Mitcheldean TWS Refurbishment

Scope of works

The project centred on the inspection, removal and replacement of the waterproofing system on a live treated water service reservoir. Ensuring the tank remained fully operational throughout was critical, requiring tightly controlled work sequencing and strict contamination prevention measures.

Existing conditions
The reservoir roof was originally protected by a loose‑lay membrane. Beneath this, the team identified:
• Teram geotextile layer
• 50mm Kingspan insulation
• A Proofex-type waterproofing membrane
• Primer layer over the concrete slab

Initial preparations
To prepare the site and protect water quality:
• Full edge protection was installed along the roof perimeter.
• Vermin mesh was fitted to prevent wildlife from entering the roof zone.

Weather management measures
Because the reservoir was live and contained treated water, the existing waterproofing could not be removed during rainfall. To mitigate this constraint:
• MCL procured and installed large inflatable encapsulation tents to create weatherproof working environments.
• These controlled spaces allowed work to continue safely during adverse weather events.

Execution of works
• Works were completed in carefully controlled sections to safeguard the exposed concrete structure.
• Continuous weather monitoring ensured that only dry conditions were used for membrane removal.
• During wet periods, all activity moved under the encapsulation tents.
• Once prepared, the new Proofex 3000 membrane was installed across the full 3,300m² roof.
• MCL undertook their own spark testing before engaging the STW‑approved third‑party assurance tester, who found minimal defects which was an excellent result for such a large area.

Following waterproofing, the team conducted a structural assessment confirming the roof could support the planned 1,600 tonnes of stone backfill, in line with STW standards. The roof was then fully reinstated, backfilled, and the asset returned to operational service.