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Realized by the force of the wrist, this buried cistern allows to recover 15m3 of rainwater to water the garden, to feed the washing machine and the toilets during almost all the year!

A rainwater cistern

Difficulty: Confirmed
Cost: 2 400€
Time: 1 month (excluding concrete drying and bad weather)
Equipment: shovel, pickaxe, trowels, buckets, metal rulers, wheelbarrow, concrete mixer, spirit level

Tank and workshop

Claude Dumont's project was twofold: build a cistern to collect rainwater and build a workshop just above. After digging a hole 4.6 x 6 m and 1 m deep, our reader has set up a formwork to delimit the floor area (3.70 x 5.20 m) needed for his future workshop. Inside, he dumped all-out to form a hedgehog that he compacted. After laying a welded wire mesh, he set up a vertical peripheral reinforcement (folded and ligated to the lattice reinforcing bars). A concrete slab (dosed at 250 kg / m3 and waterproofed in the mass) is poured on 15 cm thick.

Blocks to bancher

Blocks to bancher

While the concrete slab dries (20 days), the trenches are dug to accommodate the pipes and ducts connecting the cistern to the rainwater collection. Bancher blocks are used for the walls. The first row sealed, a waterproof concrete screed is then poured (40 mm of thickness). The following blocks are laid with bastard mortar, scraped and filled with concrete until they reach the height necessary for the high floor, made with prestressed concrete beams and slabs *.

Waterproof plaster and casing

Two holes intended to visit the interior of the tank are preserved and covered by a concrete slab (50 mm of thickness). Two layers of waterproof coating are applied to the outer walls of the tank to protect them from infiltration (Sika Foundation Mortar). Inside, two layers of pool plaster (Sika) ensure the tightness of the walls of the tank.

Digging the search

To avoid damaging the garden, the excavator was excluded. The excavation was therefore dug with shovel and pickaxe, despite the rain and the clay nature of the terrain.

Lay the welded mesh

Lay the welded mesh

The welded mesh is laid on shims 5 cm thick so that they are perfectly coated with concrete. To bring the concrete to the center of the excavation without stepping on the reinforcement, Claude Dumont has manufactured a dump chute in formwork boards.

Pour the concrete slab

Pour the concrete slab

The water-repellent concrete slab is 15 cm thick. On the outskirts, we see the concrete spacers spaced 50 cm for the vertical reinforcement of the blocks to bancher.

Arrange and cover the electricity and water ducts

Arrange and cover the electricity and water ducts

The technical ducts (red for electricity, blue to pass the pipe that will be used to pump water from the tank) are placed at the bottom of a trench 80 cm deep. They are covered with sand (10 cm thick) and a warning grid.

Driving and evacuating rainwater

Driving and evacuating rainwater

The tube (PVC Ø 50 mm) which conducts the rainwater to the tank is laid with a slope of 1 cm / m. The overflow pipe is laid with a slope of 0.5 cm / m towards the eye.

Seal the blocks to bancher

Seal the blocks to bancher

The first row of boulders is sealed around the edge of the slab. A waterproof concrete screed (4 cm thick) is then poured.

Mason the tank

Mason the tank

The walls of the tank are bricked and filled with concrete. Prestressed concrete beams are based on the last row of blocks. After reinforcement, planelles are sealed on the bank to form the concrete slab.

Ensure a constant level

Ensure a constant level

Inside the tank, you can see the float made from an aluminum tube and a piece of polystyrene. The overflow (pipe not visible) ensures a constant level.

A pump to distribute the water

A pump to distribute the water

Protected in an independent compartment, a surface pump distributes water. On the pond side, it is connected to a strainer.


Video Instruction: Culvert Cistern Tour - Rainwater Harvesting DIY