Stone column ground strengthening involves inserting vertical columns of compacted granular gravel backfill into the soil. The process does not use excavation. Instead, it relies on a displacement injection technique, which densifies the surrounding soil.
The backfill is then compacted using one of the densification techniques (often vibro-compaction), further increasing the density of the surrounding soil.
Because of their higher shear strength, the columns stiffen and reinforce the ground. The technique requires a clear site because of the gravel compaction process. It offers advantages in sandy and slightly silty sites.
Rammed aggregate piers
Rammed aggregate piers are similar to the stone column technique, except the gravel is added to the soil in increments, which forms compacted bulbs.
The technique uses a predrilled hole or a mandrel to reach the bottom of the target depth. Gravel is fed in stages down the hole or the hollow stem of the mandrel and released into the ground at depth. At each stage, the gravel is compacted, forcing it to push outwards into the surrounding soil. The process is repeated until the column is complete.
Driven timber poles
Driven timber poles, as the name suggests, involves driving treated timber poles usually several metres long into the ground to the target depth to reinforce the surrounding soil.
The full length of the pole is driven vertically into the ground using a vibrating weight or similar. The top end of the pole is driven below the surface, and the hole is filled with compacted soil. It is essential that the poles are placed in a close grid arrangement across the entire site.
The technique offers advantages for silty sites.
This technique was trialled in Christchurch by EQC as a method to remediate TC3 land so that a TC2 type foundation could be used. However, the trials revealed that this technique was the least preferred of the ground improvement options as it had the potential to increase the risk of liquefaction ejecta travelling up the side of the piles from depth.