Types of shear failure of foundation soils
When it comes to the stability of a foundation, the stiffness of the underlying soil and the depth of the foundation are important factors to consider. In some cases, the foundation soil can experience different types of shear failure depending on these factors.
One of the modes of shear failure is known as general shear failure, which is depicted in Figure 1(a). This occurs when the soil beneath the foundation undergoes shearing along a curved failure surface. This type of failure is typically observed in foundations that are shallow and situated on soft soils.
Another mode of shear failure is local shear failure, shown in Figure 1(b). This type of failure is characterized by a localized, steep-sided shear failure zone that forms directly beneath the foundation. Local shear failure typically occurs in foundations that are deeper than those that experience general shear failure and on soils that are less compressible.
Lastly, there is punching shear failure, which is shown in Figure 1(c). This type of failure occurs when the soil around the foundation is unable to resist the concentrated load imposed by the foundation. This type of failure typically occurs in foundations that are supported on soft soils or soils that have low bearing capacity.
Overall, understanding the different modes of shear failure that can occur in foundation soils is crucial in designing and constructing stable foundations that can withstand the loads imposed on them.
Fig.1: Shear failure in foundation soil
Fig: Curve in different foundation soils
General Shear Failure
General shear failure is a type of failure commonly observed in dense and stiff soil. This failure is characterized by the development of a continuous, well-defined and distinct failure surface between the edge of the footing and the ground surface. It is often seen in dense or stiff soil that has low compressibility.
In general shear failure, there is a continuous bulging of the shear mass adjacent to the footing, which is visible. The failure is usually accompanied by tilting of the footing and is sudden and catastrophic, with a pronounced peak in the curve. Moreover, the length of disturbance beyond the edge of the footing is typically large.
During general shear failure, the state of plastic equilibrium is reached initially at the footing edge and spreads gradually downwards and outwards. This type of failure is usually accompanied by low strain, typically less than 5%, in a soil that has considerable (>36o) and large N (N > 30) values, and high relative density (ID> 70%).
In conclusion, general shear failure is a significant type of failure that occurs in dense and stiff soil. Its characteristics include a well-defined failure surface, continuous bulging of the shear mass, and sudden and catastrophic failure accompanied by tilting of the footing. The state of plastic equilibrium is reached at the footing edge and spreads downwards and outwards during failure. The strain is typically low, and the soil has considerable (>36o) and large N (N > 30) values, and high relative density (ID> 70%).
Local Shear Failure
General shear failure is a type of failure that is commonly observed in relatively loose and soft soil. This type of failure is characterized by several unique features. One notable characteristic is the significant compression of soil below the footing, along with partial development of plastic equilibrium. This means that the soil is undergoing significant deformation, which can lead to the failure of the footing.
Unlike other types of failure, general shear failure is not sudden, and there is no tilting of the footing. Instead, the failure surface does not reach the ground surface, and a slight bulging of soil around the footing is observed. This indicates that the soil is undergoing a gradual failure process, rather than a sudden collapse. Furthermore, the failure surface is not well-defined, making it difficult to predict or prevent.
Another feature of general shear failure is the considerable settlement that occurs. As the soil undergoes deformation, the footing may sink or settle, which can cause significant structural damage. Additionally, there is no well-defined peak in the curve, which means that the failure process is not easily identifiable or predictable.
Finally, local shear failure is accompanied by large strain in a soil with considerably low angle of internal friction and low N-value. This type of soil also has a low relative density, which makes it more prone to deformation and failure. These factors can contribute to the overall failure process, leading to significant damage to structures built on soft and loose soil.
Punching Shear Failure of foundation soils
General shear failure is a type of failure that is commonly observed in loose and soft soils at deeper elevations. It typically occurs in soils that exhibit very high compressibility. In this type of failure, a failure pattern is not usually observed and bulging of soil around the footing is also absent. Instead, the failure is characterized by a significant amount of settlement, which can be very large.
One key characteristic of general shear failure is that it is typically characterized by continuous settlement with no increase in pressure. This is usually observed in the curve of settlement over time, where the settlement of the soil continues to increase steadily over time without any corresponding increase in pressure. This is often due to the fact that the soil in question is highly compressible, which means that it can be easily compressed without any corresponding increase in pressure.
Overall, general shear failure is an important consideration for engineers and others who are involved in designing and constructing structures that will be built on loose or soft soil. By understanding the key characteristics of this type of failure, it is possible to design structures that are better able to withstand the effects of settlement and other issues that can arise in these types of soils.
Fig. 2 presents the conditions for different failure modes in sandy soil carrying circular footing based on the contributions from Vesic (1963 & 1973)