Water Blogged

Kwanjora Primary Catchment Tank

Posted by Steve Hall on June 7, 2019

Kwanjora Primary School is located in Nyandarua County, Kenya. The climate is classified as semi-arid tropics with two rainy seasons (March-May) and (November-December). The school population is made up of 153 girls, 145 boys and 11 teachers.

Before this project, Kwanjora Primary School had one old masonry tank and one plastic tank which stored water for only one month after the rains. Thus pupils fetched water from a dam 500m from the school which posed a risk for the pupils. Other times, the pupils would carry water from home for drinking, washing hands, and dusting their mud-floored classrooms.

The new masonry tank was constructed using dressed quarry stones (12″ x 9″ x 9″ and 12″ x 9″ x 6″), which were used for the tank walls. Each masonry layer of stone is reinforced with D10 twisted bars. After completion of the tank walls, plastering was done on the inner side using a 20mm thick mortar of 1:3, sand/cement ratio. A similar mortar mix ratio was used for the 20mm thick tank floor plaster. Then the dome-shaped reinforced concrete of the tank roof, including an 18” square manhole, was constructed. The form work for dome-shape roof was made of timber (to hold concrete and reinforcement bars) and held by posts/poles from the inside. The water drawing points, which also serve as a hand-washing facility, were extended to about 6m from the tank with two taps to serve the pupils more effectively and reduce damage to the tank by pupils. The tank outlet was also fitted with a lockable manhole, which also provides the tank drain-off plug – used during periodic tank cleaning.

The following is an outline of the tank construction process (steps and activities) in a chronological order:
1. Site clearance and foundation layout
2. Excavation of the foundation
3. Laying and binding of hardcore
4. Construction of the concrete slab reinforced with heavy gauge weld mesh and D12 and D10 bars
5. Construction of tank wall, using single masonry layer reinforced with barbed wire placed after every 2 layers
6. Internal plastering of the wall and floor. Chicken wire was used to reinforce the internal wall
7. External plastering or keying the masonry joints
8. Construction of the dome-shape ferro cement roof, which start with setting up the timber form work, followed by laying of the reinforcement (using heavy gauge weld mesh, Y8 bars and chicken wire) and central pillar to hold the roof
9. Construction of water draw off points, including drain-off outlets
10. Painting of the completed tank, signage for visibility and fencing around the tank

After construction of the tank, the guttering system was installed. This involved fitting the roofs with fascia boards made of 8″ x 1″ timber along the roof edge. The 2m long gutters (4″ x 4″ x 5″ (to allow 1” overhang to reduce overshooting of runoff from the roof)), were held by metallic hooks fixed on the fascia board. The gutters were laid in such a way to allow the flow/conveyance of water from the furthest end into the tank, through the foul-flash system. The foul-flash system is necessary to allow dirt water from the first rains (after a dry spell) to be diverted from the storage tank – a simple self-cleaning system – to improve the water quality.

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