Engineering significance of three rock types

The type of rock affects its strength, durability, permeability and behaviour, so it is a basic consideration in selecting sites and materials.

Igneous rocks

• Generally hard, strong, dense, massive and low in porosity; granite and basalt are good foundation rocks for dams, bridges and heavy buildings.
• Used as crushed aggregate, road metal, building and dressing stone.
• Problems: fractures, joints and weathering in humid areas; sheet jointing; columnar jointing in basalt causes leakage; weathering of granite to clayey soil.

Sedimentary rocks

• Properties vary widely with cementation and layering.
• Sandstone (silica-cemented) and limestone are good building materials and aggregate; limestone is the raw material for cement.
• Bedding planes are planes of weakness, so slope and tunnel stability depend on the dip direction.
• Shale is weak, slakes in water and swells, and causes landslides.
• Limestone has solution cavities, which cause leakage in reservoirs.

Metamorphic rocks

• Massive types (quartzite, marble, gneiss) are hard and durable; marble and slate are used for flooring, roofing and decoration.
• Foliated rocks (schist, phyllite, slate) split along foliation, are weak, and cause slope failure and tunnel squeezing; mica minerals reduce strength.
• Some are anisotropic: strength depends on direction of loading.

Hence the engineer needs to study the type, structure and weathering of rocks before choosing foundation level, slope angle and support.