the piles fail a loading test or fail to achieve the specified ‘set’criterion when at the
design length.
Quite a different procedure is adopted when the contractor is responsible for design. The
engineer will provide the piling contractor with whatever ground information is available,
and will state either the required working load on a single pile, or he may simply provide a
building layout plan showing the column loads or the load per metre run from the load-bearing
walls. In the latter case, the contractor will be responsible for deciding the required piling
layout. In all cases the contractor will determine the type and required diameter and length
of the piles, but will be careful to quote a price for lengthening the piles should the actual
ground conditions differ from the information supplied at the time of tendering. The
contractor’s tender is usually accompanied by financial provisions to guarantee the
performance and safety of the design.
The engineer may not always specify allowable working stresses on the pile shaft, min-
imum cube crushing strengths or minimum cement contents in concrete mixes. It may be
considered the proper duty of the piling contractor to decide on these values since they
may be governed by the particular piling process employed. The need to specify allowable
working stresses and the crushing strength and minimum cement content of concrete piles
is dealt with in Chapters 2 and 10. In all cases the engineer must specify the maximum
permissible settlement at the working load and at some simple multiple, say 1.5 times or
twice the working load, either on test piles or on working piles or both. This is essential
as it is the only means that the engineer possesses of checking that the contractor’s design
assumptions and the piles as installed will fulfil their function in supporting the structure.
Only the engineer can state the requirement for settlement at the working load from
knowledge of the tolerance of the structure to total and differential settlement. It fre-
quently happens that the maximum settlements specified are so unrealistically small that
they will be exceeded by the inevitable elastic compression of the pile shaft, irrespective
of any elastic compression or yielding of the soil or rock supporting the pile. However, the
specified permissible settlement should not be so large that the safety factor is compro-
mised (see Section 4.1.4) and it should be remembered that the settlement of a pile group
is related to the settlement of a single pile within the group (Chapter 5). It is unrealistic
to specify the maximum movement of a pile under lateral loading, since this can be
determined only by field trials.
The above procedure for contractor-designed piling has been advantageous in that it has
promoted the development of highly efficient piling systems. However, they have the draw-
back that they place the engineer in a difficult position when checking the contractor’s
designs and in deciding whether or not to approve a request for pile lengths that are greater
than those on which the tendered price was based. If the engineer declines to authorize extra
pile lengths the contractor will withdraw a guarantee of performance. Nevertheless, the
engineer has a duty to the employer or client to check the specialist contractor’s designs as
far as practically possible (guidance regarding this is given in Chapter 4) to enquire as to
whether or not the contractor has made proper provision for difficult ground conditions such
as obstructions or groundwater flow, to check on site that the piles are being installed in a
sound manner and that they comply with the requirements for test loading. In the interests
of the client the engineer should not allow extra pile lengths if it is considered that the
contractor is being over-cautious in his assessment of the conditions. However, a decision
should not be made without test-pile observations or previous knowledge of the performance
of piles in similar soil conditions.8 General principles and practices