What is the consequence of increased moisture on soil shear strength?

Increased moisture in soil generally leads to a decrease in soil shear strength. This occurs because the presence of water can fill the voids between soil particles, reducing the contact area and the effective stress within the soil. Effective stress is a crucial factor affecting soil strength; as pore water pressure increases due to added moisture, the effective stress diminishes. Consequently, the soil becomes less cohesive and more prone to failure under shear stress.

When soil moisture content increases, particularly in saturated soils, the risk of erosion, landslides, or other forms of failure rises due to this decline in shear strength. Conversely, some soil types might temporarily exhibit increased strength with a slight increase in moisture, especially at specific moisture contents, but generally, for most soils, excess moisture leads to weakened shear strength. Thus, recognizing that the typical response of most soils to increased moisture is a reduction in shear strength is crucial for understanding soil mechanics and geotechnical engineering practices.