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.

 

[Terratek]

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.

 

[oldestguy]

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/mixveyor-concrete-conveyors.html

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?

 

[jayrod12]

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

 

[SlideRuleEra]

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.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[oldestguy]

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!!!

 

[SlideRuleEra]

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.

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.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[oldestguy]

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

 

[Terratek]

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.

 

[SlideRuleEra]

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:

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.

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.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[oldestguy]

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.

 

[Terratek]

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.

 

[oldestguy]

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.

 

[Terratek]

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.

 

[Mayfield571]

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.