Why undisturbed samples are required

Undisturbed samples keep the in-situ structure, density, water content and stress history of the soil. They are required because:

• Shear strength tests (unconfined, triaxial, direct shear) must be done on soil with its natural structure to get true $c$ and $\phi$.
• Consolidation tests need them to estimate the settlement of clays.
• They are needed to determine permeability, unit weight and sensitivity.
• Disturbed samples change the soil properties, so the results would be wrong and the design either unsafe or uneconomical.

They are needed mainly for cohesive soils and for final design, while disturbed samples are enough for classification.