Pile Design and Construction Practice, Fifth edition

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Critical section for shear check Critical section for flexure check
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Structural design of piles and pile groups 391
0.2 � pile width or diameter
Figure 7.12 Calculation of bending moments and shearing forces on rectangular pile cap.
The bending moments on the pile cap are assumed to act from the centre of the pile to the
face of the nearest column or column stem (Figure 7.12). When calculating bending
moments an allowance should be made for deviations in the positions of the pile heads, up
to the specified maximum tolerance (see Section 3.4.12).
In all cases the aim should be to preassemble reinforcement cages for pile caps and
ground beams in order to avoid difficulties for steel fixers working in confined conditions
in pits and trenches.
The cover to all reinforcement depends on the exposure condition, and the grade of
concrete being used in the pile cap, and reference should be made to Clause 4 of EC2 and
Clause 3.3 of BS 8110: Part 1. In particular, where concrete is cast directly against the earth,
the cover should not be less than 75 mm.
Where columns carry a compressive load combined with a unidirectional bending
moment, the line of action of the column load should be made to coincide with the centroid
of the pile group in order to obtain a uniform distribution of load on the piles.
The dimensions of a number of standardized types of cap for use in design using
computer programs are shown in Figure 7.13.
Deep pile caps are desirable for providing the stiffness necessary to distribute heavy
concentrated column loads on to a large pile cluster. However, this can sometimes cause
construction difficulties in unstable soils where the groundwater level is at a shallow depth
below the ground surface. It is desirable, on the grounds of cost, to avoid construction
expedients such as a wellpoint groundwater lowering system to enable the pile cap to be
constructed in dry conditions. Consideration should therefore be given to raising the level
of the pile cap to bring it above groundwater level or to such a level that sump pumping from
an open excavation will not cause instability by upward seepage.
The design and construction of pile caps at over-water locations is discussed in Section 9.6.3.
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