The properties of a soil mixture are influenced more by variation in moisture content than by any other cause. Saturated soils are improved in strength by drainage and dry soils lose strength by saturation. A water logged ground is undesirable because of its low bearing capacity.
Effect on Fine Grained Soil
Fine grained (clayey) soils are most likely to suffer by water absorption. It is therefore important to ascertain the wettest condition in a given case and basis of design should be the strength of the wet soil. Clayey soils are subject to a large amount of shrinkage but the rate of loss of water that causes this shrinkage is slow, and the shrinkage might amount to as much as 20% in volume.
Effects on Coarse Grained Soil
In the case of sandy soils the detrimental effect of moisture is much less than in clayey soil. Granular soils do not hold water readily and do not shrink much when drying. When such soils are saturated with water and the water is trapped, the footing may be supported on hydraulic pressure. Under such conditions the soil is without shearing strength. The seepage out of this entrapped water will cause settlement; therefore this water must be drained out.