakers. Experience has shown that anchors in the form of grouted-in bars, tubes or cables
are the most economical means of providing the required uplift resistance for piles taken
down to a shallow rock layer.
6.2.2 The uplift resistance of friction piles
The resistance of straight-sided piles in shaft friction to statically applied uplift loads is
calculated in exactly the same way as the shaft friction on compression piles, and the
calculation methods given in Sections 4.2 to 4.5 can be used. However, for cyclic loading
the frictional resistance is influenced by the rate of application of the load and the degree of
degradation of the soil particles at the interface with the pile wall. In the short term, the
uplift resistance of a bored pile in clay is likely to be equal to its frictional resistance in compression;
however, Radhakrishna and Adams (6.1) noted a 50% reduction in the uplift
Q
P H
r