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deep crawlspace in SC home below water table
2

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

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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RE: deep crawlspace in SC home below water table

(OP)
@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?

RE: deep crawlspace in SC home below water table

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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RE: deep crawlspace in SC home below water table

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?

RE: deep crawlspace in SC home below water table

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

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RE: deep crawlspace in SC home below water table

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.

RE: deep crawlspace in SC home below water table

(OP)
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?

RE: deep crawlspace in SC home below water table

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

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RE: deep crawlspace in SC home below water table

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.

RE: deep crawlspace in SC home below water table

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:

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: deep crawlspace in SC home below water table

(OP)
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.

RE: deep crawlspace in SC home below water table

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

RE: deep crawlspace in SC home below water table

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.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: deep crawlspace in SC home below water table

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.

RE: deep crawlspace in SC home below water table

(OP)
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.

RE: deep crawlspace in SC home below water table

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.

RE: deep crawlspace in SC home below water table

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.



www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: deep crawlspace in SC home below water table

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.

RE: deep crawlspace in SC home below water table

(OP)
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.

RE: deep crawlspace in SC home below water table

Clean sand or gravel would probably be okay and you probably wouldn't need to compact it if it is not supporting anything. The water will perc up through clean material easily, but if the surrounding soils are permeable, that my not matter.

RE: deep crawlspace in SC home below water table

HOW TO GET THE FILL BACK INTO THE CRAWL SPACE SOUNDS LIKE A CHALLENGE. Could be very labor intensive. However, perhaps there may be a local redi-mix outfit that has the answer with a conveyor. As a starter check out what is out there, such as: http://www.hellotrade.com/atlas-polar-company/mixv...

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

I've had pea gravel discharged into basements and crawlspaces via concrete truck before. It's actually pretty slick.

RE: deep crawlspace in SC home below water table

Pea gravel at this location wil have drawbacks. The year-round high water table means that gravel-filled "hole" under the house will contain a significant pool of stagnant water... with gravel in it. Soil will reduce the quantity of ground water and provide a certain amount of isolation between the ground water and the above-ground crawl space.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: deep crawlspace in SC home below water table

Mr. Slide Rule: I differ. Soil has much higher capillary rise than pea gravel. With several feet of pea gravel above the water table, there is much better possibility of lower humidity in air above, as compared to "soil". Who cares about the quantity of water when its the surface elevation of that water that affects humidity of air above.

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

oldestguy - I won't dispute your opinion, you have far more knowledge of soils than I do.

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.

Quote (SlideRuleEra)

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.
It is very difficult to keep the humidity in the crawl space air under control.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: deep crawlspace in SC home below water table

Farm house in western New York State, where on occasion can get hot and humid, but not like further south.

RE: deep crawlspace in SC home below water table

Mayfield,

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

Hmmm... Soils may not be my strongest field, but I'm pretty good with both geometry and geography... South Carolina geography in particular:

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:

Quote (Mayfield571)

I have a portion of my crawlspace, 19'x12' that is dug out 7 feet below ground surface.
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.

Quote (SlideRuleEra)

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.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: deep crawlspace in SC home below water table

Tertek: Now comes time for some discussion. Purpose of the filling is to avoid the cost of frequent pumping. As in any engineering venture one should balance the costs and the benefits. So some input might be: the estimated time the owner will stay there. The cost of electricity and pump maintenance vs cost of concrete or pea gravel or sand soil. As to settlement, wasn't this area excavated in order to build? If so, that was unloading and some replacement loads should be of minimal consequence. Moisture possibilities with the various alternatives can be argued both ways, especially compared with past problems or lack thereof.

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

Oldest guy, very good points, indeed.

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

A few points. Slide Rule Era reminded us the site likely is sand, so rebound is not likely. A home owner getting his "advice" via a WEB site is not going to sue any one.

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

I enjoy the use of the term "buffulo". We also find that lawyers and also doctors are the worst for slow pay / no pay. Also, good point about the rebound in sand, assuming it is, in fact, sand. The thing about giving geotechnical solutions on the internet without boring information - it could be a never ending discussion with mostly non-applicable stuff getting thrown in. But I did learn from your input. I actually don't do much work with sand as it is not common in my area.

RE: deep crawlspace in SC home below water table

(OP)
Thank you all for the input. I am still awaiting a visit from the geotechnical. He has rescheduled on me twice. In the meantime, I have had a structural engineer take a look. He advised the same as many of you though he did ask a few questions in which my answers seemed to help with. A little more background on the house. The main house was built in 1953. The addition which is where the deep crawl space is dug was added on about 10 years later or so as best we were able to estimate from speaking with neighbors and researching permits. The only thought as to why this was dug out is that there was a bit of popularity around that time to build shelters for war times. I don't know if that is the reason for it or not, just a thought that was passed on to me. Also, there was not a sump pump for a period of time prior to us purchasing the home. The pump up was installed approximately 3 mos prior to our purchase. Depending on the time of year and the rainfall amount there would be some standing water in this section of the crawl space. The sump pump was installed by the bank who owned the property through foreclosure.

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.

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