Cohesionless soils are formed due to...

Question

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The correct answer is Option A: Physical disintegration. Cohesionless soils, such as sand and gravel, are primarily formed through the physical disintegration of parent rock due to environmental weathering. This process involves the mechanical breakdown of rock into smaller fragments without altering the fundamental chemical composition of the constituent minerals.

Step-by-Step Solution & Concepts:
Cohesionless soils, such as sand and gravel, are primarily formed through the physical disintegration of parent rock due to environmental weathering. This process involves the mechanical breakdown of rock into smaller fragments without altering the fundamental chemical composition of the constituent minerals.

Working Principle: The mechanism involves physical weathering agents like freeze-thaw cycles, thermal expansion/contraction, and abrasive action of wind and water. Because there is no chemical alteration, the mineral grains remain inert and lack the surface charges (like those found in clay minerals) necessary to develop inter-particle cohesion. Consequently, the strength of these soils is derived solely from inter-particle friction.

Key Formulas: $	au = \sigma 	an(\phi)$ — Coulomb's law for shear strength of cohesionless soil where $c=0$

Key Points: Cohesionless soils possess high permeability compared to cohesive soils.; The shear strength is dependent on the effective normal stress ($\sigma'$).; Grain size distribution is a primary factor in engineering classification.; These soils do not show volume change under changing moisture contents.

Advantages: Excellent drainage properties.; Negligible settlement due to consolidation.

Disadvantages: High liquefaction potential during earthquakes.; Low bearing capacity in loose states.

Applications: Foundation base materials.; Drainage layers in embankments.

Additional Info: Chemical decomposition (hydrolysis, oxidation, carbonation) typically leads to the formation of clay minerals, which are cohesive in nature.. Common examples of cohesionless soils include sands and gravels (e.g., GP, GW, SP, SW in USCS classification).

Answer: A is correct — Cohesionless soils result from the mechanical weathering (physical disintegration) of rocks where the mineral particles retain their original chemical properties.

Exam Tip: Always remember that in soil mechanics, the presence of cohesion ($c$) usually indicates the presence of clay minerals formed via chemical weathering, whereas $c=0$ identifies cohesionless materials.

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Option B: Chemical decomposition

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Option C: Oxidation

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Option D: Hydration

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