Bearing capacity failures are characterized as foundation failures that occur when soil forces surpass the soil’s shear strength. There are three forms of shear failure: general shear failure, local shear failure, and punching shear failure. In this article, we will learn in detail about these types of bearing capacity failures of soil.
What is the Bearing Capacity of Soil?
Soil bearing capacity refers to its ability to withstand loads applied to it. The soil’s density and shear strength are the most critical considerations. The depth to which a load is applied also influences its bearing capacity: the deeper the embedding, the stronger the load.
The soil’s bearing capacity is the maximum load per unit area that the soil can safely withstand without displacement. We can calculate the soil’s safe bearing capacity by dividing its bearing capacity by its factor of safety.
Ultimate Bearing Capacity (Qu):
The ultimate bearing capacity of soil is defined as the maximum gross pressure intensity at the base of the foundation, at which shear failure of the soil does not fail in shear. It is the least amount of pressure that would produce shear failure if the supporting soil was immediately beneath and adjacent to a foundation.
Types of Bearing Capacity Failures
A base failure happens when the shear strength of the soil exceeds its bearing capability. This is referred to as a failure of bearing capacity. There are three types of bearing capacity failures of soil:
- General shear failure,
- Local shear failure, and
- Punching shear failure.
The following are the types of shear failure encountered by the foundation soil, depending on the stiffness of the foundation soil and the depth of the foundation.
1. General Shear Failure
The general shear failure is characterized by rupture of the underlying soil, followed by bulging of the soil surface around the footing. The soil is heaved up on only one side of the footing, resulting in the tilting of the structure.
In general shear failure, failure occurs at a very low strain. A well-defined failure surface is observed as the entire soil mass within the failure wedge participates. The failure is accompanied by a significant bulging of sheared soil bulk.
A strip footing that rests on soft clay or loose sand. At specific load intensity equal to Qu, the settlement abruptly increases. Shear failure occurs in the soil at such load, and the failure surface extends to the ground surface.
A heave on the sidewalls is always noticed in general shear failure. The base’s edge and the ground surface form a continuous, well-defined, and distinct failure surface. There is continuous bulging of shear mass near the footing. The failure is accompanied by footing tilting.
2. Local Shear Failure
Local shear failure is a type of bearing capacity failure in which the ultimate shear strength of the soil is mobilized only locally along the potential sliding surface when the structure supported by the soil is compromised by excessive movement.
There is significant compression of the soil beneath the footing in a local shear failure, and the failure planes are not fully formed.
Bulging of soil occurs on both sides of the footing but not as much as in the case of universal shear failure. In circumstances of pure local shear failure, the footing does not tilt.
A strip footing sitting on medium-consistency clay or medium-density sand. When the load is equal to a particular value Qu(1), the foundation moves with rapid jerks. As indicated, the failure surfaces gradually spread outwards from the foundation.
However, for the failure surfaces, significant foundation movement is required to extend to the ground surface. The load at which this occurs equals Qu. Following that, growth in legislation is accompanied by a significant increase in settlement.
Local shear failures are the most common type of failure. Heaving happens only when there is a substantially vertical component. At failure, only a tiny fraction of the soil beneath the footing participates, and well-defined rupture surfaces form only at sites directly beneath the footing.
This type of failure is common in loose, soft soil. There is significant compression of the soil beneath the footing and partial establishment of plastic equilibrium. There is no sudden failure, and there is no footing tilting.
The failure surface does not reach the ground surface, and there is some soil swelling around the footing. The failure surface is poorly defined. Local shear failure is characterized by significant settlement.
3. Punching Shear Failure
Punching shear failure occurs when there is relatively strong compression of the soil beneath the footing, as well as vertical shearing along the footing’s edges.
Punching shear failure is a type of bearing capacity failure caused by strong localized stress. This sort of failure is catastrophic since there are no obvious warning indications before the collapse.
This form of failure is common in loose and soft soil, as well as at higher elevations. Punching shear failure occurs in extremely compressible soil. In this sort of failure, the footing penetrates the soil without any surface bulging.
A strip footing that rests on soft clay or loose sand. A weight of Qu(1) causes jerks in the foundation. The footing fails at a load of Qu, and the load-settlement curve becomes steep and linear at the point at which failure occurs.
This is referred to as punching shear failure. A failure surface is not visible above ground. Punching shear failure occurs only when vertical movement is observed.
There is no failure pattern observed in punching shear failure. When the failure occurs, the soil outside the area on which the load is applied does not participate, and there is no soil movement on the edges of the footing. This type of bearing capacity failure is characterized by large settlements.
It is critical to determine the underlying soil’s carrying capability during the design stage of any construction project. Whenever a load is placed on the ground, such as from a building foundation, a crane, or a retaining wall, the ground must be able to support it without experiencing severe settlement or collapse.
Before designing the structure, the knowledge of types of bearing capacity failure is crucial to avoid failure in the future. Hope this article has given deeper insights on types of bearing capacity failure. This is why ground-bearing pressure, also known as soil-bearing capacity, is crucial.