The cistern - insulation and top access.

Since I still haven't had a chance to draw up the details I promised in "Framing the sunroom - part 1", I thought I'd take a little detour with this post and take another look at the cistern. Since I buried the tank last summer, I haven't thought about it much - briefly during the coldest part of last winter ("I wonder if the water has frozen?" - it never did) and then again this spring when I noticed that during an unusual spell of heavy rain, ground water backed up against the access lids and leaked into the tank. Ground water leakage counts as contamination so I thought I had better do something about it.

Here is what the cistern looked like last August after its burial. After some rough grading work, the tank was insulated near the surface and equipped with some basic treated wood retaining boxes.

After backfilling the house and tank, a layer of 4" XPS insulation was installed flush with the top surface of the tank with holes cut over each access lid. I then built two boxes, ~4' square, with treated 4x4s and placed them at each lid to hold back the additional fill that would cover the foam. Before winter came, the extra fill was added over the foam and some plywood squares were used to cover the retaining boxes.

Fast forward to last week. Yet another layer of fill will be added over this area before final grading around the house. During the spring melt and the subsequent heavy rains we had this spring, there was a substantial amount of settling within the backfill - which is generally a good thing, but not very good for the grading.

A close up of one of the openings into the cistern. As I mentioned earlier, heavy rain saturating the soil around the cistern made its way up against the lid where it leaked into the tank, bringing some silty material with it. After a good cleaning of the lid area, this opening was ready to have a "riser" added to it to prevent leakage in the future. If you look closely at this photo, you can see how the settling of the backfill has bent the rigid foam downwards around the tank - in some places it cracked the foam boards. Not to worry though, even with cracks the insulation will still do its job.

A closeup shot of the edge of a riser. The factory edge of the riser is not designed to create a water-tight seal against the tank so I used some EPDM "P" gasket (BG36) that I had left over. I used some tape to temporarily secure the gasket's tongue to the inside of the riser being careful to centre the round part of the gasket over the factory edge.

Here is a view of the riser with the "P" gasket completely "tacked" on...

...And here is the riser installed over the opening. While installing the riser, care needs to be taken to ensure that the gasket remains centred on the factory edge. The riser was secured to the tank using galvanized 2 1/2" roofing screws. These screws work well for this application - galvanized for longevity, self tapping to cut through the tough poly and with a neoprene washer to prevent leaks at the screw head.

I leak-checked the installation by pouring water into the space around the riser and letting it sit for a few hours. Guess what? No leaks! Looking closely, you can see how the bottom of the riser has a small step to it which is what I drove the screws through. The screws miss the gasket on the inside of the riser to pass through the top of the tank underneath.

Now that the risers have been installed, I have to find some new lids since the ones that I have don't fit the top of the risers. I'm hoping to find some large diameter garbage can lids that I can adapt for this purpose. In the meantime, I simply covered the openings with plastic bags and secured them with some twine. Now that I've addressed the leakage issue, I'll have to pump the tank out and maybe give it a little cleaning before its ready to be used for rainwater collection.

The Logical Song - Supertramp