deep crawlspace in SC home below water table 2

[Mayfield571]

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.

 

[JAE]

You should hire an engineer to look at this for you. Online engineering advice directly followed is bad news when the engineers here can't review the soils, building configuration, or site issues.

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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[Mayfield571]

@JAE Thanks for the reply. Should I direct this to a structural or geotechnical engineer? The 3 foot depth is consistent with the rest of the crawlspace and I would add vents to that section of the foundation. I will also remove the sump pump prior to filling. The load was something the other two engineers were not certain on either, but this wasn't their specialty. Since the floor is dirt and not attached to the foundation walls how does load affect the foundation? Is it that the weight of the fill dirt could compress the ground beneath it which is adjacent to the ground below the footings and therefore could cause settling under the footing?

 

[JAE]

Yes - adjacent. You would have a "bulb" of pressure gradient below the entire footprint of the crawlspace.

I would suggest you start with a geotech. They would understand the soil types and behavior better I would think.



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[oldestguy]

I'd not remove the sump pump, but would build a suitable culvert pipe section or similar so that it can be reached and operated if necessary. How do you know that you will be above a future water table? Water levels vary a lot depending on many factors. Why destroy a possible tool that may or may or may not come in handy later?

 

[JAE]

I'd rather have the hydrostatic loads the same on each side of the now-buried crawlspace foundation walls.



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[oldestguy]

Why? Un-balanced before worked OK. After filling there even is earth inside to resist movement. If it turns out to be a problem, the fix is easy.

 

[Mayfield571]

I had planned on removing the sump pump and reinstalling it with a new French drain inside the crawl space at the elevated crawl space floor. That way if there was ever water in the crawl it could still be pumped out, but the water would need to rise significantly higher before the pump would need to work so I would assume it would operate far less often. The ground here is very sandy also so the compaction rate should be minimal I would think, but I will reach out to a Geotechnical engineer. How much should I expect to pay for someone in this field?

 

[JAE]

Just depends. Tell them your situation and ask for a proposal (scope of services and fee).

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[oldestguy]

Mayfield: One caution that I hope will be followed. Any so-called French Drain has to be properly filtered. Opengraded gravel backfill is not a filter and fine sand can migrate with any flowing water. If the original drain system was properly filtered, it can still function, but if one wants to let the interior water rise some, all he needs to do is raise the elevation of the float. If the sump shows little or no sand, the current system may be OK. Another clue of bad filter is finding a delta at the discharge area of the system.

 

[SlideRuleEra]

Mayfield - I completely agree with JAE's recommendation to consult a geotech. No need for me to talk about that.

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:

http://www.SlideRuleEra.net

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[Mayfield571]

oldestguy: Thanks for the tips/info regarding the drain system. I will give you the backstory which may help. We had a significant amount of rain back on Oct 3 & 4 here and collapsed the curtain wall on one side of this deep crawlspace from all the hydrostatic pressure on the outside of that wall, which is what prompted me to look into other solutions to this area. During normal weather conditions, if you turn off the pump and let water settle down there, then you will have about 12 - 16 inches of standing water, but it does vary based on the time of year and recent rainfall. There are 1/2" weep holes drilled into every cinder block at about 16 inches above the crawlspace floor all the way around the 19x12 crawlspace. The contractors that came out and are doing the repair to the foundation wall cautioned me that we will likely always have some water pumping out of there because it seems to dug deep enough to be either below a water table or possible aquifer. He was unsure about filling it in and what issues it may cause. The pump was only dry consecutively for about 3 weeks this past summer, but runs in anywhere from 2-15 minute intervals all day everyday pretty much the rest of the year.

 

[Mayfield571]

SlideRuleEra: We are in the middle of the state, so it appears we would be on the edge of the Sandhills Region

 

[SlideRuleEra]

Mayfield - Ok, I grew up in the sandhill region. As you know, that 3-4 October storm hit hardest in your Columbia area, and is considered to be a 1000 year event. Our Charleston area was bad, but not that bad. If the geotech gives the ok, filling that deep area is a great idea, IMHO. A sump pump will be of limited or no value. Oldestguy's suggestions on French drains should help.

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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[oldestguy]

The more you explain, the more there is a the need for that geotech and do so before doing any work!!!! Contractors mean well, but many times are not qualified to solve the problem and design the fix.

 

[Mayfield571]

SlideRuleEra: Interesting how you would run the poly. I have had many people tell me when I put down the poly to cover the entire crawlspace floor, but they also weren't mentioning installing a blower in the crawlspace. I have a call in to a Geotech. I will see what he has to say and report back here.

 

[dhengr]

Mayfield571:
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.

 

[SlideRuleEra]

Mayfield - Just place the poly sheets along the foundation walls as shown in the sketch below. At the center, be sure to leave at least half the square footage of the crawl space uncovered. Overlap the poly joints, no need to tape them or to run the poly up the foundation walls. In cold climates, I understand that covering and sealing the poly over the entire crawl space works well. In central and coastal SC, summer humidity and cooling is the issue. If the crawl space is completely covered in these regions dehumidification works too well. Summer moisture is much improved... but in the winter the house drys out too much. The floors and woodwork shrink & crack, furniture comes unglued and can even fall apart. I've seen that happen. This result is not that dramatic with a modern HVAC system... but not everybody has a modern system.

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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[Terratek]

Why is the crawl space dug out the way you describe? How are you going to get the fill into and compacted in the spaced proposed for back filling? If it is a tight space you could use flowable fill.

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.

 

[Mayfield571]

Terratek: there is a storm shelter type door on the back for access with a ladder. I was going to use a cement chute to dump in the fill sand along with a wheelbarrow. I haven't thought through compacting yet, but if sand were used as fill would it need much compacting? Sand was my thought for fill since the soil at that level around here is usually very sandy.