Soil liquefaction is a natural occurrence where soil loses its strength due to external forces, especially during seismic events. When soil is partially saturated or fully saturated, it loses its ability to support the weight placed upon it, behaving like a liquid. This phenomenon, though fascinating, can also be perilous, leading to both loss of life and significant financial repercussions.
The study of soil liquefaction finds its origins in the aftermath of the Calaveras Dam failure in California, 1918. Allen Hazen, the pioneering engineer, likened this phenomenon to quicksand. The seismic events of 1964 in Niigata, Japan, and Alaska further fueled interest in this field among geotechnical engineers.
To comprehend soil liquefaction, it’s crucial to grasp the composition of soil. It comprises particles and voids. In normal conditions, these particles are closely packed, distributing the load effectively.
In saturated soil, the voids are filled with water, leaving no room for air. When external stress is applied, the water in the voids builds up pressure, known as pore-water pressure.
Under usual circumstances, this pore-water pressure shares the load with the soil particles. Effective stress measures this shared stress. However, in certain conditions, excessive pore-water pressure develops, resulting in equal distribution of force between water and soil particles. This reduces effective stress to zero.
During an earthquake, the ground shakes, disrupting the compacted soil structure. This sudden load application causes a surge in pore-water pressure, detaching soil particles. Consequently, the soil loses its strength, behaving like a liquid.
A similar occurrence was observed in the case of Calaveras Dam. The movement of embankment material accumulated pore pressure, propagating through the structure and ultimately leading to its failure. Rapid loading, blasting, vibro-floatation, and dynamic compaction can also induce soil liquefaction.
Sandy soil stands more susceptible to liquefaction than clay. Its lack of inter-particle cohesion and larger particle size make it prone to liquefaction. Loosely compacted soils, characterized by numerous voids and limited particle contact, also exhibit higher liquefaction potential.
In summary, liquefaction arises from a combination of factors, including degree of compaction, saturation level, and particle size.
The possibility of soil liquefaction hinges on several factors:
Estimations revolve around the factor of safety against liquefaction (FSL), determined by the cyclic resistance ratio (CRR) and cyclic stress ratio (CSR). If FSL is less than one, the soil is susceptible to liquefaction.
Soil liquefaction poses significant risks to buildings and infrastructure:
While it is advisable to avoid constructing on liquefiable soil, when unavoidable, various methods can be employed to reduce liquefaction potential:
Soil liquefaction has left its mark in various parts of the world, often resulting in devastating consequences. Some notable earthquakes triggering liquefaction include:
These instances serve as stark reminders of the potential dangers posed by soil liquefaction.