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The change in pore water pressure varies in different soils. In loose sands and sandy silts, the pore water pressure increases during pile driving owing to soil densification. The increase in pore water pressure reduces the soil strength. However, after piling operation ceases for a certain period of time, upon dissipation of pore water pressure the soils would result in increased strength by soil “set-up”.
For dense sands, the piling operation cause dilation and increases the pore volume. As such, water may not be fast enough to infiltrate to equalize the pore pressure and this results in a reduction of pore water pressure. Therefore, the apparent increased soil strength is temporary only and it would be reverted back when soil relaxation takes place soon.