How to Waterproof an Underground Water Tank or Sump During Construction

Underground RCC water tanks are subjected to water pressure from both the stored water inside and groundwater or seasonal moisture outside. As each stage of construction is concealed by the next stage, defects become difficult and expensive to rectify later. The following construction-stage checklist highlights the key inspections homeowners should verify to minimize leakage risks.

Construction-Stage Sequence

1. Base Preparation and PCC

Construction begins with a Plain Cement Concrete (PCC) base, which provides a stable and level foundation for the tank floor.

Where groundwater or soil moisture is expected, the approved waterproofing membrane should be installed over the PCC before the floor slab is cast. As this membrane becomes inaccessible after concreting, its installation should be verified before work proceeds to the next stage.

2. Reinforcement and Concrete Cover

The reinforcement provided in the base slab and walls should comply with the approved structural drawings. Since underground tanks are exposed to continuous moisture, maintaining the specified concrete cover is essential for protecting reinforcement against corrosion.

Before concreting, confirm that:

• Reinforcement is positioned according to the structural drawings.

• Cover blocks are correctly installed and securely positioned.

• Reinforcement remains stable throughout the concrete placement process.

3. Construction Joints

Construction joints are among the most common locations for water leakage in underground tanks.

Each planned joint should be treated using the waterstop or joint treatment system specified in the approved design. These systems must be installed during construction while the joint is being formed rather than after the tank has been completed.

4. Controlled Water-Cement Ratio

The water-cement ratio has a significant influence on the permeability of concrete. Excess water increases porosity, making the concrete more susceptible to seepage over time.

The concrete mix should therefore be prepared in accordance with the approved mix design. Water should not be added on site to improve workability unless specifically authorised by the structural engineer.

5. Continuous Concreting Where Specified

Certain portions of the tank may be designed for continuous concrete placement to minimise the number of construction joints.

Where continuous pours are specified:

• Ensure adequate labour, equipment and concrete supply are available.

• Avoid unnecessary interruptions during concrete placement.

• Prevent the formation of unintended construction joints that may later become leakage paths.

6. Curing

Proper curing is essential for developing concrete strength and reducing permeability.

The tank should be cured for the full duration specified in the approved design and project specifications before any internal waterproofing treatment is applied. Inadequate curing can reduce the effectiveness of subsequent waterproofing systems and increase the likelihood of cracking.

7. Internal Waterproofing Treatment

After the concrete has been fully cured, the internal water-facing surfaces should receive the specified waterproofing treatment.

Before application:

• Inspect the concrete for cracks, honeycombing or other defects.

• Complete all necessary repairs before the waterproofing system is applied.

• Ensure the selected internal coating is suitable for potable water storage where applicable.

The internal waterproofing system supplements, but does not replace, the protection provided by the membrane, construction joint treatment and sound concrete construction.

8. Final Water-Retention Test

A water-retention test should be conducted before the tank is backfilled or placed into service.

The tank should be filled and maintained for the duration specified by the structural engineer or project specifications. During this period, inspect the exterior surfaces and surrounding ground for signs of dampness, seepage or leakage.

Any defects identified during the test should be rectified before the tank is commissioned.

Site Sign-Off Checklist

Before backfilling and project handover, homeowners should obtain confirmation from the site engineer that:

• The PCC base has been completed and the waterproofing membrane was installed before floor slab casting.

• Reinforcement and cover blocks comply with the approved structural drawings.

• Construction joints have been treated using the specified waterstop or joint treatment system.

• Concrete has been produced using the approved water-cement ratio and mix design.

• Continuous concreting has been completed where specified in the design.

• The required curing period has been completed before internal waterproofing treatment.

• Cracks, honeycombing or other defects have been repaired before coating application.

• Internal waterproofing is certified for potable water storage where applicable.

• The water-retention test has been completed for the specified duration.

• No seepage, leakage or damp patches were observed during testing.

FAQs

Why is waterproofing important for an underground water tank or sump?

Underground tanks are exposed to internal water pressure and external groundwater pressure. Proper waterproofing at each construction stage helps minimise leakage, seepage and long-term deterioration of the reinforced concrete structure.

What is the best time to waterproof an underground water tank during construction?

Waterproofing should be integrated throughout construction. Membranes, construction joint treatments and other waterproofing measures must be installed at the specified stage before they are concealed by subsequent construction work.