Civil Engineering Reference
In-Depth Information
cause disproportionate effects on the behaviour of the pile. An obvious disadvantage
of pre-contract piles is that the main piling contract should perforce be awarded to
the pre-contract piling firm. Test piles that are not part of the permanent works,
constructed and tested prior to the commencement of the main piling programme,
avoid double mobilization charges. However, the purpose of a preliminary test pile is
often defeated if contract piling continues immediately after test pile installation and
before the loading test can be carried out. Pile testing requires some considerable time
and engineering input to carry out and to interpret the results; this process should not
be rushed at the expense of quality of the data and reliability of the results.
It is important that the test piles are loaded in the way envisaged by the designer; in
particular it maybe necessary to sleeve the pile in any overburden. Such a free-standing
length should then be designed as a long column. Any instrumentation built into the
pile should be designed to have a minimal effect on the pile stiffness and ultimate
load carrying capacity. This is not always easy to arrange. Inclinometer ducts or rod
extensometers may affect the axial and lateral stiffness of the pile shaft, and built-in
load cells may form physical discontinuities which appreciably alter the behaviour of
the pile-soil system.
The objectives of a preliminary pile test programme are usually to determine:
1
the ultimate bearing capacity of the piles, relating this to the design parameters;
2
the relative magnitudes of shaft and end-bearing capacities;
3
the stiffness of the pile-soil system at design load. A back analysis of this data will
enable the soil modulus to be evaluated, and hence the deformation of pile groups
may be predicted with greatly increased confidence.
These various objectives will necessitate a carefully chosen test procedure and instru-
mentation programme.
The preparation of a test pile will usually involve cutting down the pile to the
required level, exposing the reinforcement, and casting a small pile cap or bedding a
25-mm thick steel plate directly on to the pile. The surface of the pile cap or plate should
be flat and should be perpendicular to the axis of the pile, to minimize eccentricity of
loading. In particular circumstances it may be necessary to extend the pile by casting or
welding on an additional length. Obviously the pile should be cut free of any blinding
concrete. If the pile is to be loaded to its ultimate capacity, it may be necessary to
provide additional reinforcement at the head of the pile to absorb eccentricities of
loading and prevent bursting of the head of the pile. If additional reinforcement is
required it is obviously impracticable to select test piles at random.
A pile test arrangement is shown in Figure 9.2, and the components of the equipment
are discussed below.
9.2.2 Test equipment
9.2.2.1 Provision of a reaction
The ultimate load of a pile may range from a few tens of tonnes to as much 2500 tonnes,
and provision of a reaction against which to generate the load requires careful consid-
eration. The geometry of the pile-reaction arrangement should be such as to minimize
