Foundation settlement, effects, black cotton soil

Causes of foundation settlement

1. Consolidation of soft clay or silt (water squeezed out slowly under load).
2. Loose, weak or filled soil and poor compaction.
3. Non-uniform soil layers and unequal loads.
4. Fall in water table, or seasonal shrinkage and swelling.
5. Overloading, extra floors, or poor foundation size.
6. Nearby excavations, vibration and dewatering.
7. Tree roots, leaking pipes, erosion and underground voids.
8. Earthquake densification or liquefaction.

Effects of unequal settlement on the structure

• Cracks in walls, beams, slabs and plaster, usually diagonal and wider at the top or bottom.
• Tilting of the building or of walls and columns.
• Distortion of frames and extra bending moments and shear in members.
• Jamming of doors and windows, and uneven floors.
• Damage to services: broken pipes, drains, and electrical lines.
• Loss of waterproofing, dampness, and leakage.
• In severe cases, partial or complete failure or collapse.

Why black cotton soil is not preferred

Black cotton soil is a clayey soil rich in montmorillonite, found in parts of Nepal's Terai and India.

1. High swelling and shrinkage: It expands in the rainy season and shrinks in the dry season, a movement that can be 10 cm or more.
2. Low bearing capacity (about 50 to 100 kN/m²) and low shear strength when wet.
3. Large cracks (up to 1 m deep) in dry weather and extreme softness when wet.
4. Differential movement under the building leads to cracks and uplift.
5. Poor drainage and high compressibility.
6. Damage to foundations and floors because it exerts uplift pressure.

Hence, the foundation should go below the active zone, or the soil should be replaced or treated (under-reamed piles, sand cushion, raft).