Soil mechanics describes the mechanical behaviour of a granular material
as it is compressed or sheared and as water flows though it.
To design of structures in the ground we need to describe
- logarithmic compression
- Darcy seepage
Saturated soil contains only mineral grains and water. Both are
relatively incompressible so the volume can only change if water can drain
out. In unsaturated soil volume changes can occur as air compresses
or bleeds out. In both cases loading will bring the grains closer together
and the specific volume will reduce.
- Reduction in volume leads to
- increase in strength
- increase in stiffness
- settlement of foundations
If soil is unloaded it will swell as the grains move apart. Swelling
leads to reduction in strength and stiffness and heave of excavations.