Crawl Space Drain Questions

sequoia_2007December 25, 2007

I would like to install a drain system to intercept storm water once it makes it under the 12" x 30" spread footing for the retaining wall that separates the crawl space under our town home from our garage. We have been trying to get our HOA board and management company to come up with a solution to intercept the water before it enters our property, but we have not been successful in getting them to address the issue and we have to do something internally to protect our property since the winter rains have arrived.

One concern is to not relax the soil in front of the spread footing. Another is negotiating the piers for the posts. I only have about 6" between two of the pier footings so I am very limited as to what type of product I can use if I want to run a 24Â long trench drain parallel to the spread footing.

Because of the piers and their footings the closest the 24Â trench drain can run parallel to the spread footing is 30". We are thinking of using the 12" product from MULTI-FLOW It only requires a 4" wide trench, creates minimal spoils and uses coarse sand instead of drain rock. Anybody use this product before? Are we approaching this correctly? Is a 30" spacing between the spread footing and the 12" deep by 4" wide trench drain sufficient? I donÂt want lateral displacement of the retaining wall when the soil underneath and in front of the spread footing is saturated with water.

In addition to the drainage trench, we have already created a gutter for the spread footing and retaining wall. Based on evidence from last year, we believe one of the areas water enters the crawl space is through the mortarless vertical joints in the first course of blocks and where the sanitary drain penetrates the retaining wall. The gutter will collect the water from these areas and divert it to a basin to be pumped out.

This is what our retaining wall and spread footing looked like after we removed the soil on top.

We hired an excellent landscape company that does concrete and drainage work and I had them chisel out all of the over pour to expose the mortarless vertical openings and create a base for a concrete gutter.

This is the finished product.

Thanks for taking the time to read my post and consider my questions.


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I don't understand why you can't run a drain on the exterior of the foundation. Your second last photo shows a sump with an incoming pipe--from the exterior?

Besides filling the missing mortar, I would consider a slurry coating of a crystalline waterproofing. Anything else will only retain the water in the block.

However, as with the drain, the ideal way would be to waterproof the exterior of the block wall, again with crystalline or any of various impermeable flexible barriers.

    Bookmark   December 25, 2007 at 10:55PM
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The pipe you see drains the interior gutter and there is no way to access the other side of the retaining wall without taking out the garage floor and then digging down 4 feet to the base of the retaining wall footing. BTW, this retaining wall runs continuously through 11 units and is located 20 feet back from the front of the units. Our property is located at the lowest point in the townhouse complex so it is at the highest risk for subsurface water infiltration.

Since we have installed the gutter I donÂt think we need to fill in the missing mortar or waterproof the wall. The gutter should be able to collect this drainage. My main concern now is the water that will travel laterally under the retaining wall spread footing and surface in the crawl space in front of the spread footing.

    Bookmark   December 26, 2007 at 2:35AM
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townhouse complex That explains a lot!

Do I have this correct?

What you're calling a "gutter" is a weeper on the interior of the retaining wall. (That seems to be a particularly odd arrangement, btw.)

In addition, you have created a spillway on the interior perimeter of the crawl space on your unit. Now you want to drain this water off by diverting it to a sump and pumping it out.

But since you say "we believe one of the areas water enters the crawl space is through the mortarless vertical joints in the first course of blocks and where the sanitary drain penetrates the retaining wall" I'd still correct that. It's always better to keep the water outside the home rather than trying to collect it inside. Then proceed as you've described.

If you're getting a lot of water, I would consider running this by a soils engineer for confirmation that you're proceeding in the best way.

This may be another illustration of the downside of condominiums: a poor design of a common element causing a single unitholder problems that the other unitholders don't feel obligated to address.

    Bookmark   December 26, 2007 at 10:50AM
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Thanks for taking the time to respond.

I have found many examples on the Internet of water collection techniques on top of footings used to direct water to a basin to be pumped out. My so-called gutter does exactly that.

Regarding not sealing the mortarless joints, I'm concerned the hydrostatic pressure on the other side of the retaining wall would become to great and damage the wall since the water has no other outlet.

Let me provide more detail on my situation.

The natural grade under our property is from the front of the street to the rear of our property. I suspect one of the conduits for water is the sanitary drain trench that is lined with sand and runs from the street back to our retaining wall where the sanitary drainpipe penetrates the retaining wall.

This is what the front of the townhouse looks like. The retaining wall in question is located 20 feet back from the garage door.

Rear of garage slab with top of retaining wall exposed.

I was able to finally get our HOA to consult a geologist about installing a curtain drain in the street to intercept the water, but the report sits on the HOA lawyerÂs desk and every time I speak with our management company president he starts off the conversation "is it your problem or is it our problem". So far there is no traction on getting the HOA to address this problem out front so I finally had to try and do something internal to my crawl space to avoid complete flooding this winter.


    Bookmark   December 26, 2007 at 9:09PM
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