Which site condition is a significant factor in increasing the potential for liquefaction?

Liquefaction is a phenomenon that occurs when saturated, loose, granular soils, usually sands, experience a significant reduction in strength and stiffness due to the application of stress, often from seismic activity. This process turns the soil into a fluid-like state, which can lead to significant ground movement and associated structural damage.

Loose sandy layers are particularly prone to liquefaction because they have a high permeability and low cohesion, making them susceptible to changes in pore water pressure during seismic events. When seismic waves pass through loose sand, they can cause a rapid increase in pore water pressure, effectively reducing the contact forces between soil particles. If the pore water pressure becomes high enough to equal the confining stress, the soil can lose its ability to support weight, resulting in liquefaction.

In contrast, hardened bedrock, dense gravel layers, and dry clay layers do not exhibit the same vulnerabilities to liquefaction. Hardened bedrock typically provides a stable foundation that resists ground movement. Dense gravel layers usually have better drainage and a higher internal friction angle, making them less prone to liquefaction. Dry clay layers tend to be cohesive and do not retain significant pore water pressure, so they are not susceptible to the same liquefaction processes as saturated granular soils