Unconfined compressive strength (UCS) range for stiff clay Classify the unconfined compressive strength of a ‘‘stiff’’ clay by choosing the appropriate UCS range (kN/m²).

Introduction:
Consistency terms like soft, firm, stiff, and very stiff relate qualitatively to strength of cohesive soils. For design and site classification, it is helpful to link these terms to approximate ranges of unconfined compressive strength (UCS), aiding preliminary decisions and checks on laboratory data.

Given Data / Assumptions:

• Undisturbed specimens tested in unconfined compression.
• Short-term, undrained behavior relevant for clays.
• Standard consistency descriptors used in geotechnical practice.

Concept / Approach:

Typical guides classify cohesive soils approximately as: very soft (UCS < 25 kN/m²), soft (25–50), firm (50–100), stiff (100–200), and very stiff (200–400+). These ranges vary slightly by reference, but consistently place “stiff” near 100–200 kN/m², reflecting noticeably higher resistance to penetration and sampling disturbance.

Step-by-Step Solution:

Verification / Alternative check:

Field correlations with SPT N-values and vane shear often corroborate this strength level for stiff clays.

Why Other Options Are Wrong:

Less than 100 kN/m² corresponds to softer consistencies; above 200 kN/m² indicates very stiff to hard clay.

Common Pitfalls:

Equating “stiff” with “very stiff”; misinterpreting UCS units; using remolded samples which give lower strengths than undisturbed.

Final Answer:

100 to 200 kN/m²