Clay soils tend to shrink when the climate is dry and expand when it is humid: normally, the lower the shrinkage limit and the wider the range of variation of the plasticity index, the more there is a change in volume and the greater the extent of this change. These variations in volume can be caused by the drying of the soil after the construction of the structure or due to a loss of natural humidity following the heating of the buildings.
Variations in volume, again, can be generated by drying the soil produced by vegetation (trees or shrubs around the structures for aesthetic reasons). Finally, variations in volume can also occur as a result of an artificial increase in soil moisture below the structure (buildings for cold storage subject to thermal gradient and without adequate insulation). In any case, before starting the calculation procedures of a foundation project, it is also necessary to analyze the meteorological characteristics of the site.
Droughts tend to dry the soil; after which the rains cause significant swelling. Since there is not enough rainfall to wash away and alter the clay minerals responsible for the volume variations, the same remain unchanged near the surface of the soil and quickly become wet during rainy periods.
In these regions the land is particularly complex for construction because the water vapor, migrating from the water table (sometimes distant from the ground level) condenses on the lower sides of the slabs and foundations that require Blast Hole Drilling.
The ground below the inner areas of the buildings can, finally, become saturated due to the effect of condensation coming to swellings unless the construction is not heavy enough to counteract the (high) reaction pressure. A further difficulty arises from the fact that in arid climates the soil around the perimeter of the buildings remains in a much drier state than that of inland areas, so there are significant differential movements.
In terms of relative values, low volume variations can be assumed to be those with values ≤ 5 while they can be interpreted as very large when> 25. The structures whose foundations are placed on expansible (or expansive) land require special construction techniques for Blast Hole Drilling that is used for foundations.
Creating foundations near an existing structure the line joining the base of the new foundation with the base of the existing foundation must form an angle ≤ 45 ° with the horizontal plane. From this it follows that the distance m must be greater than the vertical distance between the two foundations; the approximation produces very strong pressures in favor of safety in the area where more than one foundation contributes to these pressures.