Expansive Soils

In many areas of North Texas, soils have high contents of expansive clays that swell when wet and shrink when dry. Damage to building foundations, swimming pools, monuments, perimeter screen walls, and retaining walls can, and often does, result from the shrink-swell pressure exerted by the soil.

The expansive clay soils cause problems for us in multiple ways. They can cause settlement, which is the problem most of us think of regarding the effect of our soils, or they can cause heave due to swelling of the soils. Think of our soils as a sponge: they swell when wet; and shrink when dry.

Settlement to any type of structure being supported by the soil can occur for several reasons including poor compaction of fill materials, lack of maintenance (maintaining a stable moisture content by watering around the structure during dry periods), and by far the greatest impact we see - trees too close to the structure. More-or-less uniform moisture levels can help preclude cyclic expansion and contraction of the soil with its resulting differential movement. Installation of an irrigation drip-line zone around the perimeter of a structure, that can be controlled, can help maintain uniform moisture levels.

Because of the highly expansive nature of the soil, trees can significantly contribute to differential settlement of any foundation or structure. The roots of trees and large plants consume the moisture from the soil, causing the soil to shrink much faster than other soil areas exposed to the weather. The soil where the moisture is lost more rapidly will sink lower than the surrounding soil, causing evidence and consequences of differential settlement in all types of structures. Research studies indicate that a tree should be at least as far away from a structure as the drip line of the tree to minimize the effect of drying caused by the tree. Clusters of trees can have an even greater effect, and some studies indicate that the impact of trees can reach a distance equal to the mature height of the tree(s).

Most people think of settlement when there is damage to a structure, however often the problem is just the opposite – heave. Heave to a structure occurs when highly expansive clay soils become wet and swell like a sponge. Structural damage can often result from the shrink-swell pressure exerted by the soil. Most often this heave results from a lack of proper drainage in close proximity to the structure. It is for this reason that building codes require a minimum 5% slope in grade away from a building. Proper grade slope doesn’t always get executed properly around buildings, as many of you know due to drainage issues. It is especially not always thought of when other structures like perimeter screen walls, retaining walls, and pools are installed. Any areas where water can pond near a structure can potentially cause damage due to swelling of the expansive clay soils.

So remember, when you encounter distress to a building, swimming pool, monument, perimeter screen wall, or retaining wall, it is likely a result of one or more of these factors. Consider the proximity of trees to buildings and other structures, maintain positive drainage away from any structure, and maintain a uniform moisture content through a consistent watering program around structures during our dry periods.