“A Phenomenon whereby a saturated or partially saturated soil substantially loses strength and stiffness in response to an applied stress, usually earthquake shaking or other sudden change in stress condition, causing it to behave like a liquid” is called Soil Liquefaction (Hazen,1918).
There are two types of soil liquefaction.
1) Flow liquefaction
2) Cyclic Mobility
How does Soil Liquefaction Work:
The soil is a mixture of soil particles that stay connected together. These particles naturally rest upon each other due to gravity and form grids based on its properties. Each particle produces its own contact force by the surrounding particle. These contact forces together hold all the individual soil particles in their place. Soil liquefaction occurs due to sudden and rapid load on the soil particle. The sudden water pressure leads to soil losing its cohesive strength. Once the soil loses its cohesion, it gets softened, weak and loses its solid properties that are converted to liquid properties.
What is the importance of Soil Liquefaction?
Earthquakes or seismic events cause number of disturbances in the ground which can harm or damage the structural stability which could turn fatal. Liquefaction causes a sudden movement shift that is out of sync with the rest of the structure. This might cause several structural damages to the property leading to casualties. Liquefaction in saturated soils generates a quicksand effect. This phenomenon occurs during liquefaction when the building or the foundation gets pulled into the diluted soil causing it to lean and eventually collapse. Construction of buildings near water bodies use retaining walls which are heavily dependent on the strength and stiffness of the soil. Once the soil gets liquefied, the retaining wall collapses which could cause landslides
Liquefaction during Seismic events
Seismic events affect ground conditions. Liquefaction of soil causes structural instability in buildings. This occurs due to various instances of structural failure. The liquefied ground cannot sustain the stresses of its load from the foundations. Foundations will sink into the sand deposit and cause the building to lean and eventually collapse. Soil liquefaction occurs only in areas which have saturated soils. Most of these areas are located near a water body such as lakes, ponds, rivers etc.
Buildings constructed in this zone must adhere strict codes and bylaws. The soil can sustain the ground forces in general conditions. But an earthquake or strong motion/vibrations in the ground, can cause water logging which increases the liquid consistency in the soil. The soil loses its rigidity and the ground cannot support the loads causing them to sink or collapse.
Effects of Liquefaction on Buildings
Buckling of Piles: Pile foundations are embedded deep into the ground because of the soil support. But if the soil is not strong, the foundations buckle which lead to collapsing of the structure.
Spreading of ground: The soil starts to move in a downward direction due to the liquefaction. Slopes starting from an angle of 3 degrees are prone to lateral spreading.
The effects of soil liquefaction on the built environment can be extremely damaging. Buildings whose foundations stand directly on the sand, which liquefies, will experience a sudden loss of support. Where a thin crust of non-liquefied soil exists between building foundation and the liquefied soil, a ‘punching shear’ type foundation failure may occur. The irregular settlement of ground may also break underground utility lines. The upward pressure applied by the movement of liquefied soil through the crust layer can crack weak foundation slabs and enter buildings through service ducts, and may allow water to damage the building contents and electrical services.
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 the earthquake induced shaking.
Soil Liquefaction during the Tōhoku earthquake in Japan that occurred on 11 March 2011
During the infamous 2011 earthquake off the Pacific coast of Tōhoku, Japanthe liquefaction occurred over hundreds of miles. Most of the structures in the affected areas were tilted and sank into the sediments, even while they remained intact. The shifts in soil destroyed water, sewer, and gas pipelines, crippling the utilities and infrastructure. With such a long-lasting earthquake, the structures continued to sink and tilt as the shaking continued for several minutes. The younger sediments, and especially the buildings constructed on the new landfill ground, were much more vulnerable (Ashford, 2011). Liquefaction caused significant damage to the Tokyo Bay coast which was located far away from the epicenter. More than 70% of Urayasu City suffered because of Liquefaction. The town was built on a land filled area in the 1960s. Soil tests later revealed that the soil in Urayasu shows less resistance to Liquefaction.
Soil Liquefaction zones in Guwahati, India
Guwahati falls under the seismic zone V as per IS 1893, considered as one of the most active seismic region in the world. This means that Guwahati is at the risk if an earthquake having 8 or a higher magnitude strikes. Guwahati is India’s biggest city that falls in zone 5. Recent developments have led to more construction and a rise in population. An earthquake measuring 8.1 scale that originated in Shillong on the 12th June 1897, started a liquefaction process in the whole Brahmaputra plain. This led to floods around the plains and plateau. In the plains, water gradually reduced and formed lakes and ponds. The earthquake led to massive destruction of property and houses. Embankments started sinking under the liquid soil. This phenomenon occurred again in 1950.
Methods to reduce damage due to soil Liquefaction:
1) By avoiding construction on saturated soils
Soil study must be conducted before construction to check whether the soil is durable for construction. Soil mapping must be made mandatory.
2) Liquefaction-proof structural system
3) Improving Soil Conditions
Methods to mitigate soil liquefaction have been designed to improve soil strength and quality. Methods such as Vibro compaction, dynamic compaction, and use of vibro stone columns are preferable