Understanding how a foundation behaves under load is crucial in geotechnical engineering. When a footing is placed on soil and loaded, the soil can fail in different ways depending on its properties and the foundation's characteristics. One such failure mode is known as general shear failure. Let's delve into what general shear failure is and where the failure surface typically develops.

What is General Shear Failure?

General shear failure is a mode of bearing capacity failure that occurs when the soil beneath the footing is relatively strong and incompressible, typically dense sand or stiff clay. In this type of failure, a well-defined and continuous shear failure surface develops in the soil mass supporting the footing.

Key characteristics of general shear failure include:

• A sudden, catastrophic failure.
• Well-defined plastic equilibrium zones develop in the soil.
• Significant bulging or heaving of the ground surface adjacent to the footing.
• The footing tilts and settles significantly.
• Failure is accompanied by a clear peak resistance followed by a sudden drop or constant resistance.

Development of the Failure Surface in General Shear Failure

In general shear failure, the continuous failure surface extends from the edge of the footing downwards and outwards through the soil mass. It curves outwards and upwards, eventually reaching the ground surface at a significant distance from the footing. This creates a distinct wedge of soil directly beneath the footing that moves downwards with the footing, pushing the adjacent soil sideways and upwards.

The failure surface in general shear failure consists of different zones:

1. A triangular elastic wedge zone immediately beneath the footing.
2. Radial shear zones extending from the edges of the footing.
3. Linear passive shear zones extending from the radial shear zones upwards to the ground surface.

The continuous failure surface connects these zones, starting from the edge of the footing and extending to the ground surface. This forms a continuous path of soil rupture.

Analyzing the Options for General Shear Failure

Let's look at the given options concerning the location of the continuous failure surface in general shear failure:

• Edge of the footing and ground surface: This describes the characteristic path of the failure surface in general shear failure, starting at the edge of the loaded area and extending up to the free ground surface.
• Below the ground surface: While part of the failure surface is below ground, stating it's entirely 'below the ground surface' is inaccurate as it clearly emerges to the surface.
• Foundation and the ground surface: This is too general. The failure originates specifically from the edge of the load application area (the footing), not just anywhere under or around the entire foundation structure.
• Ground surface and footing: This phrasing is vague and doesn't accurately describe the origin (edge of footing) and path (extending from edge to surface) of the continuous failure surface.

Based on the mechanics of general shear failure, the continuous failure surface develops connecting the edge of the footing to the ground surface.

Revision Table: Foundation Shear Failure Modes

Additional Information: Bearing Capacity in Shear Failure

The ultimate bearing capacity of a soil is the maximum pressure a foundation can withstand before shear failure occurs. Different theories, such as Terzaghi's bearing capacity theory, are used to estimate this value for various soil conditions and failure modes, including general shear failure.

Terzaghi's general bearing capacity equation (for a strip footing at ground level) is often given by:

\(q_u = c'N_c + qN_q + 0.5 \gamma B N_\gamma\)

Where:

• \(q_u\) is the ultimate bearing capacity.
• \(c'\) is the effective cohesion of the soil.
• \(q\) is the surcharge pressure (\(\gamma D_f\)).
• \(\gamma\) is the effective unit weight of the soil.
• \(B\) is the width of the footing.
• \(D_f\) is the depth of the footing.
• \(N_c\), \(N_q\), \(N_\gamma\) are bearing capacity factors which depend on the soil's effective angle of internal friction (\(\phi'\)).

For general shear failure, these bearing capacity factors are typically higher than those used for local shear failure, reflecting the higher strength and stiffness of the soil involved in general shear failure.

Understanding the concept of general shear failure, its characteristics, and the path of its failure surface is essential for designing safe and stable foundations.