(1) Where soil of low bearing capacity of signiﬁcant depth is underlain by strong strata.
Piling which transfers the foundation load to the strong strata is frequently a more economic solution than alternative foundations (see Fig. 14.1).
|Fig. 14.1 Stiff strata at depth.|
(2) Where the surface strata is susceptible to unacceptable settlement, and is underlain by stiff material (see Fig. 14.2).
|Fig. 14.2 Compressible strata.|
On a low-cost site the authors’ practice has installed 18 m long in situ concrete piles to support two-storey domestic housing. The increased cost of the piles was more than compensated for by the low cost of the site.
(3) Where surface foundation would impose unacceptable increase in bearing pressure, or surcharge, on
existing foundations (see Fig. 14.3).
|Fig. 14.3 Load transfer below existing foundation.|
(4) Where the foundation is subject to lateral loads which can be more economically resisted by raking piles (see Fig. 14.4).
|Fig. 14.4 Lateral resistance.|
(5) Where variations in the compressibility of the soil would lead to excessive differential settlement of surface foundations (see Fig. 14.5). The leaning tower of Pisa is a classic example of differential settlement.
|Fig. 14.5 Variable compressibility.|
(6) Where excavation to ﬁrm strata would prove ex- pensive and difﬁcult, e.g. soft waterlogged alluvial deposits. On one important contract, near the coast in North Wales, it was found that the proposed structure
was sited over a glaciated channel ﬁlled to a depth of 15 m with a soft, highly saturated silt which was impossible to dewater. Excavation and foundation construction would have been difﬁcult and expensive.
(7) Where, on sloping sites, it is necessary to transfer the additional load to a level below the possible slip circle (see Fig. 14.6).
|Fig. 14.6 Load transfer below critical slip circle.|
(8) Where anchoring of a ﬂotation foundation by tying down or tension pile is necessary (see Fig. 14.7).
|Fig. 14.7 Anchorage against ﬂotation.|
(9) Where heave and swelling of clay could exert excessive forces and movements on surface spread foundations.
(10) Whenever piling is a more economic solution. In the past few decades there have been advances in piling manufacture and construction which have considerably reduced the cost of piling. Piling is no longer a last resort but can be considered as an economic alternative in foundation design.
Piles should be used with caution, if at all, where the ground is subject to signiﬁcant lateral movement, e.g. in areas affected by mining, as such movements can shear off the piles leaving the structure unsupported.