Explain the phenomenon of soil-liquefaction and its impact. (10 marks)
- Introduce with what Soil- Liquefaction is
- Explain the phenomenon
- List out various types of impact due to liquefaction
- Conclude appropriately
Soil- Liquefaction is 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.
The phenomenon of liquefaction:
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.
Impact of Soil-liquefaction
- Loss of bearing strength: Liquefaction causes a sudden movement shift that is out of sync with the rest of the structure. The ground can liquefy and lose its ability to support structures. This might cause several structural damages to the property leading to casualties.
- Lateral spreading: The ground can slide down very gentle slopes or toward stream banks riding on a buried liquefied layer.
- Sand boils – Sand-laden water can be ejected from a buried liquefied layer and erupt at the surface to form sand volcanoes; the surrounding ground often fractures and settles.
- Flow failures: Earth moves down steep slope with large displacement and much internal disruption of material.
- Ground oscillation : the surface layer, riding on a buried liquefied layer, is thrown back and forth by the shaking and can be severely deformed.
- Flotation : light structures that are buried in the ground (like pipelines, sewers and nearly empty fuel tanks) can float to the surface when they are surrounded by liquefied soil.
- Settlement : When liquefied ground re-consolidates following an earthquake, the ground surface may settle or subside as shaking decreases and the underlying liquefied soil becomes more dense. .
Though Soil Liquefaction is a natural phenomenon it can’t be fully avoided, but it can be reduced by constructing Liquefaction-proof structural system, avoiding construction on saturated soils and improving soil strength and quality using methods such as Vibro compaction, dynamic compaction, and use of vibro stone columns etc.