Draining the Mega-Footing
Stockwell
| Posted in General Questions on
I have gotten my engineering drawings on my new house(Zone 4, NC, steep slope). In places, the footing for the uphill concrete wall is over 13′ wide! I have attached a representative cross section. How should I handle drainage? I plan to use a spray on membrane with a dimpled drain over that. How far down should that go past the footing/wall junction? Where should the drain-tile go, still at the edge of footing?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Kevin,
First of all, questions about drainage should be directed to your engineer.
On such a steep slope, drainage matters -- but not necessarily for the reasons you think.
The footings for a basement need drainage to make sure that the basement stays dry. But it looks to me as if all of the rooms in the building in your drawing are above grade -- so the footings don't need drainage.
That said, you still need a site plan that depicts how you will handle the water that runs down this steep slope during a heavy rainstorm.
I was just looking for some insight from those who might have dealt with this before.
You make an interesting point. However, if water leaked through the footing, it would enter the crawl space which would not be a good thing despite encapsulation. I imagine it would work its way down to the downhill wall as well.
Kevin,
It's hard to tell from the drawing whether the area with the triangular cross-section under the lowest floor of the building is outdoors (which would indicate that this is a pier foundation) or indoors (which would indicate that this is a crawl space foundation). Your most recent comment tells us that it is a crawl space.
Also, would a capillary break do me any good in this situation? There will be soil on the crawl space side of the wall which might negate any benefit from a capillary break, but then again, the soil on the crawl space side should remain dry(drier?)???
The drawing is a bit confusing as to where the grade outside the crawlspace will fall as well as the grade inside the crawlspace. That is assuming the crawlspace is the triangle area under the lower truss joist.
Given the astounding footer width on the upper wall I suspect that fill is intended to be place against it for nearly the entire height. The object dangling to the left near the top of the very thick wall profile appears to be a concrete pad for the front entry(?). It would certainly appear that a substantial drain path would be desirable for the up hill wall. Martin notes quite correctly that the engineer really should be the one to advise on drainage. Local soil conditions will dictate how water behaves, but it is possibly instructive to remember that sponges hold water over their entire profile until full. Over saturation will cause water to flow out wherever a cut in soil layers make for an easy exit. Like a soft sponge layer sitting on a hard sponge or table.
Where I live the soil is very stratified as layers of sandstone and soils of widely varying porosity. Even relatively small slopes contained inside a crawlspace can end up with water weeping out and collecting against the downhill wall; a condition I was warned about when I first proposed doing a full basement. Also a very hard condition to correct after the house is built. In my particular case, the lot slope was almost four feet across the footprint. Water from up-slope on the lot generally soaks through about five feet of mixed profile then hits and travels most quickly along the sandstone bedrock at that point. I have a full drain system around the footing perimeter for that upper soil born water. The entire footer is set on the bedrock, so inside the foundation perimeter I insisted on all washed stone under the floor slab to level the pitch presented in the bedrock.
Surprisingly, the water that does collect below my basement slab ( drained to light on my down slope corner) is not directly from the upper soil levels, but rather from the fissures in the bedrock. Water is very persistent and will make its way into and through most any layer. Originally the foundation person wanted to use compacted fill with a small layer of stone under the slab as a capillary break. However, despite everyone thinking I was being weird, I had washed stone the whole way. If I had dirt fill, the sandstone would weep water which would mostly soak in and pretty much stay. The stone acts as much less of a sponge, letting the water move on through. At any rate, I have the dry basement of my dreams albeit on three levels to accommodate the bedrock.
At risk of being wrong in my interpretation of the drawing you supplied, I would suggest that at minimum you will want to have a full width drain in stone against the inside of the second down slope wall exiting to a lower point down slope. I will leave it for you to know if there are HOA or wetland concerns for where the water goes. I would recommend capping the crawl soil with plastic sealed to the walls like a radon installation. Vents in the foundation would very likely not be able to handle the moisture load soil can produce, especially in a warmer, more humid environment like NC. I suspect the difference in soil moisture levels inside the crawl would pretty much match the outside given the slope all around you. If you are actually on one side of a knob like hill maybe it could be drier. If you are on a long slope then I think it is more likely water moving in the soils would equalize the values. Think again of a sponge and attempting to keep a small spot drier.
As for the up slope grading, I would suggest as much planning as possible to direct water away to the sides, around the house site. Hard surface collectors, like driveway, roadway and roofs all need to be reviewed carefully for water exit points. Obviously downspouts should get their own drain pipes separate from footer drains. A mid-depth drain set to the left of the up hill wall and exiting well to the sides of the footprint might be warranted if the soils are very quick to pass water. Waiting for all the up hill water to get to the drain at the bottom of the first wall might be too much. Budgeting it in now might be a lot cheaper than having to disturb walkways and landscaping in the future should the dimple and footer drain not suffice.
Hope this rather long discourse has been of some help.
Roger--I appreciate the long response! The grade of the crawl space will be roughly the same as the grade outside the footprint of the house.
The crawl will definitely be encapsulated in plastic. We have already started to plan the drainage as you suggested, diverting water from the road/driveway well before it hits the wall of the house.
I have attached the foundation detail drawing from the engineer. I don't get why drain tiles are 4' above the footing. Seems like a quick way to get 4' of water against the footing! He also specifies a crazy amount of rock and virtually no soil. That's very bad for my pocketbook and for my gardening aspirations. I have a bunch of questions in to the engineer. I appreciate any comments from the experts here.
oops--here is the file
Mr. Spellman, Hope you don't mind my adding another inexpert two cents worth. The engineer may be aware of just how much water your local soil can pass. The sorta midline drains correspond with what I tried to describe. If anything I would like to see another bigger drain line at the footing/wall intersect to alleviate pooling below the interior crawlspace soil line as you seem to be continuing the down slope inside the crawl space after a short "level" section. I would also urge that a drain be placed against the inside downhill wall/footer intersect to take care of water that will be passing under the mega footer. Maybe it is specified, but the section posted doesn't show that area. As a mental exercise, imagine pressing a cookie cutter (your foundation) into a sloped wedge of foam that you then saturate with water. Then try to imagine the area inside the cutter remaining perfectly dry.
I agree the amount of washed stone is dismaying. You are sort of faced with the vertical equivalent of what I faced with my sloping bedrock mess. I am a bit puzzled by the compacted in 12" lifts note as I was told by my cement contractor that washed stone couldn't or needn't be compacted. Consequently leveling my basement hole for the slab came out as a near wash in cost since we eliminated the man hours running a compactor on areas at three levels. Topping with enough stone to form a capillary break under the slab would have still meant dumping and raking stone so dumping an ungodly amount in and raking it the once balanced out. I won't go into all the work I did setting 3" foam over all the stone across three levels or the radon pipes I set too. By the way if you have a radon problem in your area, do put the pipes under the sealing plastic sheet scheduled for the crawlspace. If you need more on that, ask and I will scare up some notes on what was done on a different house I renovated.
The stone, aside from providing a massive gap between soil and foundation, may be the only practical way to fill in the excavation without enormous fussing with special fill and compaction. More immediate on my radar is the rather extreme cut angle shown. The footer trench will be almost 12 feet below the sloping cut shown and not many soils can be cut that vertically without risking collapse. Particularly if heavy rains visit you during the excavation process. They always will and I had to pump out my cellar hole four times before we wrapped up the drain lines. Anyway, for insurance reasons and safety, be sure to verify who is responsible and liable for a cut that deep and steep.
Whether or not you really need to have stone all the way up as high as shown is a good question to pose. The soil thickness shown is about 8" or so and grass might do okay, but as you note, not shrubs. If the surface run off is being handled well, I can't see why the "topping" layer over the stone can't be more like 3'. May not saye much if any money but you will be able to landscape a bit more freely and without the plants going dry super fast.
One thing puzzles me greatly. What will you be doing about the grade line on the exterior side of the foundation? The original slope line is shown, but what happens at the corners of what I am guessing is a rectangular plan form. Will there be wings off the 16' tall wall that controls the soil? stepped terracing? Just asking in case there might be some other issues to address now before the answer becomes one of the dreaded un-budgeted add-ons.
Lastly, I think the people on this site/forums are not big fans of bitumenous waterproofing. It's your pocket book but the rubbery versions might be worth considering. The cure time before application might be a controlling factor, I am not sure. For my part I used a product called Xypex which is mixed into the concrete. A bit pricey, especially when adding it to 50-60 yards of concrete. I also have Grace watershield membrane over my exterior foundation insulation so my belt plus suspenders may be overkill not to be repeated. One thing I was told about Xypex additive; it is approved as a substitute for basement waterproofing coatings in some localities. You might also want to look up the bumpy dots membrane stuff. I can't recall the brands. Not sure how that much rock will interact with the drainage board specced.
Kevin,
When I looked at your first drawing, I guess I wasn't paying attention. I thought this was a house on piers, and I missed the fact that the lowest level of the house is below grade, with a concrete wall on the up-slope side.
This is a very tricky site. Crushed stone is the right type of backfill, although Roger is possibly right that you can get away without compacting the crushed stone. My guess is that the engineer is worried about settling because of the small slab at the entry door -- the slab on top of the 10 feet of crushed stone.
Anyway, this is all tricky. A miscalculation will result in all kinds of problems, so I'm not going to second-guess your engineer.
Ok--talked with the engineer. I got the amount of stone greatly reduced to just the area straight up from footing. That cuts it down to a mere 450 tons or so. He said the compacted crusher run stone is essentially impermeable and it would be useless to put a drain tile at the footing. I don't know if that is true.