The maximum permissible differential settlement, in case of foundations in clayey soil, is usually limited to 20 mm. This is because clayey soils are relatively incompressible, and therefore any differential settlement can cause significant damage to structures. A differential settlement of 20 mm is the maximum that can be tolerated without causing significant damage to most structures.
Option A: 10 mm is too small a differential settlement to be of any concern.
Option B: 20 mm is the maximum permissible differential settlement for clayey soils.
Option C: 30 mm is a larger differential settlement than is usually acceptable for clayey soils.
Option D: 40 mm is a very large differential settlement that is likely to cause significant damage to most structures.