Variations in pile lengths can be more easily accommodated using segmental liners. The piles can be cast accurately to the required length and the driving or liner tube can be driven in short lengths. In some cases the tube is left in position permanently and in other cases the tube is withdrawn and used to tamp the concrete by lifting and dropping the liner tube. In other situations the tube is with-drawn and vibrated as the concrete is poured and additional compaction achieved by impact to the surface of the wet concrete. Driven piles can therefore have a smooth or irregular side surface depending on the method of driving and this results in differing friction and mechanical keying to the surrounding soil which varies depending on the pile type and sub-soil conditions. Again large rigs are required for driving cast in situ piles and hardstanding requirements can prove expensive.
In piling systems where the liner tubes are withdrawn there is a danger that the tube can lift the upper portion of in situ concrete leaving a void a short distance below the surface or squeezing during withdrawal can cause necking. This can happen where the mix is not carefully controlled or where the liner tube is not withdrawn at a steady slow rate.
Driven cast in situ piles, however, can prove to be economic for sands, gravels, soft silts and clays, particularly when large numbers of piles are required. For small numbers of piles the on-site cost can prove expensive. Driven precast piles and driven cast in situ piles can prove particularly suitable where groundwater or soft inclusions occur in the sub-strata.