What is the effect of increased normal stress on the shear strength of soil?

In soil mechanics, shear strength is defined as the resistance of soil to shear stresses. One of the critical factors that influence shear strength is the effective normal stress acting on the soil. According to the principles of effective stress, as normal stress on a soil increases, the effective stress—which is the difference between the total stress and pore water pressure—also increases.

When normal stress increases, if the soil is not saturated to the point that pore water pressure is significantly influencing the conditions, it usually leads to an increase in the inter-particle force interactions within the soil structure. This enhanced force interaction improves the soil's ability to resist shear stresses, thus increasing its shear strength.

Another important factor is that as normal stress increases, the soil grains are more firmly packed together, which leads to higher frictional resistance. This phenomenon is described by the Mohr-Coulomb failure criterion, which states that shear strength depends on the normal stress and the cohesion of the soil. Therefore, in most practical scenarios, increased normal stress correlates with increased shear strength, especially in granular soils.

In summary, the relationship between normal stress and shear strength is generally such that as normal stress increases, the shear strength of soil increases as well due to enhanced inter-particle forces and