Soil mixing, also known as mass stabilisation, mechanically combines the soil with a binding agent, often dry cement powder or a cement/lime mix, to create columns of higher-cohesion soil through the liquefiable soil layer. As well as stabilising the treated material, the mixed soil also reinforces the surrounding soil.
Soil mixing is most often used with soft silty soils. Because they are easier to mix, the technique is easier to use with cohesionless soil than cohesive soil. Large rocks and boulders can also be a problem on some sites.
The mixing process may be carried out in three ways – off-site, rotovated or in situ – or in any combination of the three.
Off-site soil mixing
The off-site soil mixing process involves excavating the site to the target depth, transporting the excavated soil off site, mixing it with the binding agent, then replacing the stabilised soil back into the excavated site.
The material is replaced and compacted in layers to form a stabilised crust over the full area of the site.
Rotovated soil mixing
Rotovated soil mixing also involves excavating the site to just above the target depth. The binder is then spread over the exposed layer, and a rotary mixing tool (rotovator) is used to mix the soil and binder together before the stabilised soil is compacted.
When complete, another layer of soil and binder is added, and the process repeats until the remediation is complete. Like the off-site process, rotovated mixing remediates the full area of the site.
In situ soil mixing
In situ or horizontal soil mixing is similar to other soil mixing techniques but works by drilling and mixing horizontally. The technique creates horizontal columns of higher-cohesion soil within the liquefiable layer and helps reinforce the surrounding material.
A drill is used to extend through the target layer and out to a dug trench where a rotary mixing tool is fitted. As the tool is drawn back through the hole, the binding compound is pumped along the drill shaft and mixed with the soil.
This technique offers advantages in situations where an existing building is to remain in place or where vibration or space constraints exist due to close proximity to neighbouring properties.