Weathering and residual soil

Weathering breaks down rock at and near the ground surface. Physical weathering, by temperature cycles, wetting and drying, frost and root growth, opens cracks and splits blocks. Chemical weathering, by water, oxygen and acids, alters minerals: feldspars turn to clay, iron minerals oxidise and stain, and calcite dissolves. In warm, wet climates such as much of India, chemical weathering dominates and can reach tens of metres below the surface.

Weathering produces a profile that grades from fresh rock at depth to soil at the surface. Engineers describe it in grades, commonly six: fresh, slightly, moderately, highly and completely weathered rock, and residual soil, in which the rock fabric is no longer recognisable. The boundaries are irregular, and corestones of fresh rock can float in weathered material, which can mislead a borehole drilled into one.

Residual soils formed in place differ from transported soils. Their properties depend on the parent rock and the climate. Laterites and lateritic soils, rich in iron and aluminium oxides, are widespread in Kerala, Karnataka, Goa and Odisha; some are strong enough to cut into building blocks, while others soften when saturated. Black cotton soils over Deccan basalt are rich in expansive clay minerals and swell and shrink strongly with moisture, cracking light buildings and roads.

Weathered zones control many works: foundation levels, rippability for excavation, slope stability in hill roads, and quarry selection. Slake durability and point load tests help judge how weathered rock will behave.