Question: Moist Soil in Sub Crawl Space
I had an interaction with a homeowner experiencing one of those unfortunate cases of unlocate-able moisture in the basement area. If you have such problems, you'll know that it can be a maddening puzzle. His experience goes far beyond the all-too-familiar case of water appearing on basement walls or a slow trickle from below.
Nothing is concluded in this interaction.
But if you're desperate for any kind of information on moisture in sub-crawlspaces, it does introduce a couple of concepts, namely the ideas of soil heave and how void material works under slabs.
The homeowner writes:
At 90 days of ownership, it was discovered that the structural basement floor was defective, and the builder has now completely demoed the floor. Once it was removed, we discovered that the clay dirt in the sub-crawl space was very moist to the point that you could easily make mud balls out of it. We have a perimeter drain around the outside of the foundation tied to a sump pit and it runs pretty often.
The builder’s engineers have evaluated the situation and determined that the perimeter drain was installed properly and the drain and sump are operating properly. Their findings indicate that “there is no evidence that any free water is bypassing the existing drain system. The soils are absorbing moisture and wicking that moisture beneath the basement floor system. The moist soils observed beneath the floor system are not unusual or excessive for these conditions.
Installing an interior drain with laterals beneath the slab would not affect the moisture content of the subgrade soils, given that the presence of free water was not observed, and therefore would not be intercepted by the interior drain system.
They go on to say that “the structural basement floor was constructed over a 6-inch void material which does not require ventilation nor a vapor retarder, since there is no crawl space or accessible area.” Do you have any suggestions on a need for mitigation of this condition, and most importantly, do you feel that this moist soil poses any danger of potential degradation of the foundation, floor support piers, or the actual basement floor over time?
Answer:
I am answering more as a fellow homeowner who can sympathize with a roughly similar experience than as an expert who can expound on soil engineering. The water-in-crawlspace problem in a former house was mitigated enough to satisfy the subsequent buyer of the home.
But the neighborhood of houses itself was located at the base of a hill, and the water problem was much larger than something that could be solved by mere perimeter drainage. Patio and driveway slabs were heaving as a result of this deeper water problem.
It's implied that the defective basement floor was a result of the moisture. You don't say it directly, but that's what I'm deriving from this.
I'm not sure what kind of void material was installed. If the void material was something like Dynavoid, it's specifically designed for the kind of time-frame you mention. It supports the concrete long enough for the concrete to cure, then its legs intentionally fracture upon the first major soil heave. Does that sound familiar?
The engineers have continually claimed that the slab defect was not related to the moisture, but instead was the result of misplaced rebar over the caissons, and the rebar issue was true. They claim that no heaving or compromised integrity of the caissons has happened.
An independent repair person advocates for an interior drain system, but the builder's engineers say it won't do any good, since there is no "free water" apparent. Nor do those engineers advocate for ventilation or vapor retardant system.
I had an interaction with a homeowner experiencing one of those unfortunate cases of unlocate-able moisture in the basement area. If you have such problems, you'll know that it can be a maddening puzzle. His experience goes far beyond the all-too-familiar case of water appearing on basement walls or a slow trickle from below.
Nothing is concluded in this interaction.
But if you're desperate for any kind of information on moisture in sub-crawlspaces, it does introduce a couple of concepts, namely the ideas of soil heave and how void material works under slabs.
The homeowner writes:
At 90 days of ownership, it was discovered that the structural basement floor was defective, and the builder has now completely demoed the floor. Once it was removed, we discovered that the clay dirt in the sub-crawl space was very moist to the point that you could easily make mud balls out of it. We have a perimeter drain around the outside of the foundation tied to a sump pit and it runs pretty often.
The builder’s engineers have evaluated the situation and determined that the perimeter drain was installed properly and the drain and sump are operating properly. Their findings indicate that “there is no evidence that any free water is bypassing the existing drain system. The soils are absorbing moisture and wicking that moisture beneath the basement floor system. The moist soils observed beneath the floor system are not unusual or excessive for these conditions.
Installing an interior drain with laterals beneath the slab would not affect the moisture content of the subgrade soils, given that the presence of free water was not observed, and therefore would not be intercepted by the interior drain system.
They go on to say that “the structural basement floor was constructed over a 6-inch void material which does not require ventilation nor a vapor retarder, since there is no crawl space or accessible area.” Do you have any suggestions on a need for mitigation of this condition, and most importantly, do you feel that this moist soil poses any danger of potential degradation of the foundation, floor support piers, or the actual basement floor over time?
Answer:
I am answering more as a fellow homeowner who can sympathize with a roughly similar experience than as an expert who can expound on soil engineering. The water-in-crawlspace problem in a former house was mitigated enough to satisfy the subsequent buyer of the home.
But the neighborhood of houses itself was located at the base of a hill, and the water problem was much larger than something that could be solved by mere perimeter drainage. Patio and driveway slabs were heaving as a result of this deeper water problem.
It's implied that the defective basement floor was a result of the moisture. You don't say it directly, but that's what I'm deriving from this.
I'm not sure what kind of void material was installed. If the void material was something like Dynavoid, it's specifically designed for the kind of time-frame you mention. It supports the concrete long enough for the concrete to cure, then its legs intentionally fracture upon the first major soil heave. Does that sound familiar?
Homeowner Answer:
The house was new, but had been here a year before we bought it. And yes, there was Sure Void under the concrete slab and 4 caissons. So when the slab was removed, all that remained was flattened soggy cardboard and moist clay soil.The engineers have continually claimed that the slab defect was not related to the moisture, but instead was the result of misplaced rebar over the caissons, and the rebar issue was true. They claim that no heaving or compromised integrity of the caissons has happened.
An independent repair person advocates for an interior drain system, but the builder's engineers say it won't do any good, since there is no "free water" apparent. Nor do those engineers advocate for ventilation or vapor retardant system.
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