In Riyadh the underground tank is the household's water reserve when supply is interrupted or the tanker is late. It is also one of the elements most often built in a hurry: dig, walls, lid, done. The trouble is that it sits underground, so if water leaks out, or contaminated groundwater seeps in, you may not know until the surroundings are damaged or the water quality changes. This guide covers how a reinforced concrete underground tank should be built and what to check at each step.
Before digging: location and capacity
The tank's position is a design decision shown on the approved drawings, but it helps to understand what drives it:
- Tanker access to the filling point without parking on the tank or on paving not designed for the load.
- A safe distance from the septic tank and drainage lines, as required by the regulations, because any sewage leak could reach the tank.
- Away from tree roots, which can press on walls or find their way into cracks.
- Clear of heavy vehicle routes unless the roof slab is designed for them.
- Close to the pump, to keep pipe runs short.
- Not bearing on the villa or boundary-wall foundations unless the engineer has designed it that way.
Capacity is set by the engineer from occupancy, expected use and how regular supply is in the neighbourhood, then converted into internal dimensions and an effective water depth, leaving an air gap below the roof slab.
Excavation and soil
The tank is excavated to the level on the drawing, with safe side slopes or shoring if it is deep or the soil is loose. A soil report helps here just as it does for foundations, showing whether the ground can carry the full tank, whether groundwater will push on it from outside, and whether soil salts call for a particular cement or stronger waterproofing. See foundation types and Riyadh soil.
The usual below-ground steps follow: trim and compact the base, lay the polyethylene sheet against moisture, then pour blinding concrete to give a clean, level surface for the base slab reinforcement.
The concrete shell
A villa tank is normally reinforced concrete. Unlike ordinary structural elements, it has to do more than carry load: it must stop water passing in either direction.
Base slab
Thickness and reinforcement as drawn, with the larger concrete cover appropriate for elements in contact with water and soil, as specified by the engineer.
Waterstop at the construction joint
The weakest point is the construction joint between base and walls, since walls are usually poured after the base. A waterstop, a PVC, rubber or hydrophilic strip as specified, is fixed in this joint at mid-thickness of the wall. It must stay straight and secure during the pour, not bent over or displaced.
Walls
- Tight formwork that doesn't leak cement paste, which leaves honeycombing.
- Formwork ties suitable for water-retaining walls, so they don't leave a path for water once stripped.
- Thorough vibration in layers, especially at corners and around pipes.
- Inlet and outlet pipes cast in with puddle flanges, rather than cored through after the pour.
Roof slab
A reinforced slab with an access manhole whose upstand sits above the surrounding ground, so wash water and rain can't run in, with a tight, lockable cover. Add a vent with insect mesh and an overflow that discharges away from the tank.
Curing
Concrete poured in Riyadh heat needs continuous wetting and covering. It matters even more in a tank, because shrinkage cracks from rapid drying can become leak paths. Read pouring and curing concrete in hot weather.
The fill test before backfilling
This step separates a carefully built tank from a rushed one. Once the concrete has gained strength, and before final external waterproofing and backfill:
- Fill the tank to its operating level.
- Mark the water level and monitor it for a period set by the engineer.
- Inspect the outside of the walls and base for damp patches, especially at joints, pipes and tie holes.
- Repair any seepage as the engineer directs, and retest if needed.
With the walls still exposed, any seepage is easy to see and fix. After backfilling, finding a leak means digging again.
Waterproofing, outside and inside
| Layer | Typical materials | Purpose | Check |
|---|---|---|---|
| External waterproofing | Bitumen or membranes as specified | Keeps soil moisture and salts off the concrete | Full cover of walls, roof and corners |
| Protection board | As specified by the engineer | Stops backfill stones damaging the membrane | In place before backfill |
| Internal lining | Coating approved for potable water | Watertightness and water quality | Potable-water approval certificate |
Never use ordinary bitumen, or any product not approved for drinking water, inside the tank. The internal lining must be made for the purpose, applied to a clean, dry surface as the manufacturer instructs, and fully cured before filling.
Backfill and commissioning
Backfill around the tank in compacted layers with clean material free of sharp stones and debris. Connect the filling pipe, pump line, overflow and vent. Before use, wash, clean and disinfect the tank according to approved guidance, then fill.
Common mistakes
- No waterstop at the base-to-wall joint, or one fixed crooked or folded.
- Pipes cored through walls after the pour.
- A low manhole upstand that lets in wash water and dust.
- Backfilling before the fill test.
- An internal lining not approved for drinking water.
- Poor curing in summer, leaving surface cracks that seep.
- A loose cover that admits insects and dust.
How Marafi builds tanks
We treat an underground tank as a concrete element that needs more precision than usual, not as a hole with walls. We follow the structural drawings for dimensions and reinforcement, lay the polyethylene sheet and pour blinding before the base, fix formwork tightly, check construction joints and pipe positions before each pour, and cure the concrete properly. Owners receive photos at each stage, from reinforcement to the fill test and waterproofing.
Tanks are part of our Boundary Walls & Water Tanks service, with the concrete work carried out under Reinforced Concrete Works. Building the boundary wall at the same time? Read building boundary walls, and see our process on How we build.
Owner's checklist
- Tank position suits tanker access and is clear of drainage and trees.
- Capacity and dimensions are on the structural drawing.
- Waterstop checked at the base-to-wall joint before pouring.
- Pipes cast in, not cored later.
- No backfill before a successful fill test.
- Internal lining certified for potable water.
- Raised manhole, tight cover, meshed vent.
Planning an underground tank for your villa or alongside a boundary wall in Riyadh? Contact us to arrange a site visit, and we will send an itemised proposal covering excavation, concrete, waterproofing and testing.
Frequently asked questions
Where should the underground tank go on a villa plot?
Somewhere a water tanker can reach easily, a safe distance from the septic tank, drainage lines and tree roots, and in line with the approved drawings and the relevant authority's requirements.
What capacity do I need?
The engineer sets it from the number of occupants, expected consumption and how reliable the water supply is in your area, then turns it into internal dimensions on the structural drawing.
How do I know the tank doesn't leak?
Through a fill test before final waterproofing and backfill: the tank is filled, the level and outside walls are watched for a period set by the engineer, and any damp spots are repaired before backfilling.
What lining should be used inside a drinking-water tank?
A coating approved for contact with potable water, applied to the manufacturer's instructions. Ordinary bitumen must never be used inside.
