To cure liqufaction soil improvement techniques, as described below can be applied:
Densification:
To reduce liquefaction hazards improve the soil by improving the strength, density, and/or drainage characteristics of the soil. Densifiction is performed by dynamic compaction. This is performed by dropping a heavy weight of steel or concrete in a regular grid pattern. The impact of the free fall creates stress waves that help in the densification of the soil. These stress waves can penetrate up to 10m. In cohesionless soils, these waves create liquefaction that is followed by the compaction of the soil, and in cohesive soils, they create an increased amount of pore water pressure that is followed by the compaction of the soil.
The weight and the height determine the amount of compaction that would occur. The weight that is used, depends on the degree of compaction desired and is between 8 tonne to 36 tonne. The height varies from 1 m to 30 m or 30 to 100 feet. It provides an economical way of improving soil for mitigation of liquefaction hazards.
Vibroflotation:
Vibroflotation involves the use of a vibrating probe that can penetrate granular soil to depths of over 100 feet. The vibrations of the probe cause the grain structure to collapse thereby densifying the soil surrounding the probe.
Drainage techniques:
Liquefaction hazards can be reduced by increasing the drainage ability of the soil. If the porewater within the soil can drain freely, the build-up of excess pore water pressure will be reduced. Drainage techniques include installation of drains of gravel, sand or synthetic materials. Synthetic wick drains can be installed at various angles, in contrast to gravel or sand drains that are usually installed vertically. Drainage techniques are often used in combination with other types of soil improvement techniques for more effective liquefaction hazard reduction.
Compaction Grouting:
Compaction grouting is a technique whereby a slow-flowing water/sand/cement mix is injected under pressure into a granular soil. The grout forms a bulb that displaces and hence densifies, the surrounding soil
The following types of foundations should be used in liquefied zones:
- Deep pile foundations (expensive)
- Mat or Raft foundations
- If both above are too expensive or not feasible, use the soil improvement methods described above.
Stone Columns:
Stone columns are columns of gravel constructed in the ground. Stone columns can be constructed by the vibroflotation method. They can also be installed in other ways, for example, with help of a steel casing and a drop hammer as in the Franki Method. In this approach the steel casing is driven in to the soil and gravel is filled in from the top and tamped with a drop hammer as the steel casing is successively withdrawn.
Verification of Improvement:
A number of methods can be used to verify the effectiveness of soil improvement. In-situ techniques are popular because of the limitations of many laboratory techniques. Usually, in-situ test are performed to evaluate the liquefaction potential of a soil deposit before the improvement was attempted. With the knowledge of the existing ground characteristics, one can then specify a necessary level of improvement in terms of insitu test parameters. Performing in-situ tests after improvement has been completed allows one to decide if the degree of improvement was satisfactory. In some cases, the extent of the improvement is not reflected in in-situ test results until some time after the improvement has been completed
