Liquefaction of sands and soils has been an extensively studied field of research in geotechnical engineering. The process of liquefaction occurs when soils that are normally solidundergo a process of mechanism whereby the pore pressure increases beyond the confining pressure and the grains of the soil lose their inter-granular strength and form a suspension, a liquid suspension. The resultant mechanical properties of the soil such as strength and stiffness result from this process.
Liquefaction of soils can be caused from a variety of external forces such as earthquakes, cyclic loading, rapid draw down of reservoir water levels and even loading of heavy vehicles.The processof liquefaction of sand and soils can result in severe damage to structures such as buildings, bridges, embankments, etc.The process involves the following steps;
1. Initial yield of soil
When a soil is stressed, it initially responds through elastic deformation where the soil grains get compressed and the entire soil element becomes dense.However, ifthe strain passes beyond this threshold, the soil element enters a plastic deformation stage where additional flow or permanent deformation takess place.
2. Strain softening
With further increase in strain, the strength of soil drops due to loss of contact between the grain particles. This is known as strain softening and it eventually leads to a stage where the soil behaves like a suspension with no strength.
3. Formation of pore water pressure
The above processes of initial yield and strain softening lead to formation ofpore water pressure. This pressure results from the increase in the amount of water in the pore spaces of the soil and it eventually overcomes the confining pressure of the soil element and causes liquefaction.
4. Loss of soil strength
The last and the most important step in liquefaction involves the total loss of strength of the soil element. This loss of strength occurs because of the soil grains being reduced to a suspension-like state and hence losing their intergranular binding forces. The results of this may be large lateral displacements in the soil elementwhich can damage structures built over them.
Thus, it can be seen that liquefaction is a complex process involving several steps. In order to prevent liquefaction and its associated hazards, proper measures should be taken such as proper site characterization and providing reinforcing measures to affected soils. Several techniques are available for these purposes such as deep soil mixing, compaction grouting, freeze-drying, etc. However, proper understanding of the basic process of liquefaction is essential for these steps to work.
