Pile Design and Construction Practice, Fifth edition

6.2 Uplift resistance of piles

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 com-

pression; however, Radhakrishna and Adams(6.1) noted a 50% reduction in the uplift

308 Piles to resist uplift and lateral loading

Pile cap

Tension pile B

Compression

pile A

Triangle of

forces

p q

H

r

Q

R

P H

Figure 6.5Restraint of horizontal force by raking piles.