Trained Eye

 
 
 

Sewer Backup Prevention

Question:

My house is situated at the bottom of a hill, just above the Assiniboine River. It is really a beautiful location, except for the fact that all the precipitation that falls on the neighbourhood above us (one city block) tends to end up in our back yard. For the most part that is not a problem, either. There is now a ditch that leads the water to a city storm sewer intake, which conducts the water away through an eight-inch pipe. However, on those rare occasions (three times in 25 years) when a downpour overwhelms the existing drainage, especially when the soil is already saturated, the water will come up as high as the window wells on the house. The resulting water pressure fills the weeping tile, which discharge into the catch basin in the basement.

Our observation has been that on occasions of such prodigious rainfall, the city sewer is also taxed to capacity with the result that the existing backup valves become operative to protect us from sewer backup. While we do have a sump pit, with the requisite immersion pump, it is no match for the volume of water delivered through three, 3 inch weeping tile openings in the catch basin. The result is, on the last occasion this happened, water in the basement to the height of 34 inches.

What would be the result if we were to modify the catch basin in such a way that the water from the weeping tile would be prevented from entering the basement during such an event? In discussing this with friends, none of whom are experts, the following concerns were suggested.

Considerable damage could be done to the weeping tile because of the pressure of the water standing in the yard and in the weeping tile. Seepage might occur at the footings of the basement because the weeping tile were not performing their intended function for the duration of the event.

It should be noted that any modification made would be operative only during he duration of the emergency described. Our past experience suggests that this would last between two and four hours.

I appreciate the opportunity to present this problem to you.

Answer:

Firstly, I would like to commend you for one of the most complex and articulate questions I have ever received. It made the cumbersome task of keying the wordy inquiry into my computer, from the printed and mailed letter, much less of a chore. I am not clear on how you plan to rig up the apparatus to temporarily block your weeping tile terminations in the catch basin, but I will ignore that question and deal with the possible problems that may occur if you were successful in your attempt.

The answer may depend, somewhat, on the age and type of the weeping tile installed in your home. The fact that they are running so freely leads me to believe that they are either modern plastic weeping tile or concrete tile. Older clay tile would likely be blocked with soil or broken and only partially effective, at best. The fact that they are flowing so easily tells me that the system is working well, and you don’t want to do anything that might disrupt this important drainage collection system. The focus of your modifications should not be to stop the water from entering the catch basin, but look for an improved way to get rid of the excess moisture when a big storm hits.

Blocking up your weeping tile when they are filled due to major rain runoff may cause the excess water to flow into the soil under your basement floor and can cause damage. You are correct that it could seep through the joint between the floor slab and the footing, but it could also force its way up through cracks or openings in the slab through hydrostatic pressure. If this is severe enough, it may cause the floor slab to heave significantly, which can cause cosmetic or structural damage to the interior walls in the home.

The better approach is to improve the means of getting rid of the excess water in the rare case of a major storm. This may be accomplished by installation of a second sump pit, with a large overflow pipe from the catch basin and a much larger pump and discharge hose. This pit could also be installed in a location that redirects the water from one of the weeping tiles directly into it, eliminating a portion of the water that flows to the catch basin. Alternatively, you could store a large capacity sump pump and discharge hose in the basement, which could be placed in or on top of the catch basin during a storm, just in case of a major overload of the existing pit and pump. The only drawback to this idea is that you would have to be home at the time to plug it in and run the discharge hose outside to a location where the water could easily drain.

The bottom line is that your friends are correct that you could do damage to weeping tile system by trying to block it up during heavy discharge. It is a better approach to try and find a solution to deal with the large influx of water before it can flood your basement.

 

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