520 Ground investigations, contracts and pile testing
to the comprehensive set of tables which detail the information and frequencies required
under this Standard.
If the boreholes are free of water, the conditions at the base of small-diameter piles in dry
boreholes can be checked by shining a light down to the bottom before placing the concrete.
In the case of large-diameter piles the base of all piles should be inspected from a safety cage
of the type shown in Figure 11.6 or by lowering the inspector in an approved chair or harness
(see BSEN 813). The safety precautions should follow the procedure described in BS 8008
(safety precautions and procedures for the construction and descent of machine-bored shafts
for piling and other purposes). The procedures and problems in placing concrete in pile
boreholes are described in Sections 3.4.5 and 3.4.6. An essential factor in controlling these
operations is the maintenance of records of the quantity of concrete placed in each pile, and
preferably in addition, the level of the concrete in the shaft as each batch is placed. Tests to
control the quality of materials and mixing of concrete are, of course, required and must
be specified.
11.4 Load testing of piles
EC7-1 Clause 7.5 defines when pile tests are to be considered mandatory, in summary:
when using a type of pile or installation method for which there is no comparable
experience
where piles have not been tested under similar conditions
where theory and practice are insufficient to give confidence in the design and
when observations during installation indicate behaviour which deviates from
anticipated behaviour.General requirements are given for static and dynamic tests, trial piles, and tests on
working piles.
11.4.1 Compression tests
Two principal types of test are used for compressive loading on piles. The first of these is
the constant rate of penetration (CRP) test developed by the Building Research
Establishment,(11.18)in which the compressive force is progressively increased to cause the
pile to penetrate the soil at a constant rate until failure occurs. The second type of test is
the maintained load (ML) test in which the load is increased in stages to some multiple, say
1.5 times or twice the working load with the time/settlement curve recorded at each stage
of loading and unloading. The ML test may also be taken to failure by progressively
increasing the load in stages.
The CRP method is essentially a test to determine the ultimate load on a pile and is
therefore applied only to preliminary test piles or research-type investigations. The method
has the advantage of speed in execution and because there is no time for consolidation or
creep settlement of the ground the load/settlement curve is easy to interpret. BS8004 states
that penetration rates of 0.75 mm per minute are suitable for friction piles in clay and
1.55 mm per minute for piles end bearing in a granular soil. The CRP test is not suitable
for checking the compliance with the specification requirements for the maximum settle-
ment at given stages of loading. There is also the difficulty of pricing tenders for this form