Liquefaction is the phenomena when there is loss of strength in saturated and cohesionless soils because of increased pore water pressures and hence reduced effective stresses due to dynamic loading. It is a phenomenon in which the strength and stiffness of a soil is reduced by earthquake shaking or other rapid loading. One of the main risks in low densified sandy soils with the presence of water and an external force such as the earthquake is the generation of liquefaction. This phenomenon was studied for the first time in 1964 after the earthquake in Niigata, Japan. The main objective of this paper is to present a review about methods of analysis and mitigation in soil liquefaction, especially in those of sandy and loose density soils. The experience of different researchers shows that the most suitable methods of analysis are those that are based on in situ tests.
Liquefaction – Soil Engineering
Liquefaction for soil engineering is a process in which the strength and stiffness of a soil is reduced by earthquake shaking or other rapid loading. Liquefaction and related phenomena have been responsible for tremendous amounts of damage in historical earthquakes around the world. Liquefaction occurs in saturated soils, that is, soils in which the space between individual particles is completely filled with water. This water exerts a pressure on the soil particles that influences how tightly the particles themselves are pressed together. Prior to an earthquake, the water pressure is relatively low. However, earthquake shaking can cause the water pressure to increase to the point where the soil particles can readily move with respect to each other.
According to the National Research Council’s Committee on Earthquake Engineering (1985), soil liquefaction is defined as the phenomena in which there is a loss of shearing resistance or the development of excessive strains as a result of transient or repeated disturbance of saturated cohesionless soils. Sladen et al (1985) stated that “Liquefaction is a phenomena wherein a mass of soil loses a large percentage of its shear resistance, when subjected to monotonic, cyclic, or shocking loading, and flows in a manner resembling a liquid until the shear stresses acting on the mass are as low as the reduced shear resistance”. After initial liquefaction if large deformations are prevented because of increased undrained shear strength then it is termed, “limited liquefaction” (Finn et al. 1994). When dense saturated sands are subjected to static loading they have the tendency to progressively soften in undrained cyclic shear achieving limiting strains which is known as cyclic mobility (Castro 1975). Cyclic mobility should not be confused with liquefaction. Both can be distinguished from the very fact that a liquefied soil displays no appreciable increase in shear resistance regardless of the magnitude of deformation (Seed 1979). Ground failures associated with the phenomena of liquefaction under cyclic loading can be classified in a broader sense as follows (Robertson et al. 1992): (1) Flow failures-It is observed when the liquefaction of loose, contractive soils (i.e. the soils where there is no increase in strength at larger shear strains) results in very large deformations., (2) Deformation failures-It is observed when there is a gain in shear resistance of the liquefied soil at larger strain, resulting in limited deformations but no loss of stability.
The process that causes liquefaction begins when four key elements:
- Soil particles are loose and cohesionless and will move closer together when shaken.
- Soil particles are sized between coarse silt to fine sand approximately 0.01–1.00 mm in diameter. (The effect has been observed in other soils under specific conditions.)
- Ground is saturated (particularly material that is below the water table).
- Sufficient shaking occurs (the level of shaking to cause liquefaction depends on several site-specific factors).
When all four conditions are present, the loose material begins to compress under the force of gravity, closing the spaces between the grains. However, the water already occupying the spaces resists the change, and pressure begins to build in the material. Eventually, the pressure rises enough that the grains become buoyant and float in the water. At this point, the strength of the soil is completely, and it begins to act like a liquid. Soil can remain liquefied for several hours after the earthquake shaking has stopped, although it will gradually solidify and regain bearing strength as the pressure within the material disperses.
Examples of liquefaction around the world:
Recent case of soil liquefaction – Indonesia – 2018
In 2018 the city of Palu on the Indonesian island of Sulawesi, buildings collapsed as the ground slid beneath them. Soil liquefaction is thought to have occurred as a result of the recent 7.5 magnitude earthquake, which also triggered a devastating tsunami. Footage has emerged from the stricken city of Palu showing people running to find solid ground as structures were swept away and destroyed by waves of undulating earth.
In the Palu neighborhood of Balaroa, about 1,700 houses were swallowed up when the earthquake caused soil to liquefy, the national rescue agency said.Satellite images of the Petobo district, south of Palu’s airport, showed another large area of urban development seemingly wiped clear of buildings.Mud with such large mass volume drowned and dragged the housing complex in Petobo.Liquefaction is most likely to happen in reclaimed land. Areas with shallow water tables and close to the sea or rivers are also susceptible to liquefaction.
Factors Affecting Soil Liquefaction
- Soil Type
- Grain size and its distribution
- Initial relative density
- Vibration characteristics
- Location of drainage and dimension of deposit
- Surcharge load
- Method of soil formation
- Period under sustained load
- Previous strain history
- Trapped Air
Consequences of Liquefaction
The effects of soil liquefaction on the built environment can be extremely damaging. Buildings whose foundations bear directly on sand which liquefies will experience a sudden loss of support, which will result in drastic and irregular settlement of the building causing structural damage, including cracking of foundations and damage to the building structure itself, or may leave the structure unserviceable afterwards, even without structural damage. Bridges and large buildings constructed on pile foundations may lose support from the adjacent soil and buckle, or come to rest at a tilt after shaking. Earth embankments such as flood levees and earth dams may lose stability or collapse if the material comprising the embankment or its foundation liquefies. The major effects of liquefaction are:
- Lateral spreads
- Lateral flows
- Loss of lateral support
- Loss of bearing support
- Flotation of bearing supports
Liquefaction Determination and Mitigation Methods for Soil Engineering
In-situ tests to determine soil susceptible to liquefaction
Standard penetration test (SPT): The most accepted field method to evaluate susceptibility to liquefaction is the Standard Penetration Test (SPT). Seed and Idriss (1967) proposed a simplified semi-empirical procedure for the determination of the susceptibility to the phenomenon of liquefaction in saturated loose sands. It is based on observation and recording of cases where this phenomenon has occurred, in addition to the evaluation of the results of standard penetration tests (SPT) and shear stresses induced in soil during a seismic event. This method consists in finding a safety factor (FOS). Soil strata that have FOS<1 are liquefiable.
Shear Wave Velocity (Vs): The procedure is based on the evaluation of liquefaction potential, based on the SPT test, and in the measurement of the cutting wave velocity (Vs). The advantage of Vs is that it is applicable in sites with uncontrolled sanitary landfills and gravel deposits where it is not possible to apply SPT or CPT (Youd and Idris, 2001). However, this method alone is not reliable (Andrus et al. 2004).
Dynamic Penetration Test (DPT): This test allows measurement and analysis of the resistance of the different strata of a soil before the possible risks of liquefaction through correlations and probabilistic methods. This method has many advantages: low economic cost, easy execution, data and codes available for verification of correlations, simple interpretation of results and relatively short application time. The advantage of DPT over other in situ tests is that it can be used in gravel soils (Cao et al. 2013)
Cone Penetration Test (CPT): Boulanger and Idriss (2016) explained a probabilistic method that evaluates the liquefaction induced by a penetration cone test (CPT) in non-cohesive soils. This probabilistic relation was developed using a maximum likelihood method. The CSR is calculated based on the resistance to penetration ( 1? ) obtained from the CPT test .
Methods to mitigate the effects of soil liquefaction have been devised by earthquake engineers and include various soil compaction techniques such as:
- Gravel drains encased with geo-synthetics: In this mitigation method, the strata of saturated sand are encased with geo-synthetics in columns of gravels. Even if the columns do not mitigate the liquefaction, it effectively reduces the displacement and does not generate permanent deformations on the soil.
- Air injection to reduce deformation under surface foundations: This method is considered reliable and of an effective cost-benefit, in addition to being ecological in comparison with others. It is based on the saturation degree of the soil. By reducing the degree of saturation of the soil (Sr), the potential to liquefy is also reduced. The Sr can be attributed to the presence of air bubbles retained in the voids of the soil, as well as the dissipation of the pore pressure. The reduction is achieved by the introduction of artificial air for a long period of time in which the air bubbles are easily dissipated.
- Silica injection using curved grouted penetration technique (PGM): This method consists in the injection of chemical products to improve liquefiable soils. This method is implemented to improve the liquefaction resistance of the soil by injecting the chemicals through the vertical holes by means of injection tubes.
- Vibro Compaction (Compaction of the soil by depth vibrators):
- Dynamic Compaction & Vibro Stone Columns
These methods result in the densification of soil and enable buildings to withstand soil liquefaction. Existing buildings can be mitigated by injecting grout into the soil to stabilize the layer of soil that is subject to liquefaction. The methods of liquefaction analysis based on the “in situ” tests have also better reliability due to the abundant correlated data and practicality. SPT is the most commonly used. Different methods of mitigation of liquefaction in soils have been developed. Densification using gravel drains is still the most indicated solution due to practicality, low cost and availability of equipment, as well as the implications that it generates in the improvement of the resistance, behavior as drain and densification of the soil. New methods required to be developed that assure to keep in operation the structures over the soils during the interventions.
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Authored by – Dr. Siddhartha Sengupta, Associate Professor, Civil & Environmental Engineering, Birla Institute of Technology, Mesra, Ranchi & Deepak Kumar, Research Scholar, Civil & Environmental Engineering, Birla Institute of Technology, Mesra, Ranchi
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