Expansive Clay Minerals – Identify the Incorrect Property of Montmorillonite-Rich Soils Which of the following statements about soils containing significant montmorillonite (smectite) minerals is <em>incorrect</em>?

Introduction / Context:
Montmorillonite clays are well known for their expansive behavior due to interlayer water adsorption. Designers must understand their mechanical traits to mitigate heave, shrinkage, and low shear strength in foundations and earthworks.

Given Data / Assumptions:

• Soils are dominated by montmorillonite (smectite) clay minerals.
• Typical index properties include high liquid limit and plasticity index.
• Shear strength arises from both cohesion and friction angle.

Concept / Approach:

Montmorillonitic soils have expansive lattices leading to large volume changes with moisture. They are very plastic and compressible, and their effective stress friction angle is usually low to moderate, not high, especially at higher water contents. Therefore a statement claiming a high internal friction (large phi) contradicts typical behavior and is incorrect.

Step-by-Step Solution:

Verification / Alternative check:

Index and strength correlations (e.g., between plasticity and phi) and numerous triaxial/UU-CD tests on expansive clays confirm modest phi values compared with dense sands.

Why Other Options Are Wrong:

(a), (b), and (c) accurately describe expansive clays; 'None of these' cannot be correct since one statement is indeed incorrect.

Common Pitfalls:

Assuming all clays have similar friction behavior; overlooking moisture sensitivity of strength parameters.

Final Answer:

They possess a high coefficient of internal friction (phi is large)