Pile Design and Construction Practice, Fifth edition

526 Ground investigations, contracts and pile testing

Where piles have been designed by the methods described in Chapter 4 it is very helpful
to provide devices whereby the shaft and base loads can be evaluated separately. The load
on the base of the pile can be measured by inserting load measuring devices in a cylindrical
unit interposed between the pile base and the shaft. A typical installation consists of a ring
of pillar-type load cells around the periphery of the unit recording to a data logger at the
ground surface(11.21). The distribution of shaft friction on the pile shaft can be measured by
fixing electrical-resistance strain gauges onto the interior surface of a hollow steel pile, or
to a steel pipe embedded in a precast or cast-in-place concrete pile. Gauges of this type can
withstand the impact of pile driving and have given satisfactory service on piles which have
remained in the ground for a year or more.
A simple method described by Hanna(11.22)for obtaining the distribution of shaft friction
on the shaft of long hollow-section piles consists of installing metal rods down the interior
of the pile. The rods are terminated at various levels as shown in Figure 11.11 and are free
to move in guides as the pile settles under load. By means of dial gauges mounted on the
heads of the rods the elastic shortening of each length of pile between the toes of the rods
can be measured. Thus the load reaching the pile shaft at the toe of each rod is given by the

Figure 11.11Use of rod strain gauges to measure load transfer from pile to soil at various levels

down pile shaft.

Load

Dial gauges

to measure

settlement

at toes of rods

relative to

pile head

Hollow tubular

or box pile

Rod 1

Rod 2

Rod 3

P 3

P 2

P 1

l^3

l^2

l^1

P