Assessing Drainage of Residential Lots in Areas With Extensive Soils

Proper lot drainage is more critical in areas with expansive clay soils. Foundation performance requires that soil moisture is controlled and moisture equilibrium maintained under and around slab-on-ground foundations and pier-and-beam foundations. However, slab-on-ground foundations and pier-and-beam foundations are not the only options.

Expansive soils can contain many minerals, including smectite and clays that absorb water. These soils are mostly vertisol clay soils with montmorillonite. They form deep cracks (desiccation fractures) during dry seasons and years in Texas. They absorb water and increase in volume. In the opposite direction, soil volume decreases as it dries. It is not unusual for soil to expand or contract by 10-25%. This can cause serious and expensive damage to a building’s foundation if it is not applied uniformly.

It is possible to build deep foundations that can withstand the seasonal changes of expansive soils. Properly powered foundations can be used without soil moisture is an issue. They can be pier and beam (a raised foundation with crawlspace, cast-in-place concrete perimeter/grade beam and interior piers/piles) or structurally supported slabs on the ground. These foundations should not be confused with pier and post foundations, including wooden posts.

The more common slab-on-ground foundations of today have footings that seldom exceed 30 inches deep. The zone of influence of expansive soils can extend to 10-20 feet deep in many places. Some areas of Dallas-Fort Worth are not suitable for drilling. Drillers cannot reach solid substrate (usually shale) in these areas.

A common myth is that homeowners can prevent foundation damage by properly watering their lawn and foundation. Although this is a good idea for areas with dry soils, it’s an oversimplification. Even the most skilled engineers at Texas A&M University don’t know how to do it.

Even with expertly programmed sprinkler systems, the soil around foundations can dry faster on steeply sloped or sunny sides than on those on less sloped or shaded sides. Trees and other landscaping near the foundation can also be a problem. They can absorb significant amounts of water and cause soil dehydration. In reality, the homeowner is responsible for preventing foundation failures by properly designing, building, and landscaping the foundation. However, this seldom happens.

It is more expensive to build solid foundations on large soils. The professional skills and judgment required to build solid foundations on expansive soils are part of the cost. The cost of repairs is often much more expensive than good design and construction. Also, the results are not always satisfactory. In arid areas where droughts are frequent and mandatory water conservation, homeowners should not spend billions of dollars on replacing poor building practices.

Topography

Topography is a geoscience field that studies the Earth’s surface features and shape. Surveyors used to be the only ones who could perform this task. They would mount their optical transits (or theodolites) on wooden tripods with their range poles and linemen. Today, you’re more likely to find robotic total-station GPS stations and sophisticated, computerized geographic information systems (GIS) equipment. Uncrewed aircraft vehicles (UAVs), or drones, are being explored by some companies. No matter what tools are used, the goal is to measure the Earth’s surface in any given location and record this information in topographic maps. These maps can be used by end-users for various purposes, not least to design a building’s lot drainage.

Before inspecting the property on large soils, inspectors might want to research. It is important to determine the drainage patterns of any subdivision or area. Google’s USGS Topographical Maps site allows one to identify the drainage patterns in an area easily. This might not be obvious if you stand in front of a commercial or residential building.