What Is Soil Liquefaction?
Soil liquefaction is a phenomenon of soil losing its’ strength due to externally applied stress. Partially saturated or saturated soil under the seismic event loses firmness and strength.
As a result, it doesn’t support the load over it. The soil behaves like a liquid, and hence the name soil liquefaction. Soil liquefaction is a precarious process that causes loss of life and money.
In 1918, Allen Hazen first defined the soil liquefaction process about the failure of Calaveras Dam, California. He stated that the phenomenon is equivalent to the quick sand condition. The 1964 earthquake in Niigata, Japan & Alaska sparked an interest in the geotechnical engineers’ community.
Why Does Soil Liquify?
The soil mass consists of soil particles and voids. Under normal conditions, the soil particles are packed together. The compact soil structure distributes the load among particles.
In saturated soil, the water fills the void and leaves no space for air. Under the externally applied stress, the void water develops the pressure in it. It is known as pore-water pressure.
The pore-water pressure shares the externally applied load with the soil particles. Effective stress is a measure of the stress shared by soil particles.
In some conditions, the pore water pressure increases so much that the external force distributes equally between the pore water and the soil particles. At this instance, the effective stress reduces to zero.
The earthquake shakes the ground that disturbs the compact soil structure. The sudden load application (or shaking) builds up the excessive pore-water pressure that detaches the soil particles from each other. The soil loses its’ strength at this instance, and the soil firmness reduces to zero. It behaves like a liquid.
The same phenomenon had occurred in Calaveras Dam, which Allen Hazen had studied. The dam embankment material movement accumulated the pore pressure at a point. It transferred from one point to another. Hence, triggering a chain reaction that caused the failure of the Dam.
Rapid loading, blasting, vibro-floatation, and dynamic compaction can also trigger soil liquefaction.
Which Type Of Soil Is Prone To Liquefaction?
Sandy soil is more susceptible to liquefaction than clay. The sandy soil has less or no inter-particle cohesion. Also, sand has a bigger particle size that offers a lesser degree of compaction when compared to fine-grained soil. Thus, they liquefy easily.
Loosely compacted soils are also easily liquifiable. The voids are more in number, and the soil particles have lesser contact between them. Larger particle sizes and loosely packed structures both yield higher permeability.
To conclude the reasons for liquefaction, it is evident that it occurs as a combination of multiple factors. The factors are the degree of compaction, saturation level, and particle size.
Types of soil liquefaction
1. Flow Liquefaction
Flow liquefaction is a phenomenon of static equilibrium disturbance caused by the external load. The initial trigger is relatively small but, it converts to a stronger event that overtakes the inter-particle bonding.
After the start of the phenomenon, the soil has no shear strength and behaves like a thick liquid. The flow liquefaction is identifiable by large and rapid movements.
The ground movement leads to the failure of bearing capacity. The structure under the influence of flow liquefaction may tilt or overturn. Historically, it has been observed, that a large flow of soil liquefaction takes place with rapid movement.
2. Cyclic Liquefaction
The cyclic loading causes soil liquefaction. This phenomenon is triggered in soils having lesser shear stress than the soil’s shear strength.
The deformations in cyclic liquefaction are in the lateral direction. The lateral deformations are commonly observed in roads, rivers, and lakes.
The extent of cyclic liquefaction depends upon the period of load application. The pore-water pressure developed in soil subsides slowly. Thus, the deformations are large.
Estimation For Soil Liquefaction Possibility
The estimation takes the following factors into account.
- Soil type.
- Soil particle size.
- Water saturation level.
- Degree of soil compaction.
- Seismic activity in the region.
- Historical data of the region for liquefaction.
The estimation of soil liquefaction is based on the factor of safety against liquefaction. The factor takes into account the earthquake loading and the resistance of soil against liquefaction.
FSL = CRR/CSR
Where CRR means cyclic resistance ratio and CSR is cyclic stress ratio.
If the FSL is smaller than one, the soil is susceptible to liquefaction.
Effects Of Soil Liquefaction
The soil liquefaction has disastrous effects on the buildings and facilities. The structure built on liquefiable soil may face foundation settlement, cracks, tilt, and overturning. Some of the major effects of soil liquefaction are as follows.
1. Building Failure
The building foundation is designed as per the bearing capacity of the soil. In soil liquefaction, the bearing capacity of soil reduces to nil.
As a result, the building settles into the ground. It may also tilt or overturn. The differential foundation settlement can also cause cracks in the structure.
2. Foundation Crack
The uneven settlement of a raft foundation may damage it. The cracks in the foundation are dangerous for the structure’s safety. A cracked foundation fails to distribute the load evenly on the ground.
3. Slope Erosion
The liquefaction destroys the soil slopes. The soil flows down and settles at the flat ground. The sliding soil causes a large fissure formation on the slopes. The slope failure is a potential threat to the structures in the vicinity and utilities.
4. Retaining Wall Failure
A retaining wall fails by sliding, tilting, or overturning. The liquified soil exerts an additional force on the retaining wall, and hence it may cause the failure of the wall.
5. Damage To Pile Foundation
The pile foundation acts as a column that transfers the structural load to the deeper strata. The liquified soil exerts lateral stress to the pile that causes buckling. The pile may distort, and its’ load- carrying capacity decreases.
6. Damage To Utilities
The utilities like water lines, sewers, gas lines, etc., are disturbed due to liquefaction. The sewer manholes vertically uplift in the liquefaction phenomenon.
It is suggested to avoid the construction on liquifiable soil. If unavoidable, soil condition improvement methods, reduce the soil liquefaction potential.
The liquefaction mitigation methods make the ground safer to carry the structural load. The cost of the mitigation method is 5-10% of the building cost.
The deep vibrators compact the liquefaction susceptible soil. The loosely packed strata are compacted up to the required depth.
2. Dynamic Compaction
The dynamic compaction method involves the dropping of weight from a height. A crane drops the weight at every point of the ground, thus compacting the entire area.
3. Drainage Columns
The drainage columns are the boreholes in the ground at suitable spacing. The permeable material (gravel) fills the boreholes that help dissipate the pore water pressure quickly.
Soil Liquefaction Cases Around The World
Soil liquefaction has been observed in several countries around the world. The damages due to the soil liquefaction have been devastating. Some of the earthquakes that triggered the liquefaction are as follows-
- Shillong (India) Earthquake, 1897. An earthquake of 8.1 (on the Richter scale) liquified the Brahmaputra plain. Massive floods destroyed the plains and plateau.
- The Great Alaska Earthquake (magnitude of 9.2 on Richter Scale), 1964.
- Christ Church (New-Zealand) Earthquake, 2011.
- Palu (Indonesia) Earthquake (magnitude 7.1 on Richter Scale), 2018. More than 800 people died in buildings collapse due to liquefaction.