Too many times, I see inadequate surface drainage conditions around a foundation that can potentially cause foundation movement. Because of the presence of expansive clay soils, poor drainage conditions around a foundation in North Texas can increase the likelihood of soil expansion (with the resultant upheaval of a slab foundation). The expansion of clay soils is caused by an increase in the soil moisture content.
Some clays in North Texas are highly expansive while others not so much. In areas that have highly expansive clays, soil expansion can cause a slab to heave upwards several inches (I have seen 12” of upheaval in Irving, Texas). However, in other areas, having water ponding near the foundation does not cause a noticeable foundation problem but the water is more of a nuisance. It is a good practice to always maintain excellent drainage conditions around a foundation, regardless of what the problem is.
Some builders make it nearly impossible to have good drainage conditions because they did not install the slab foundation high enough around the ground surface. This seems to be especially the case in many rural areas.
Rural Residential Developments
I got a call from a homeowner in a rather rural community (a few dozen homes in a non-incorporated area) that said he had a drainage problem and asked me to inspect it and design a repair. Once at his house, I noticed that not only was the ground surface around his home flat but so was the entire neighborhood, so there was no place for the water in his yard to drain. He then showed me photos of his neighborhood after a heavy rain and there was a foot or so of water in the streets (the streets were somewhat lower than the front yards) but the water came up very close to his house (and his neighbors). I told him (the obvious) that improving the drainage conditions around his home was meaningless until the neighborhood drainage was repaired.
The homeowner ended up suing the builder and the developer (who installed the streets, water, lots, etc.). Both the builder and developer took bankruptcy and the homeowner took a real bath on the house value.
The foundation should be installed high enough so that when (if) a flower bed is installed adjacent to the house, the ground surface can still slope enough so water can run rapidly away from the foundation. (Refer to the Drainage Guidelines below).
Swimming Pool Drainage
If I were to build a swimming pool, I would specify that any drainage catch basins or drainage channels had to be a minimum of 9” diameter for the catch basins and 4” wide for the drainage channels. Not the absurdly small drainage systems normally used that get clogged up easily.
Over the years, I have seen several french drains situated immediately adjacent to the foundation. Depending on the drainage system construction and soil characteristics, it is possible that the soil moisture content can be increased because too many times, water pools around the drainage pipe. This can be like a sub slab plumbing leak and under the “right” soil conditions can actually cause a slab to heave up. To prove this would require some geotechnical testing. But to be safe, the drain system should be installed a few feet away from the foundation.
Engineering Drainage Guidelines
The “Guidelines for the Evaluation and Repair of Residential Foundations” adopted by the Texas Section of the American Society of Civil Engineers (ASCE) in 2002 with an effective date of January 2003, states the following concerning the drainage conditions around a foundation: “Where practicable, for adjacent ground exposed or vegetative areas, a minimum slope of 5% (i.e. 6 inch drop over a 10 feet distance) away from the foundation should be provided for the first 5 feet all around (the foundation). (Drainage) swales should have longitudinal slopes of at least 2% (i.e. 6 inches over a 25 foot distance), if practicable, and 1% (3 inches over 25 feet) at a minimum.”
The ASCE also developed a guideline for the “Recommended Practice for the Design of Residential Foundations”. In that document, they state the following concerning drainage: “For adjacent ground exposed or vegetative areas, provide adequate drainage away from the foundation (minimum five percent slope in the first ten feet and minimum two percent slope elsewhere). The bottom of any drainage swale should not be located within four feet of the foundation. Pervious planting beds should slope away from the foundation at least two inches per foot. Planting bed edging shall allow water to drain out of the beds.”