Pile testing - Piling Engineering - page 304

Figure 9.2 A 2500-tonne pile load test using tension piles to provide reaction. (Photograph courtesy
of Cementation Piling and Foundations Ltd., Rickmansworth.)
interaction between the pile and the reaction, and to avoid movement of datum beams
used as references to measure settlement.
Kentledge is commonly used to provide reaction, and more or less any material
available in sufficient quantity can be used (Figure 9.3). Specially cast concrete blocks
or pigs of cast iron may be hired and transported to site. The cost of transport to
and from the site is a significant factor. Regular-shaped blocks have the advantage
that they may be stacked securely, and are unlikely to topple unexpectedly. Sheet steel
piling, steel rail, bricks or tanks full of sand or water have to be adopted as kentledge
from time to time. The important criterion is that the mass of material is stable at all
times during and after the test.
Clearance is required above the pile cap for the jack, load cell and reaction beam,
and it is necessary to support the whole weight of the kentledge on a timber or other
appropriate cribbage. The area of the cribbage platform should be sufficient to avoid
bearing failure, a severe problem on soft ground. The kentledge is usually supported on
a deck of steel beams or a timber mattress. The cribbage should also be spaced at least
3 or 4 diameters from the pile to avoid an unacceptably large interaction between the
soil and the pile as load is transferred to the pile. This spacing determines the minimum
length of the reaction beam required.
The reaction beam is subjected to high bending and buckling stresses and needs to be
designed to carry the maximum load safely. The maximum safe load should be clearly
marked on the beam so that it is not inadvertently exceeded during a test. A thick