Firm, compact shrinkable clays suffer appreciable vertical and
horizontal shrinkage on drying and expansion on wetting due to seasonal changes. Seasonal volume changes under grass extend to about 1 m below the surface in Great Britain and up to depths of 4 m or more below large trees.
The extent of volume changes, particularly in firm clay soils, depends on seasonal variations and the proximity of trees and shrubs. The greater the seasonable variation, the greater the volume change. The more vigorous the growth of shrubs and trees in firm clay soils, the greater the depth below surface the volume change will occur.
As a rough guide it is recommended that buildings on shallow foundations should not be closer to single trees than the height of the tree at maturity, and one-and-a-half times the height at maturity of groups of trees, to reduce the risk of damage to buildings by seasonal volume changes in clay subsoils.
When shrubs and trees are removed to clear a site for building on firm clay subsoils there will, for some years after the clearance, be ground recovery as the clay gradually recovers moisture previously withdrawn by the shrubs and trees. This gradual recovery of water by the clay and consequent expansion may take several years. The depth at which the recovery and expansion is appreciable will be roughly proportional to the height of the trees and shrubs removed, and the design and depth of foundations of buildings must allow for this gradual expansion to limit damage by differential settlement. Similarly, if vigorous shrub or tree growth is stopped by removal, or started by planting, near to a building on firm clay subsoil with foundations at a shallow depth, it is most likely that gradual expansion or contraction of the soil will cause damage to the building by differential movement.
At the recommended depth of at least 0.9 m it is not generally economic to use the traditional strip foundation and hence the narrow strip or trench fill foundation (Fig. 9) has been used. A narrow trench 400 mm wide is excavated by machine and filled with concrete to just below the surface. If the concrete is placed immediately after the excavation there is no need to support the sides of the trench in stiff clays, the sides of the trench will not be washed away by rain and the exposed clay will not suffer volume change.
The foundations of buildings sited adjacent to past, present or future deep-rooted vegetation can be affected at a considerable depth below the surface by the gain or removal of ground moisture and consequent expansion or shrinkage. Appreciable expansion, following the removal of deep-rooted vegetation, may continue for some years as the subsoil gains moisture. Significant seasonal volume change, due to deep-rooted vegetation, will be pronounced during periods of drought and heavy continuous rainfall.
Many beds of clay consist of combinations of clay with sand or silt in various proportions. The mix of sand or silt to clay will affect the behaviour of these soils as a foundation. In general where the proportion of sand or silt to clay is appreciable the less dense the soil will be. Because of variations in the proportion of clay to sand or silt and the general loose or soft nature of the soil it is practice to assume that their bearing capacity is less than that of clay.