Sand compaction piles and stone columns

Sand compaction piles are columns of compacted sand formed in loose sandy or soft clayey soil. A casing is driven or vibrated into the ground, filled with sand, and withdrawn in stages while being re-driven to compact the sand. The surrounding soil is densified, and the sand pile increases the strength and acts as a drain, reducing settlement and liquefaction potential. They are also used in soft clay.

Stone columns are vertical columns of compacted crushed stone or gravel (size 20 to 75 mm) installed in soft clay or loose silty soil, usually by vibro-replacement (a vibrator is jetted into the ground and the hole is backfilled with stone in lifts) or by a driven casing (rammed stone column). Diameters are 0.6 to 1.0 m, spaced at 1.5 to 3 m in a triangular or square grid.

Functions: they increase the bearing capacity, reduce settlement, speed up consolidation by acting as drains, and reduce the liquefaction potential. The capacity of a column depends on the lateral confinement given by the soil: $q_{ult} \approx \sigma_{r}K_p$, where $\sigma_r$ is the limiting radial stress in the soft soil (bulging failure) and $K_p$ is the passive earth pressure coefficient of the stone.