When clay soils expansion occurs, foundation movements can result. It has been my experience that whenever an older home (say 15 to 20 years or more) that is constructed on highly plastic clay suddenly experiences cracking in brick or sheetrock, after many years of crack-free service, and there’s no other obvious answer to the problem, it’s probably related to drying shrinkage of underlying expansive clay subsoils. These subsoils probably remained wet each year, throughout the life of the home. Or, at least, the moisture change that did occur each year in the underlying massive clay soil was not sufficient to cause enough building distortion to, in turn, cause building material cracking. If the home suddenly develops cracks, however, there’s probably been some major change in the drainage conditions or landscape surrounding the home. For example, if the house originally had gutters and these served their purpose for many years, then, for some reason (frequent leaf/debris clogging hassles and subsequent rusting deterioration?), the owner removes them, the moisture conditions around the house suddenly change. My personal experience suggests that the effects of water falling beside the home will eventually begin to take its toll by eroding more and more surface soil– thus exposing the foundation footing and/or the underlying plastic clays to more and more solar radiation. At some time, say 20 years after construction, the effects of normal summertime drying shrinkage may finally cause soil shrinkage sufficient to create enough foundation distortion to bring about enough building distortion to in turn cause sudden building material cracking, such as cracks in brick or sheetrock.
Another problem that I’ve noticed over the past three decades of inspections, is that if a particular part of the crawl space or foundation remains moist or under standing water during most or part of the year, as evidenced by dark, water/algae stains on the concrete or concrete block foundation system, then the adverse effects of a summer drought are most pronounced on the underlying bearing soils in these wet areas. In other words, these soils go from the extreme wet end of the moisture scale to the dry end. Imagine: the larger the moisture change, the more the soil shrinkage and, therefore, the greater potential for foundation settlement.
A third problem is related to large trees. If hardwoods are planted near a home soon after construction, it is likely that in 20 to 30 years, the trees will overhang the house-which means that their roots underlie the house. These roots are going to contribute to soil desiccation during a drought by robbing the soil of all available moisture. Hence, whenever a hardwood or pine tree overhangs a home/roof built on a highly plastic clay soil site, the tree should probably be removed.
Regardless of trees and poor drainage conditions, if a house is initially built on a shallow foundation system which bears on expansive clay soils, then the house will likely move and distort from day one. Moreover, it is unlikely that this movement will be uniform because the moisture content around and beneath the home will not be uniform. It will vary because of different drainage conditions that exist, the varying degree of solar exposure and the presence of large trees or shrubs. As stated, trees are especially capable of desiccating (completely drying) soil during drought periods. In time, the effects of uneven foundation movement, or differential foundation movement/settlement, or simply building distortion, will take its toll on the home and cracks will begin to form in the more brittle materials. The cracks form to relieve the built-up stress of distortion.
What can be done to control the distortion?
The most economical way to deal with expansive clay soils underlying a home is to try and maintain a uniform moisture content in the ground (clay soil) throughout the year. This is best accomplished by letting nature takes its course throughout the wet winter, when rainfall is greatest and evaporation is lowest — then adding water via irrigation, as required, during the dry periods of the year. Drip irrigation tubing laid alongside the foundation footings, covered with thick beds of mulch, typically works well to do this. By applying water slowly, via the drip tube, the ground can absorb water previously lost or given up to evaporation and plant root uptake. One has to be careful, however, not to over-water or saturate the ground because as stated, this can soften the bearing soils and lead to foundation settlement. Any irrigation contractor should be able to come up with an automated drip irrigation system. In other words, moisture detectors can probably be buried below/beside the foundation and utilized to activate (automate) a drip irrigation system using a conventional lawn irrigation control panel. The buried sensors tell the irrigation control panel when to turn valves on and off. The irrigation contractor should work closely with a professional soil scientist to determine the range of soil moisture content readings that should be utilized to activate/deactivate the irrigation system.
In some instances, when the amount of differential foundation movement has been so severe that a foundation component has cracked and faulted, when doors and windows start to bind, or when the brick veneer has cracked, faulted and/or pulled away from the home, it might become necessary to underpin the distorted foundation so that future movements do not make matters worse. Foundation underpinning can also be used to uplift and/or relevel a settled (distorted) foundation.
Underpinning typically entails digging or drilling a vertical shaft into the ground, alongside and beneath an isolated part of the foundation, then filling the excavation with concrete. These shafts are usually spaced several feet apart so as to not undermine the entire foundation, which would obviously lead to collapse. In some instances the concrete is simply placed into the shaft until it contacts the footing, thus preventing future or additional settlement. In other cases, the concrete placed into the excavation is stopped a foot or more beneath the footing and allowed to harden or cure for several days. Large hydraulic jacks are then placed beneath the footing, on top of these concrete piers or shafts, and the footing is carefully jacked upward, extending the jacks in proportion to the amount of settlement at each point. It’s a real art (and danger) to relevel a settled house/foundation, so this type of repair should be left only to professionals.
A similar method of foundation repair entails the use of helical augers which are turned or twisted into the ground and stopped whenever the driving torque reaches a certain level — indicating that the blades on the auger have reached a sufficient bearing capacity with the deeper soil strata to provide a substantial amount of vertical support. Steel brackets are fitted over the auger shaft and under the foundation. The auger shaft is usually threaded, so large nuts placed below the bracket are turned in order to lift the footing upward.
There are many firms which specialize in this type of repair: Foundation and Structural Renovations (Huntsville), Hollis Kennedy House Moving (Athens), Alabama RamJack (North Alabama), AFS (North Alabama), Steve Reeves Construction (Decatur), and others.