Basic Soil Classification – Identifying Cohesionless Soil In the context of soil mechanics, which of the following is generally classified as cohesionless (i.e., negligible true cohesion under drained conditions)?

Introduction / Context:
Soils are broadly classified by the presence or absence of true cohesion. Cohesionless soils develop shear strength primarily through friction and interlocking, whereas cohesive soils show significant apparent cohesion due to electrochemical and water-related bonding mechanisms, especially under undrained conditions.

Given Data / Assumptions:

• Standard drained condition understanding.
• Clean, well-graded sand is the reference cohesionless material.
• Silt can exhibit minor apparent cohesion when wet, but is often treated separately from clean sands in engineering practice.

Concept / Approach:

Sand and gravel are the classic cohesionless soils; their shear strength is governed by the friction angle φ, contact roughness, and density. Clay, by contrast, possesses significant cohesion and plasticity. Silt is intermediate; many codes still handle it differently from sands because of capillary and plasticity effects at certain moisture contents. Therefore, among the options provided, 'Sand' most clearly represents a cohesionless soil in the strict geotechnical sense.

Step-by-Step Solution:

Verification / Alternative check:

Unified Soil Classification System (USCS) designations for clean sands (SP, SW) are cohesionless; typical direct shear tests show negligible intercept c′.

Why Other Options Are Wrong:

Silt (b) may show small apparent cohesion; clay (c) and (d) are cohesive; peat (e) is highly organic with very different behavior.

Common Pitfalls:

Assuming silt behaves like sand under all conditions; silt can exhibit capillarity and sensitivity that complicate classification.

Final Answer:

Sand