Liquefaction is the process by which a mass of saturated soil loses its shear strength and behaves as a fluid when subjected to external forces. This phenomenon can occur during earthquakes or other dynamic loading events, and it can have serious consequences for structures and infrastructure built on the affected soil.

Explanation:

Liquefaction occurs primarily in saturated soils, where the spaces between soil particles are filled with water. When external forces such as seismic waves from an earthquake are applied to the soil, the water pressure within the soil increases, causing the soil particles to lose contact with each other. As a result, the soil loses its shear strength and behaves as a fluid.

The process of liquefaction can be understood through the following steps:

1. Saturation: The soil must be saturated with water for liquefaction to occur. This means that the void spaces between soil particles are completely filled with water.

2. External Forces: External forces, such as those generated by an earthquake, are applied to the soil mass. These forces cause the soil particles to move and rearrange.

3. Increased Water Pressure: As the soil particles move, the water trapped between them is squeezed out and the water pressure within the soil increases. This increase in water pressure reduces the effective stress between soil particles, leading to a loss of shear strength.

4. Particle Displacement: The soil particles start to move and rearrange themselves due to the increased water pressure. This causes the soil mass to behave as a fluid, with the particles flowing past each other.

5. Effects: The liquefaction of soil can result in a number of effects, including settlement of the ground surface, tilting or sinking of structures, and the formation of sand boils or sand volcanoes. These effects can lead to significant damage to buildings, infrastructure, and other man-made structures.

In conclusion, liquefaction is the process by which a mass of saturated soil loses its shear strength and behaves as a fluid when subjected to external forces. It is an important consideration in geotechnical engineering, particularly in earthquake-prone areas, as it can have severe consequences for the stability of structures and infrastructure.