Densification techniques

Gravel rafts

A gravel raft forms a flat pad structure built at ground level. When placed above liquefiable soil, this prevents liquefiable material from ejecting to the ground surface and helps to control the amount of differential settlement.

The technique involves excavating a shallow layer of soil on the site and incrementally refilling it with layers of compacted high-strength materials, such as gravel, crushed concrete or hardfill.

Depending on the size and depth of the excavation and the proximity of other structures, the sidewalls may need to be reinforced to prevent collapse.

The technique is suitable for very soft ground and can remediate a large area but produces a relatively large amount of spoil.

Dynamic compaction

Dynamic compaction involves repeatedly lifting and dropping a heavy weight (typically 5–20 tonnes) onto the surface of the ground to increase the density of the soil directly below the impact site.

As the soil compacts, large impact craters can form on the surface. These are repeatedly filled and recompacted until the soil reaches its target strength.

Dynamic compaction has been widely used on projects in New Zealand, and it can provide an excellent degree of ground improvement . However, the high level of vibration generated during the procedure means it may not be suitable for vibration-sensitive areas or sites near existing structures.

Rapid-impact compaction

Rapid-impact compaction is a smaller-scale version of dynamic compaction. It uses a pile-driving hammer attached to a large steel end plate installed on the arm of an excavator. This end plate is driven into the ground with rapid hammering.

This increases soil density by compacting the surface layers as the plate is hammered and through vibro-compaction of lower soils.

Rapid-impact compaction can improve ground density down to several metres and is particularly suited for sandy soils in areas where there is adequate distance from neighbouring buildings.

Rapid-impact compaction.

Rapid-impact compaction works by vibrating the soil particle matrix to increase its density.


Vibro-compaction involves lowering a weighted vibrator or vibroflot into a hole at the lowest depth of soil to be compacted. The vibrator is then activated and either repeatedly raised and lowered through the target soil, raised at a set speed or raised in set increments. The process is usually repeated at several locations, depending on the size of the site to be compacted.

Water or air jets or excavation are sometimes used to help the vibrator reach its target depth. As the soil compacts, a void is created around the vibrator, and material must be added to the hole to compensate.