General shear failure is a critical concept in geotechnical engineering, describing a specific mode of bearing capacity failure in soils. This type of failure occurs when the soil supporting a foundation is strong and relatively incompressible, allowing a distinct and well-defined failure surface to develop.

General Shear Failure Explained

General shear failure is characterized by a sudden and complete rupture of the soil beneath the foundation. This failure is often accompanied by several distinct features:

• A clear and continuous failure surface extends from the foundation outwards to the ground surface.
• Significant bulging or heaving of the ground surface occurs adjacent to the foundation, indicating that the soil has been pushed upwards.
• The load-settlement curve for the foundation shows a sharp, well-defined peak load at which failure occurs, followed by a sudden drop in resistance.
• Comparatively small settlements are observed before the ultimate bearing capacity is reached and the sudden failure occurs.

Soil Conditions for General Shear Failure

General shear failure is typically observed in soils that possess high shear strength and are dense or stiff. These conditions allow the soil mass to act as a relatively rigid body until the shear strength is fully mobilized along a distinct failure plane.

• Dense Sand: When a foundation is placed on dense sand, the sand particles are closely packed, leading to high friction and interlocking. This gives dense sand high shear strength and stiffness. As a load is applied, the dense sand resists deformation significantly until the shear stresses exceed its capacity, causing a sudden and catastrophic general shear failure with a visible failure surface extending to the ground.
• Stiff Clay: Similarly, stiff clay possesses high undrained shear strength due to its cohesive properties and low water content. This makes stiff clay relatively incompressible and resistant to deformation under load. Like dense sand, stiff clay can support substantial loads until a critical point, where a sudden general shear failure occurs, characterized by a distinct failure surface and ground heaving.

Conversely, soils like loose sand or soft clay tend to exhibit different failure modes, such as local shear failure or punching shear failure. In these cases, the soil is more compressible, and a clear failure surface extending to the ground surface typically does not form. Instead, large settlements occur gradually without a sudden collapse or prominent ground heaving.

Comparing Soil Bearing Capacity Failures

To further differentiate general shear failure from other types, consider the following comparison:

Therefore, based on the characteristics of soil strength and behavior under load, general shear failure occurs in both dense sand and stiff clay.