Soil cohesion and soil shear strength are two important parameters that civil engineers need to consider when designing foundations, slopes, retaining walls, and other structures that interact with the soil. In this article, you will learn what these terms mean, how they are measured, and how they differ from each other.
Soil cohesion is the property of soil that describes how well the soil particles stick together. It is influenced by factors such as water content, clay content, organic matter, and cementation. Soil cohesion is usually expressed in units of force per unit area, such as kilopascals (kPa) or pounds per square inch (psi). Soil cohesion helps to resist deformation and failure of soil under external forces.
Soil shear strength is the property of soil that describes how much force the soil can withstand before it fails or slides along a plane of weakness. It is influenced by factors such as soil cohesion, internal friction, pore water pressure, and stress history. Soil shear strength is also expressed in units of force per unit area, such as kPa or psi. Soil shear strength determines the stability and bearing capacity of soil under different loading conditions.
There are various methods for measuring soil cohesion and soil shear strength, depending on the type and condition of the soil. The direct shear test involves placing a soil sample in a box with a horizontal plane of separation and applying a normal load and a horizontal load to the sample, with the horizontal load increased until the soil fails along the plane of separation. The triaxial test involves placing a cylindrical soil sample in a cell filled with water and applying a confining pressure and an axial load to the sample, with the axial load increased until the soil fails along a circular plane. The unconfined compression test involves placing a cylindrical soil sample in a loading device and applying an axial load to the sample, with the axial load increased until the soil fails along a vertical plane. From these tests, the soil cohesion and shear strength can be calculated or estimated from various parameters such as normal load, horizontal load, confining pressure, axial load, cross-sectional area, and internal friction angle.
Apart from all the above tests there is one field test known as the field vane shear test. A laboratory vane shear test apparatus is also there. But out of all laboratory tests, triaxial tests are more accurate.
Soil cohesion and soil shear strength are related but not identical properties of soil. Soil cohesion is a component of soil shear strength, but not the only one. Soil shear strength also depends on the internal friction angle, which is a measure of how much the soil particles resist sliding past each other. The internal friction angle is affected by the shape, size, and arrangement of the soil particles, as well as the water content and stress history of the soil. Therefore, two soils with the same cohesion may have different shear strengths depending on their internal friction angles.
Another difference between soil cohesion and soil shear strength is that soil cohesion is usually constant for a given soil type and condition, while soil shear strength varies with the direction and magnitude of the applied stress. For example, a soil with high cohesion may have high shear strength under vertical loading, but low shear strength under horizontal loading. This is because the vertical loading increases the normal stress and the horizontal loading reduces the shear stress on the failure plane.
Soil shear strength is represented by cohesion and angle of internal friction. While cohesion is a contributor to shear strength, it is not the only factor. There can also be shear strength in non cohesive soils such as beach sands.
Soil cohesion and soil shear strength are fundamental concepts for civil engineering structures that interact with the soil. For instance, these concepts influence the bearing capacity of foundations, the stability of slopes, and the design of retaining walls. Specifically, they affect the maximum load a foundation can support, the ability of a slope to resist sliding or collapsing, and the wall height and thickness. As a civil engineer, it is important to understand and apply these concepts to ensure an optimal and safe design of your projects.