What causes settlement in soil?

Settlement in soil refers to the downward movement of the ground surface due to the compression of soil layers, and this often happens when the volume of voids in the soil decreases. The correct answer highlights the process where air or water trapped in the soil experiences a reduction in volume. This "squeezing" effect leads to a consolidation of the soil particles, resulting in settlement.

When pressure is applied to soil, either through additional loads or changes in effective stress, the moisture or air may be expelled from the voids between the soil particles. As these voids decrease, the soil grains become more tightly packed, which leads directly to a decrease in height or settlement of the soil layer above. It is a fundamental concept in soil mechanics, emphasizing the importance of effective stress and the behavior of saturated soils.

Understanding this mechanism is crucial for engineers and constructors who work with soil structures, as it enables them to predict how soils will behave under various loading conditions, thus ensuring the stability and longevity of the structures built upon them.