Which of the following is not one of the characteristics of a local shear failure?

Question

Examoogle · Accepted Answer

The correct answer is Option C: Failure is sudden. Local shear failure is a mode of soil failure characterized by progressive development of plastic equilibrium. Unlike general shear failure, which is sudden and catastrophic, local shear failure involves significant settlement and tilting of the foundation, with the failure surface not extending to the ground surface.

Step-by-Step Solution & Concepts:
Local shear failure is a mode of soil failure characterized by progressive development of plastic equilibrium. Unlike general shear failure, which is sudden and catastrophic, local shear failure involves significant settlement and tilting of the foundation, with the failure surface not extending to the ground surface.

Working Principle: This mode occurs typically in loose sands or soft clays where the soil exhibits high compressibility. As the load increases, the shear zones develop beneath the foundation but remain contained within the soil mass, resulting in a continuous, gradual settlement without a clear peak in the load-settlement curve.

Key Formulas: $q_u = \frac{2}{3} c N_c' + q N_q' + 0.5 \gamma B N_{\gamma}'$ — Modified Terzaghi bearing capacity equation for local shear failure where strength parameters $c$ and $\phi$ are reduced to $c_m = \frac{2}{3}c$ and $	an(\phi_m) = \frac{2}{3}	an(\phi)$.

Key Points: Associated with loose sand (relative density; Significant settlement occurs before the soil reaches its ultimate capacity.; The failure surface is localized and does not manifest as a heave at the ground surface.; The load-settlement curve does not exhibit a well-defined peak (unlike general shear).; It is a gradual failure process, not a sudden collapse.

Advantages: Predictable in soft soils; Warns through large settlements

Disadvantages: Large differential settlements can damage structures; Requires larger factor of safety in design

Applications: Foundations on loose sandy soils; Foundations on soft, saturated cohesive soils

Additional Info: In local shear failure, the soil undergoes high vertical compression, leading to an ill-defined ultimate bearing capacity value.. Option C is correct as 'sudden' is the hallmark of general shear failure, whereas local shear is defined by its progressive and gradual nature.

Answer: C is correct — Local shear failure is a progressive, gradual process characterized by significant settlement, whereas sudden failure is the defining trait of general shear failure.

Exam Tip: When calculating bearing capacity for loose soils, always remember to use the reduced cohesion $c' = 0.67c$ and angle of internal friction $\phi' = \arctan(0.67 	an \phi)$ to account for the local shear failure mode.

Examoogle · Answer

Option A: Failure is defined by large settlements

Examoogle · Answer

Option B: Failure surface do not reach the ground surface

Examoogle · Answer

Option D: Ultimate bearing capacity is not well defined

Feature	General Shear	Local Shear
Failure Nature	Sudden/Catastrophic	Gradual/Progressive
Soil Density	Dense Sand/Stiff Clay	Loose Sand/Soft Clay

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