Seasonal (natural) fluctuations in the moisture content of the soil are caused by periods of moisture evaporation during dry periods and by periods of moisture increase due to precipitation events. These alternating periods of drying and wetting can result in shrinking and swelling cycles of the soil near the perimeter of a foundation causing soil degradation due to the up and down fashion of the perimeter beam. Long-term cycling of the soil at the perimeter can lead to some permanent deformation of these soils causing the perimeter to be down relative to the center. This characteristic is commonly referred to as edge settlement. The cycles can be intensified by nearby downspout discharges, poor surface drainage, trees or vegetation near the foundation perimeter, and plumbing leak(s).
Residential slabs are supposed to be installed sufficiently above site grades to allow proper post-construction surface drainage. The primary function of good drainage is to prevent ponding near, or intrusion of water, under the structure, which would increase seasonal moisture fluctuations, or migration of water. In addition, providing constant, uniform soil moisture next to and under a foundation is the single best thing to reduce or minimize the effect of continual differential foundation movement. Chemical injection can reduce the movement to acceptable tolerances.