Identifying cohesive soils: Which of the following is most appropriately classified as a cohesive soil for geotechnical design purposes?

Introduction / Context:
Cohesive soils exhibit significant plasticity and apparent cohesion due to clay mineral surfaces and adsorbed water. Correctly identifying a soil as cohesive versus cohesionless guides the choice of laboratory tests (e.g., Atterberg limits) and strength parameters (c, φ) for analysis.

Given Data / Assumptions:

• Cohesive soils: clays and some silts with plasticity.
• Cohesionless soils: sands and gravels with little to no plasticity.
• Naming should refer to a soil type, not a construction condition.

Concept / Approach:

“Clay” is the canonical cohesive soil, often showing plasticity, shrink–swell, and low permeability. Black cotton soil is an expansive clay, also cohesive, but the option set requests a single best identification. “Compacted ground” is not a soil type, and lateritic red earth can vary widely, sometimes behaving as granular or weakly cohesive depending on cementation and fines.

Step-by-Step Solution:

Verification / Alternative check:

USCS classes CL, CI, CH describe cohesive clays with plastic fines; expansive variants (black cotton soils) fall within CH/CI categories but the general term “clay” is the baseline cohesive descriptor.

Why Other Options Are Wrong:

(a) Red earth may be lateritic and not consistently cohesive; (c) is cohesive but is a specific sub-type causing ambiguity; (d) is not a soil type; (e) well-graded sand is cohesionless.

Common Pitfalls:

Equating color names with engineering behavior; treating construction state (“compacted”) as a classification.

Final Answer:

Clay