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cistern block mortared walls

Summer 2015: Rainwater Cistern Construction

light-posts-comparisonBefore delving into the big project of the summer, the rainwater cistern, I decided to finally bury the electrical and network wiring running to the cabins in flexible conduit.  I also put in 4 cedar light posts that also act as junction boxes.  The lights are attached to a light activated timer that turns the path lights on for a couple hours after it gets dark.  There’s also now a light in the composting toilet that’s got a timer switch so it can’t be left on.

Eric helped a lot with digging the trenches for the conduit.  The project cost a lot more than I’d hoped.  Thick copper wiring and conduit are expensive.  They also get you with all of the connector pieces.  Outlets, junction boxes, a breaker box, breakers, etc. It all added up to a pretty penny.

cistern putAfter that it was on to dealing with gaping mud pit that the cistern hole had become after 3 years.  Last Fall we spent a lot of effort cleaning it up again only to have the monsoons this Spring deluge it and cause the sides to collapse.  Any hope I had for letting it dry out to make work easier evaporated unlike the water in the foot or more of clay mud at the bottom, so we sucked it up, pulled our shoes off and jumped into the muck.

It took Eric and I probably 2.5 weeks of hard work to get it all cleaned up and leveled out.  The next step was laying sub base gravel for underneath the concrete slab.  The cistern will hold almost exactly 4,000 gallons or 16 tons of water, not to mention the weight of the concrete-filled cinder block walls and wooden cap, so having a thick sub base and thick foundation slab were important.  We tamped down and leveled 8″ of 1″ minus gravel (~10 tons).

cistern leveled with sub baseNext week built a square form for the slab using 2x8s, bracing the corners and putting stakes in to hold the sides in place.  We had to get it pretty level for the concrete, and we also needed to leave as much space as we could around the outside to remove the frame and be able to maneuver.

We then used a couple of pipes and an angle grinder to cut and bend the 1/2″ rebar so that base and the walls would be tied tightly together. We used broken bricks and rocks to hold the rebar up off the ground so it would end up roughly in the middle of the slab.  We used wire to hold the rebar together where it crossed and held all of the ribs upright by laying down a piece of rebar on the ground with screws in it that we could tie each piece of rebar to.

pouring cistern slab concreteAt last it was time to call in the cement truck.  We had to drag a bunch of tables and other things out of the way and clear out some brush to make a path for the truck.  I believe I calculated that we’d need 3 yards and it turned out to be almost exactly right, with just a little left over to spare.

We let the slab set for a week or more before we started in on the block walls.  I should have gotten concrete blocks not cinder blocks since cinder blocks contain fly ash which is toxic, but I didn’t realize it until I’d already bought, hauled, and unloaded all of them.  To get around that I’ll be putting several more coats of potable cement coating than I’d planned and then do testing before I use the water.

cistern block mortared wallsI’d never mortared blocks before and Eric had only watched, so the first few courses were slow going.  Getting the right amount of mortar on the blocks to keep it level without too much overflowing, and also getting the mortar to be a good sticky consistency were the main tricks that we had to get worked out.  We used Type S mortar, which is essentially a mix of cement and hydrated lime.  It probably took us 3 weeks to do the walls.

Ultimately I believe we used about 8 90-lb bags of mortar mix and a pick-up load of masonry sand.  I believe we used about 300 cinder blocks altogether in the 12 courses of the wall.  We turned 2 half-blocks on their side, one a couple feet down which will allow the 3-inch PVC inlet pipe in and the other is in the top course for the 3-inch overflow pipe.  I also used a masonry bit and drilled about a 1-inch hole 2 feet down to run the outlet pipe from the cistern into the common house where it wouldn’t freeze.

cistern cement paintOnce the blocks were in place we filled in all of the block holes with concrete that we hand-mixed.  It took us 4 days to mix and fill all of the blocks.  We also placed some anchor bolts as we went to attach the wooden roof cap later.  I also went around the outside of the cistern and put 3 coats of potable cement paste on the outside of the cistern to minimize any toxins leaching out from the cinder blocks.

Next we back-filled dirt around the cistern to about 2 feet below the surface.  The frost line in our area is about 20 inches, so I want to make sure there’s good drainage down to that point to avoid wet soil freezing and pushing against the cistern, potentially cracking it.  Once we got the ground back-filled and tamped we made sure that it was all sloping down towards the lowest corner and then dug a trench from there downhill and put in a loop of 4-inch perforated drain tile around the cistern to collect water and then non-perforated drain tile to carry it from the lowest point out and away downhill.  At this point in the year Eric’s time was up so I carried on by myself for a few weeks.

cistern backfilled with insulation, gravel, and drain tileI used some 2″ extruded polystyrene boards that someone gave me to insulate the outside of the cistern from 2′ down and up to the top of the cistern.  This is to help keep the water from freezing.  After getting the insulation in place I back-filled with 1″ clean gravel on to the drain tile so water would drain down.  Once I got that tamped down to a little above ground level I put landscaping fabric down and then piled up dirt against the side of the cistern.

That’s as far as I got with it this summer.  I laid some boards across and then put plywood on top of that with an empty 55-gallon drum on it and then tarps laid over that so the rain and snow would run off.

The plan for next year is to build an insulated wooden roof cap over the top, which will essentially act as the floor of a gazebo.  I’ll put several cedar post timbers in and then a gable roof with galvanized steel roofing on it.  This will protect the roof cap and also provide a nice flat outdoor space for people to do yoga or other activities.  I’ll also add some retractable clotheslines.

After that I’ll need to add the system that filters the water coming off the roof by filtering out bigger debris like leaves and sticks, and then dumping the first 20 gallons, which is the dirtiest.  Then the water will go through a couple of fine screens before entering the cistern.  I’ll describe that system more in a future post.