During periods of what moisture can expansive soil lift a building?

Expansive soils, often referred to as "shrink-swell" soils, consist primarily of clay minerals that expand when they absorb water and contract when they dry out. This characteristic behavior can lead to significant movement in the soil, which can lift or shift structures built on top of it.

When there is high moisture content, expansive soils absorb water, leading to an expansion of the soil volume. This swelling pressure generated can be substantial enough to lift foundations and cause structural damage to buildings. High moisture conditions, therefore, significantly increase the movement potential of expansive soils.

Given this understanding, it’s clear that the correct response highlights a critical factor affecting the stability of structures built on expansive soils. Building on or near these soils requires careful consideration of moisture levels and often specific engineering solutions to mitigate the risks associated with soil expansion.