6.1 The occurrence of uplift and lateral loading
When vertical piles are installed beneath buoyant structures such as drydocks, basements
and pumping stations they are required to resist uplift loads. Where the hydrostatic pressure
always exceeds the downward loading, as in the case of some underground tanks and
pumping stations, the anchorages are permanently under tension and cable anchors may be
preferred to piles. However, in the case of the shipbuilding dock floor in Figure 6.1, for
example, the anchorages may be under tension only when the dock is unwatered before the
commencement of shipbuilding. As the loading on the floor from ship construction
increases to the stage at which the uplift pressure is exceeded, the anchor piles are required
to carry compressive loads. Cable anchors might not then be suitable if the dock floor was
underlain by soft or loose soil.
Vertical piles are used to restrain buildings against uplift caused by the swelling of clay
soils. Swelling can occur, for example, when mature trees are removed from a building
site. The desiccated soil in the root zone of the trees gradually absorbs water from the
surrounding clay, and the consequent swelling of the clay may amount to an uplift of 50 to
100 mm of the ground surface, causing severe damage to buildings sited over the root zone.
In sub-tropical countries where there is a wide difference in seasonal climatic conditions,
i.e. a hot dry summer and a cool wet winter, the soil zone affected by seasonal moisture
changes can extend to a depth of several metres below the ground surface. In clay soils these
changes cause the ground surface to alternately rise and fall with a differential movement of
50 mm or more. The depth to which these swelling (or alternate swelling and shrinkage)
movements can occur, usually makes the use of piled foundations taken below the zone
of soil movements more economical and technically more suitable than deep strip or pad
foundations.
Vertical piles must have a sufficient depth of penetration to resist uplift forces by the
development of shaft friction in the soil beneath the zone of soil movements (Figure 6.2).
Uplift on bored piles can be reduced by casting the concrete in the upper part of the pile
within a smooth polyvinylchloride (pvc) sleeve, or by coating a precast concrete or steel
tubular pile with soft bitumen (see Section 4.8.3). Uplift can be further reduced by supporting
the superstructure clear of the ground surface, or by providing a compressible layer beneath
pile caps and ground beams (see Figure 7.15). Piles in large groups may also be lifted due
to ground heave, as described in Section 5.7.
In countries where frost penetrates deeply below the ground surface, frost expansion of
the soil can cause uplift on piles, resulting in severe effects in ‘permafrost’regions, as
Chapter 6