deep crawlspace in SC home below water table
deep crawlspace in SC home below water table
(OP)
I have a portion of my crawlspace, 19'x12' that is dug out 7 feet below ground surface. It has a French drain system which runs to a sump pit. The sump pump runs every 2-5 minutes most of the year. I have spoken to a couple of engineers(one structural and one civil) who were unsure about the complications in what I want to do. I want to bring in dirt or gravel and fill in the crawlspace to a depth of only 3' to keep the crawlspace floor above the water table. What drawbacks and/or complications might I encounter? I can get fill dirt very cheaply so it is the most cost effective approach I know of if it will work.





RE: deep crawlspace in SC home below water table
Having said that some initial thoughts:
1. Adding fill adds load and might compress the underlying soils supporting the home...thus some possible settlements.
2. Some settlements might be irregular (more at one end vs. the other and resulting cracking, damage to the home.
3. Eliminate the sump pump prior to filling.
4. The crawlspace soil surface will now be much closer to wood framing (assuming you have wood framing above) and I'd be concerned with moisture damage. Need to ensure good venting of the space.
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RE: deep crawlspace in SC home below water table
RE: deep crawlspace in SC home below water table
I would suggest you start with a geotech. They would understand the soil types and behavior better I would think.
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RE: deep crawlspace in SC home below water table
RE: deep crawlspace in SC home below water table
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RE: deep crawlspace in SC home below water table
RE: deep crawlspace in SC home below water table
RE: deep crawlspace in SC home below water table
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RE: deep crawlspace in SC home below water table
RE: deep crawlspace in SC home below water table
However, depending on where the house is located in the state what in needed to keep crawlspace moisture from causing problems will vary. Per this map, where is the location:
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RE: deep crawlspace in SC home below water table
RE: deep crawlspace in SC home below water table
RE: deep crawlspace in SC home below water table
The biggest problem is the combination of hot, humid summers and fairly high water table. The air outside the crawlspace is as humid as the air inside the crawl space. For this reason, passive vents don't help. The best way to handle the moisture is with a powered "intelligent" crawl space blower. It will sense inside and outside humidity and ventilate the crawl space at the optimum time - probably late at night in the summer.
Another help is 6 mil polyethylene sheet placed on top of the crawl space ground, around the inside perimeter of the entire crawl space. Keep the sheets within, say 3' to 6' of the foundation wall. Don't cover the entire crawl space; that will effectively trap moisture under the poly... but poly is a barrier not a sealant. Summer time moisture will continue to be a problem. By keeping the center of the crawl space uncovered, moisture can escape from the soil and be expelled by the blower.
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RE: deep crawlspace in SC home below water table
RE: deep crawlspace in SC home below water table
RE: deep crawlspace in SC home below water table
I agree with SRE about the venting issue being difficult to resolve, because often you are introducing air witch is wetter than what you are exhausting. But, I would go a couple steps further than he suggests. Can (or has) the water table ever risen to the elev. of your intended new fill? You might want an outside found. drainage system, to a sump and pump, which would keep the water table some distance below your new fill elev. Certainly, grading and exterior drainage are important, but your problem sounds like something more than that. Then, depending upon the conditions and details of your found., I would consider conditioning the crawl space. Fill up to within about 6" of the intended finished grade, and then do the last 6" with nice clean sand, which is a nice soft cushion and can literally be screeded level/flat. Put the 6mil poly on this sand cushion and seal (tape) the lapped joints. Bond some rigid foam insul. board to the exterior found. walls, including doing something to insul. and seal the sill pl. and the rim joist space. Run the 6mil poly up the found. wall and seal it to the sill pl. area. Then, at least dehumidify the crawl space, or run a couple conditioning ducts into the crawl space. Allow these ducts to be adjustable depending upon the season. If you don’t go all the way, you don’t appreciably solve the moisture problem. Because it is slightly cooler down there during the most humid periods, you can actually get condensation. And, of course, the further (deeper) you can keep the water away from the finished grade the better, but you will still get some moisture percolating upward (capillary action) to the surface.
RE: deep crawlspace in SC home below water table
With all that said, the intelligent blower (and appropriate perimeter inlet vents) is most important. I would try just that first. The poly can always be added later, if needed.
dhengr - I have the utmost respect for your judgment, but I have to disagree with you. In the Sandhill Region of SC, no sump pump is going to be able to lower the water table significantly. It takes well points to do that. Insulating the crawl space is not economically justified - it does not get cold enough for long enough to make that and the additional steps attractive. No doubt there would be some improvement, but an intelligent blower is almost always enough - even in the Outer Coastal Plane where water table is much higher than the Sandhills.
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RE: deep crawlspace in SC home below water table
For your own protection, you need to retain a local, well qualified geotechnical engineer. It would be interesting to know what recommendations the geotechnical engineer provides to you.
RE: deep crawlspace in SC home below water table
RE: deep crawlspace in SC home below water table
RE: deep crawlspace in SC home below water table
Maybe some agricultural screw conveyor or those coal screw conveyors on the last of the cola fired locomotives. What do coal miners do in low overhead areas?
RE: deep crawlspace in SC home below water table
RE: deep crawlspace in SC home below water table
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RE: deep crawlspace in SC home below water table
I lived for a while in an old farm house with no basement,but much of the crawl space was a pond, due to high water table in the area. Not much of any venting there wither, but up stairs there was no noticeable affect of this water, year around. No fun working on the plumbing there!!!
RE: deep crawlspace in SC home below water table
Keep in mind that there will not be "several feet of pea gravel above the water table", it will be more like 1 to 2 feet... after each rain event probably a lot less. Central South Carolina can expect 2 to 4 rain events (scattered afternoon and evening thundershowers) per week in the summer.
Was the farm house in your example in a location where the humidity was 90%+, temperature 85 degrees Fahrenheit+, for several months at a time (say June though September)? That's what central South Carolina is like.
It is very difficult to keep the humidity in the crawl space air under control.
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RE: deep crawlspace in SC home below water table
RE: deep crawlspace in SC home below water table
I've had some time for this to gestate and below are the things that I have concluded with the information you have given.
Here is the condition and some facts:
1) You said the soils are sandy...that implies 30 to 50 percent sand content. So, assume the other component is clay, therefore the native soils are sandy clay.
2) In a perfect world, you would replace the native condition.
3) Even with the same soils (sand clay) used as backfill, your compacted soils won't be the same as native in terms of strength and permeability.
4) You will not cost effectively compact sandy clay in the confined space you describe, particularly the upper lifts that come near the floor.
5) The issue with mold and mildew may or may not be a concern if you use a clean sand or gravel. Regardless, there will be a bathtub effect.
6) There could be could settlement issues by placing all that fill below the existing structure. The severity of this potential settlement is not know without exploration of the site.
Solution:
Backfill with lean concrete (flowable fill). This eliminates the questions about permeability, compaction and strength to resist the adjacent soils currently pushing on the walls.
Regarding potential settlement; what can you do about it in practical terms? Well, not much really. It will likely be minimal settlement considering the load (less than 1,000 psf for 7 ft of backfill). Can it still occur? Yes. So what then? Do a relative elevation survey of the residence before backfilling. Monitor settlement (using relative elevation surveys) after backfilling both immediately and a couple times over the next few months. Shim the foundation as necessary to re-level. Make sure the client has a full understanding of the risks and costs associated with this procedure, including the cost to repair cosmetic damage. In the worst case of settlement, even pluming and underground electrical connections can be affected, but my gut tells me that this is a minimal concern.
RE: deep crawlspace in SC home below water table
Geography:
The Sandhills Region of the Carolinas are.... well, sand - no clay. It is a strip of ancient beach dunes... and is the evidence of a former coastline. Here is another reference: Sandhill Fact Sheet
and a couple of typical photos of the Sandhill Region:
Geometry:
Even if the 19'x12' portion is filled with a 100% impermeable material, the remaining portion of the crawlspace will contribute moisture to the entire crawlspace area. The idea is to control the moisture in the crawlspace... not to eliminate it. Using the poly around the perimeter allows soil moisture (after rain events) to slowly migrate toward the center of the crawlspace and be released over time - not is a sudden "spike" shortly after rainfall. At the same time, the high water table is a constant source of crawlspace moisture - whether it rains or not.
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RE: deep crawlspace in SC home below water table
For what practical reason would one want to fill with what was there before and why would one want to compact it? I could go on with more, but this reminds me of why I was so successful in getting calls for work from many that were unhappy with their former consultants.
RE: deep crawlspace in SC home below water table
As to the financial analysis, lets just assume it costs more to "fix" it than to leave it as is. Thread over....
Okay, now lets consider the question of settlement. If the soil has rebounded, it can re-compress, taking adjacent footings down that were constructed on the rebounded soil. This assumes the space was excavated before the rest of the house was built. Is that a practical assumption? We don't do basements where I live. Will the settlement be much? I doubt it and have indicated so. Can I say so strongly? Not without some borings or experience in the area (like in the neighborhood - not the region). I don't have either. But I've provided a way to address that concern if one is concerned.
Regarding replacing native conditions as ideal - its just that - ideal. Providing uniform conditions is rarely NOT preferred. Not necessarily practical, but also not recommended in my post.
You make an excellent point about compaction, though. Does it really need to be compacted? Maybe not. And as I think about it more.....probably not. Some nominal compaction would occur during placement and even when the new fill settles under its own weight, the top surface would still probably be above the groundwater if it only settled 6 inches. This is a palm face for me. Very good point.
It still may not be practical to deliver sandy clay (again working with the assumption of sandy clay) to the space.
My recommended process, while maybe not the cheapest route, eliminates most of the questions arisen from this thread compared to any other suggestion. It also delivers the most painless method of construction. I have found the more pain in construction, the more prone to errors and shortcuts and general butt pains the contractors become.
While I respect trying to find the cheapest solutions, I also recognize residential work as prone to the most sensitive clients and the highest source of liability. So, I tend to approach residential work with hesitancy. This particular project, and projects like it would not be one I would relish having on my backlog.
You said you could go on with more. I'd be interested in learning from the litany. Please present.
RE: deep crawlspace in SC home below water table
The new clients I picked up through the years were not home owners who had been "advised" by others. They were businesses, architects, engineers and contractors. Of course home owners can be fussy, but in my work with a consultant firm and later as self employed from '83 and on included home owners at times. No complaints or law suits, but reverse. Usually it was thanks for the money saving recommendations. I did have one attorney who would not pay the bill for my work at her house, thinking she could "Buffalo me". The tables were turned when I took her to small claims court. As most of us know, attorneys on their own projects are the worst clients to pay their bills.
RE: deep crawlspace in SC home below water table
RE: deep crawlspace in SC home below water table
The main concern the structural engineer had with filling it in was settling of the foundation from water sitting at the footings without anywhere to escape, however, he mentioned if the sump pump was not always there then water would have standing beside the footing already so the future settling possibility would be less likely, but still possible.
Lastly, this information and "advice" provided is simply an exchange of ideas based on an issue I brought forth. I am thankful for the advice and in no way intend to take it for a 100% guaranteed fix. No lawsuits or attorneys will be involved at any point based on the information found here.
So far, the solution the SRE provided is the most attractive as it would be the most economical and based on the soil and humidity here.
Terratec's solution would be the best solution I think, but much more costly and would definitely require a more in depth cost recoupment calculation.