Replacement techniques
Replacement involves removing liquefiable material from the site and replacing it with a non-liquefiable material.
Well compacted and well graded gravel or soil mixed with cement or other additives is commonly used as the replacement material.
Replacement techniques can be used to treat both sands and silts. It is particularly suited for areas with a shallow liquefiable layer, but replacement can also be used to form a uniform stiff platform for new structures where the desired ground performance can be achieved with only partial replacement of the liquefiable soils.
On sites with suitable ground conditions, it may also be possible to remove liquefiable material, compact it to a higher density and replace it in its original location.
The depth of treatment is typically limited by the feasibility of excavating materials below the water table and, where the site is near existing structures, the cost of temporary supports to protect neighbouring structures from damage.
Replacement proved effective at mitigating differential subsidence for lightweight structures in the ground improvement trials undertaken by EQC in 2013 and has been a common method of ground improvement in the Christchurch rebuild.
Gravel mats
A gravel mat forms a flat pad structure built at ground level. When placed above liquefiable soil, this prevents liquefied 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 replacing it with layers of compacted high-strength materials such as gravel, crushed concrete or other hardfill.
Depending on the size and depth of the excavation and the proximity of other structures, the side walls 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.