The Leaking Basement Car Park

The pattern of leakage at the slab-wall construction joint (not random cracking) strongly points to a defect at the joint itself - likely inadequate waterbar installation, a construction joint that has opened under thermal or settlement movement, or honeycombing at the kicker. The presence of a permanent sheet-pile wall behind the concrete means there is no accessible external positive-side waterproofing and possibly no external drainage layer; groundwater is therefore in direct contact with the structure. The PU resin repair failed because (a) hydrophilic PU foams re-swell and shrink with wet/dry cycles and degrade under sustained hydrostatic pressure, and (b) injection of a joint without addressing the joint's ongoing movement and without a continuous, load-bearing seal is a local fix to a systemic problem. The musty odour and efflorescence indicate ongoing moisture ingress and salt migration, not just a cosmetic issue. A 10-year guarantee cannot responsibly be offered until the water source and joint geometry are properly understood.

1. Drainage and groundwater survey - CCTV of perimeter drains and a trial pit to establish whether the sheet-pile wall has a drainage path or is acting as a dam. This determines whether the design intent was 'tanked' or 'drained'. 2. Moisture mapping - non-invasive moisture meter readings and thermal imaging across the 40 m length to distinguish active leaks from residual damp and to locate cold bridges. 3. Concrete condition survey - covermeter, half-cell potential and carbona-tion testing on the retaining wall to establish whether reinforcement is corroding (which would change the repair strategy entirely). 4. Joint inspection - local breakout of finishes at 2–3 locations to expose the joint, identify the waterbar and measure joint width and movement. 5. Water testing - sample the leak and test for chlorides/sulphates to confirm it is groundwater, not a services leak. 6. Services trace - pressure test or isolate any rainwater/surface water drainage running near the wall. Each investigation must be justified by what decision it informs; a blanket 'condition survey' is not enough.

Given no external access, an occupied building and a tight budget, a positive-side (external) membrane is not feasible. The defensible strategy is a combined internal approach: (a) re-inject the joint with a flexible, movement-accommodating resin (e.g. a hydrophilic PU or acrylic gel) after establishing joint geometry; (b) install a proprietary drained cavity system or a cementitious crystalline waterproofing system to the internal face, with a drained channel at the base; (c) reinstate a serviceable sump and pump if the drainage survey shows groundwater is present. Critically, the repair must be designed as a drained system, not a fully tanked one, because the structure cannot be made watertight from the outside. I could offer a manufacturer-backed materials warranty (typically 10 years on the membrane/system) but I must be explicit that this is a materials and workmanship warranty, not a guarantee that no water will ever enter. I would recommend a 12-month post-completion monitoring period with moisture readings before any long-term guarantee is discussed, and I would document the residual risk in writing to the client.