Earthquakes cause liquefaction of soil, transforming granular material from a solid to a liquefied state as a consequence of increased pore-water pressure and reduced effective stress. The generation of excess pore pressure under undrained loading conditions is a hallmark of all liquefaction phenomena. This occurrence was brought to the attention of engineers more so after the Niigata (1964) and Alaska (1964) earthquakes. Liquefaction causes building settlement or tipping, sand boils, ground cracks, landslides, dam instability, highway embankment failures, or other hazards.
Liquefaction changes the amplitude and frequency content of ground motions. Ground surface displacement increases when ground acceleration decreases, causing damage that is of great concern to public safety. Site-specific evaluation of liquefaction susceptibility of sandy and silty soils is the first step in liquefaction hazard assessment.
Liquefaction is divided into two groups—flow liquefaction and cyclic mobility. The former occurs only in loose soil and produces massive flow slides, sinking and tilting of heavy structure, floating of light buried structures and failure of retaining structures. Cyclic mobility occurs in loose and dense soils, causing slumping of slopes, settlements of buildings, lateral spreading and retaining wall failure. Level-ground liquefaction occurs when cyclic loading is sufficient to produce high excess pore pressure. The existence of sand boils is often taken as evidence of level-ground liquefaction.
The determination of behaviour of soil due to an earthquake is an imperative task in disaster mitigation. Liquefaction of soil depends on the following parameters:
- Intensity of earthquake and its duration
- Location of ground water table
- Soil type
- Soil relative density
- Particle size gradation
- Particle shape
- Depositional environment of soil
- Soil drainage conditions
- Confining pressures
- Aging and cementation of soil deposits
- Historical environment of soil deposit ; and,
- Building/additional loads on the soil deposit.
Poorly-graded soils are generally more susceptible to liquefaction than well-graded ones; similarly, soils with rounded particles are more susceptible to liquefaction. A site that is close to the epicenter of fault rupture of a major earthquake or a site that has a ground water table close to ground surface is prone to liquefaction.
Liquefaction potential is evaluated by comparing equivalent measure of earthquake loading and liquefaction resistance. Earthquake loading characterisation is generally done by using cyclic shear stress. By normalising the cyclic shear stress amplitude by initial effective overburden stress, a cyclic stress ratio (CSR) is defined. CSR represents the level of cyclic loading induced at different depths in a soil profile, which corresponds to a specific earthquake. Resistance, mostly characterised on field observation and based on the potential for liquefaction, is classified by comparing CSR with the liquefaction resistance, and cyclic resistance ratio (CRR). In cyclic strain approach, liquefaction is expected at locations where the cyclic strain amplitude induced for a particular number of cycles by an earthquake is greater than the cyclic strain amplitude required to initiate liquefaction in the same number of cycles. There are five earthquake zones in India and the above techniques can be used in zone 2, 3, 4 and 5.
The liquefaction hazard can be minimised by using the following construction methods for specific locations. However, most of these mitigation methods are yet to be extensively adopted in India.
Vibroflotation: This 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.
Dynamic Compaction: Densification by dynamic compaction is performed by dropping a heavy weight of steel or concrete in a grid pattern from heights of 30 to 100 ft. It provides an economical way of improving soil for mitigation of liquefaction hazards.
Stone Columns: These are columns of gravel constructed into the ground. Stone columns can be constructed by the vibroflotation method.
Compaction Piles: Installing compaction piles is an effective way of improving soil. Compaction piles are usually made of prestressed concrete or timber. Installation of compaction piles both densifies and reinforces the soil.
Compaction Grouting: This is a technique whereby a slow-flowing water/sand/cement mix is injected under pressure into granular soil. The grout forms a bulb that displaces and hence densifies the surrounding soil. Compaction grouting is a good option if the foundation of an existing building requires improvement, since it is possible to inject the grout from the side or at an inclined angle to reach beneath the building.
Drainage techniques: Liquefaction hazards can be reduced by increasing the drainage ability of the soil. If the pore water 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.