Underground Tank Waterproofing in Riyadh

A buried tank is surrounded by soil, and soil can hold water for several reasons: a locally high water table, garden irrigation repeated beside the wall, a leaking supply line nearby, or rain that collected around the cover. When the surroundings saturate, water pressure builds on the outer wall, known as hydrostatic pressure, and it rises with depth.

This means the lower walls and the floor carry the greatest load, and a fine crack low down can let in or lose more water than the same crack near the top. The direction of flow follows the difference between the two sides: a full tank pushes water out into the soil, while a low level can let soil water in.

Because the water level in the ground differs from plot to plot and district to district, we assume nothing before inspection. We ask about irrigation, rain and any excavation or networks nearby, and we read the inner walls for the trace of an outside water level: a horizontal salt line, or a damp band that repeats at one height.

• Frequent irrigation beside the cistern wall
• A leaking water line nearby
• Rain pooling around the cover or in the courtyard
• A locally high water table
• Neighbouring excavation or building that redirected water

In waterproofing terms the positive side is the face the pressure comes from, and the negative side is the opposite face. In a ground cistern the roles swap: stored water presses on the inner face outwards, while soil water presses on the outer face inwards. So whatever is applied from inside sometimes sits on the negative side with respect to soil water.

In practice, reaching the outside means digging beside foundations, paving and pipework, and it is only chosen when the inside route fails or the wall is already exposed. From inside, crystalline cementitious systems and some mortars rated for reverse pressure are designed to work in that position, while an epoxy may blister or peel if water pushes at it from behind.

So the usual inside route is: treat the points of entry first by plugging or injection, then a base coat that tolerates reverse pressure, then a finish if required. If water is clearly entering in volume, that points to a problem outside that deserves an external fix or drainage around the tank, because the inside alone cannot shut off a flowing source.

Uniform concrete is only slightly permeable, but a cistern is not a single pour: the floor is cast first, then the walls, then the roof, and between each pour sits a cold joint that never fully fuses. These joints, the pipe penetrations and the access-frame are the first water paths, and seepage starts there before any crack shows in the wall itself.

At the wall-to-floor junction stress concentrates, because a sharp internal corner stops the coating wrapping around it at even thickness, leaving it thinnest and weakest exactly there. The answer is a cove fillet: a curve of shrink-resistant repair mortar run along the corner, so the coating passes from wall to floor in a gentle bend instead of a hard break.

Joints are treated according to condition: a dry joint is opened and filled with mortar, a weeping joint is plugged with fast-setting mortar or injected, and the area is then covered by the general coat. Around pipe penetrations the concrete is chased out deeply and packed with non-shrink mortar, as this is the most frequent weak point in existing cisterns and painting over it is not enough.

• Floor-to-wall joint
• Wall-to-roof joint
• Inlet, outlet, overflow and washout pipes
• Access frame and neck of the shaft
• Wall-to-wall corners

A level-drop test gives a measurement that can be compared, unlike an impression. The cistern is filled to a set level, valves are closed and the float is isolated so no fresh water enters, then a fixed reference is set at the surface and the level is read at regular intervals. Readings are logged with date and time, not from memory.

To avoid a false reading, an open container of water floats in the cistern as an evaporation control, and the outlet valves are checked for passing through. If the fall stops at one height it indicates the height of the defect, so we know where to look instead of treating every wall.

The test is run twice: before the work, to confirm the cistern is the source and estimate the rate of loss, and after it, as a condition of handover once the curing period is over. The duration is not measured in minutes, since small leaks need a full day or more to show clearly, so we ask that the valves stay shut for the whole test.

Rising damp is water climbing through wall pores by capillary action, showing as patches near the base of a wall along with flaking paint and white salt. When a ground cistern sits near a room or a garden wall, the dampness may be blamed on it when it really comes from saturated soil, and the reverse is just as possible.

To tell them apart we ask three questions: does the dampness follow the tank filling? does it change with irrigation or rain? does it show a horizontal line at a constant height? Dampness tied to filling points to the tank, dampness tied to watering points to the soil, and a repeating horizontal line points to an outside water level touching the wall.

Salts matter because they show direction. Water evaporating at a surface leaves its salts behind, so white salt on the inner wall of a cistern means water is moving from the concrete inwards, even if only slowly. Paint alone does not cure that, because a coating over the salts traps water behind it and blisters.