Draintile for concrete block wall...where to place?

[hydroponder]

I am rebuilding my garage and am wondering what is the ideal depth to place the draintile. The existing garage is completely demolished and I am getting concrete today. I have an alley garage and I walk about about 6 feet to my back yard to the east. The north wall will also be buried approximately 6 feet. I also have a wing wall on the south wall that heads approx. 6 feet east toward my house that will be used for the steps. I am planning on placing draintile along this wing wall, then along the east wall and also along the north wall and then daylighting it to the alley at the northwest corner of the garage. See the attached PDF for a better idea of what I have going on. I am wondering what depth I should place the draintile. A couple things to consider: the alley at the northwest corner is about 1 foot above slab grade and this is the only spot I can daylight the draintile. Should I place the draintile as low as possible and then use crushed stone (or pea rock?) filled up all the way to near ground level? Or just use about a foot of crushed stone? Or, am I better off placing the draintile a couple feet below grade?

To me, I think I should place the draintile as low as possible, and then fill a 1 foot trench with crushed rock up to about a foot below finish grade. Then I will probably put some fabric over the rock and fill the remainder with topsoil for a planting medium. Can somebody either confirm this or recommend a better solution? Thanks.

 

[ACtrafficengr]

Have you considered using a sump pump?

Do you have frost-susceptible soils?

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

 

[hydroponder]

I did consider a sump pump, but opted against it. This is a detached garage and am hoping to keep it as simple as possible. The concrete is being poured as I type, so that option is off the table. The soils are primarily silt, with some areas of sand, so yes, the soils are frost-susceptible. And, I am in Minnesota, so we get some pretty heavy frost. The plan I attached is all but set in stone, so I need to pick the draintile location that will be the most effective at moving water away from the foundation walls.

 

[ACtrafficengr]

I'm a traffic engineer that doesn't often get below the yellow paint, but you've got no frost walls, finished floor below grade, deep frost and frost susceptable soils.

Bozhe moi

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

 

[MiketheEngineer]

Put as low as possible and still drain to day light.

Put crushed rock ~3/4'' about 4-6'' below and on top within about ~6'' of grade. Put geoteck "sock" on tile FIRST. You could even put a layer of poly over rock and under soil - if you think you will be there more than about 20 year.

Fill in with soil and grass....

 

[OldPaperMaker]

hydroponder,
Did you get a building permit for this structure? A few things that you might want to consider:
1. On the west side of the building the footing is an 8" thickend slab. If you are in Minnestoa I think that the frost depth is 4'.
2. On the north & east sides of the building the CMU walls stop at 1'-8" below the floor joists so they have to act as a cantilevered retaining wall instead of like vertical beams spaning between the floor slab & the upper floor joists. The footings shown won't act like a heel for a cantilevered wall.

 

[hydroponder]

I appreciate everyones concern regarding the wall, footings, frost, etc. Believe, I would have designed it differently if the space would allow. I do not have room to excavate for frost footings or I would have put them in. I did get a building permit and in this area of Minneapolis, MN, you build what you can with the space provided. This typically consists of a thickened slab (floating) as shown. When I was designing the garage, I put the 1'-8" wall on the north and east side soas to prevent the lateral earth pressure from tipping the hole structure, which I think would be more likely if I were to attached the floor joist directly to the CMUs. This short wall should allow some lateral movement without tipping the whole wall. This made sense when I was designing it, but I am not a builder so let me know if you think this is amiss. I am using 12" block with core fills every 4 feet and a bond beam. I am doing what I can with the knowledge that the structure might move some.

So, despite the poor overall design of the structure, what is the ideal location for the draintile to minimize the chance of foundation wall failure?

 

[cvg]

surface drainage may be a bigger concern. it appears that your house, roof and yard all drain towards the garage, putting a lot of water on the east side of the garage. grade to drain that water away from the garage and not into your new foundation drain.

 

[OldPaperMaker]

With 7' of silt/sand backfill you will get around 45 pcf of lateral soil pressure pushing against the wall. With a 22' long bond beam on the top of the wall I suspect that you will push the wall over when you backfill the wall. I suggest that you check out running the CMU wall up to the upper floor diaphragm on the north & east walls.

 

[hydroponder]

OldPaperMaker,

Are you suggesting placing the floor trusses directly on the CMUs? It doesn't seem to me that this will add much, if any, strength the wall. Am I missing something here?

 

[JAE]

hydroponder - I'd agree with OPM above - this design isn't very good. Very bad in fact. The lateral earth pressure (whether you drain it or not) will push the wall sideways and there is no structural mechanism to resist that other than horizontal bending of the top portion of the wall.

Normally, any wall that has earth against it must resist that lateral earth pressure. The horizontal forces, usually, in a basement wall are resisted by vertical bending in the wall - the floor of the basement and the floor of the house act to resist those lateral forces.

In your case, you do not have anything at the top of your block wall to push back against the earth. Thus, the wall has to span horizontally instead of vertically and the distance appears way to far for typical 8" block to span - even with a bond beam on the top course of the wall.

I'd suggest you get some help on this as you are going to have problems with it.

 

[OldPaperMaker]

hydroponder,
No, I am suggesting that you build your CMU wall up to the bottom of the TJI's (wood I joists) that are supporting the floor above the garage. Once you put the sheathing on the joists the floor diaphragm will become a horizontal beam that can be used to resist the lateral load from the soil at the top of the wall.

Of course, you have to design the anchor bolts to plate, diaphragm chord and plate to joist connections.

 

[hydroponder]

I am supposed to be getting the block work done on Thursday of this week, so I guess I better get this dialed in asap. I am thinking of having my block guy do two extra courses of block on the east and north wall so I can attached the I joist to the bond beam (with a plate). I will have him put anchor bolts every 4 feet in the bond beam, attached a 2x6 treated plate and attached the I joist to the plate. Should I just nail the I joist or should I use some type of joist bracket?

For those that are curious, I demolished the existing garage that was there. The reason for this was the east wall was failing. This wall was a 4" to 6" poured concrete wall. It only lasted about 100 years, with no rebar or any other type of reinforcement. It was framed with 2x6 walls, 2x4 trusses and 2x8 floor joist. I think the new design is a bit of overkill, but I don't want to have to replace it again. Attached is a picture of the old garage during demo. Again, this lasted for about 100 years.

Back to the original question, what's the ideal depth to place the draintile?

 

[OldPaperMaker]

hydroponder,
MiketheEngineer answered the question about drainage.

About the old garage, it looks like the east wall was supported by studs that were balloon framed from the garage slab to the upper floor level. These studs were probably what kept the concrete "shoring" from pushing the wall over.

You should heed JAE's advice and get a structural engineer to design this entire new structure for you or drainage will be the least of your problems.

 

[oldestguy]

Not sure if you have covered this, but your use of silty soil for backfill will not only cause an earth pressure due to its weight, but that stuff will heave sideways pushing against your wall much greater than any earth weight, even if you have a drain at the bottom. By the way, one sized gravel ain't a good backfill for permanent drainage , since it can plug up easily.

So, how to minimize both of these things. Well, first all backfill within about what you show on your cross section should be non-frost susceptible granular soil. Then, to avoid plugging the drain, use a clean sand or a clean sand and gravel. That will not heave and it will not plug the drain. The sand fraction of the sand and gravel acts as a filter against silt movement with water flow.

I'd also opt to seal the ground surface and divert surface water around the garage with a swale up there. A clayey topsoil would help a lot, even though it will heave some.

On holding that block wall agaist earth pressure, well, if you had the chance, built it as a mechanically stabilized earth wall (MSE) (do a search for that in Google or here), with geo-grid laid back into the soil towards the house. Then it is designed to hold that earth pressure. Even though the wall may be up, as long as it is not backfilled, you might be able to make it work.

so you see it is not only that drain pipe that you need help on

 

[aeoliantexan]

As others have said, you have a lot of potential issues. Here are some of my thoughts:

You said the alley at the NW corner is above slab grade, but there must be some place lower than the floor slab or surface water will stand in the garage. If so, put the drain tile along side the thickend slab below the floor elevation and take it to the low point so it can drain. Put a heat tape inside and insulation outside to keep it open during cold weather. Or run it to a dry well (a hole full of gravel - deeper than frost depth) if regulations allow.

The second floor diaphragm may not resist the lateral soil pressures because you have two walls with no backfill and the front wall is nearly all door. I'm with Oldestguy on this; get some good geogrid, lay it into the block joints and run rebar thru it, make it as least 8 feet wide and use plenty of layers. It won't work well unless you compact the backfill, but just use a gas-engine wacker and don't try to compact within 2 feet of the walls. Or you can use crushed stone for all the backfill. Do the same with the wing wall and detach the wing wall from the garage because it is going to move.

I like concrete sand-gravel mixture for backfilling around the drain and up the wall to about 12 or 18 inches from the surface. Be sure at least 25% by weight is particles larger than the perforations. If you do this, you shouldn't use a sock.

Frost will penetrate the backfill horizontally several feet. If you put two inches or more of styrofoam against the wall before backfilling and the sand-gravel is at least two feet wide, you might be OK as far as frost pressures against the walls. Keep the styrofoam well below the wood framing and siding because termites can use it to access the wood.

Your floor slab and exposed footings are likely to experience frost heave. You can bury more styrofoam sloping down outside the footings to reduce the exterior cold source. The only way I can think of to prevent frost under the floor is to insulate and heat the interior. Your wife will love getting into a warm car with clear windows, so this will pay for itself.

Here's the disclaimer, my friend: these are concepts to consider. You need local professional advice for the specific dimensions and details.

Good luck