Introduction
Most waterproofing methods need a dry surface. Injection grouting with polyurethane is the rare exception — it is the only practical way to seal a crack while water is still pouring out of it. That makes PU injection the go-to technique for the hardest waterproofing problems: gushing basement cracks, leaking tunnel segments, lift pits, water tanks, and underground walls.
This guide focuses specifically on active, flowing leaks: how PU resin works, which type to choose, and the exact field sequence to shut the water down.
How Polyurethane Stops Water
Water-reactive polyurethane resins cure on contact with water. As the resin meets moisture in the crack it reacts and expands into a closed-cell foam, growing to many times its original volume and physically filling the leak path. Because the reaction is triggered by the very water you are trying to stop, more water simply means a faster, more aggressive seal.
Our Polygrout hydrophobic PU expands up to 3000% on contact with water — a dramatic illustration of how a small amount of resin can fill a large void.
For the broader comparison with rigid epoxy systems, see PU vs Epoxy Injection Resins.
Hydrophobic vs Hydrophilic vs Two-Component
Not all PU resins behave the same way. Picking the right one is half the battle.
Hydrophobic PU Foam — the Default for Leaks
Hydrophobic PU repels water as it cures, forming a tough, closed-cell foam that does not re-absorb water and stays stable when the crack later dries out. This is the workhorse for most active leaks.
- Polygrout — single-component hydrophobic PU, huge expansion, semi-rigid closed-cell foam.
- Seal Injection PU-101 — fast-reacting PU that cures into a flexible, closed-cell hydrophobic foam for cracks, joints, and voids.
Two-Component PU — for Flooded Cracks
When a crack is completely water-filled or under pressure, you may need a resin that foams hard regardless of contact conditions. Seal Injection PU-201 is a two-component, CFC-free PU that rapidly foams up in water-filled cracks to give an immediate cut-off.
Elastic Polyurea — for Continuous Exposure
Where the structure is permanently wet and you need long-term elasticity, ChemShield Polyurea delivers exceptional adhesion and durability under continuous water exposure.
The Equipment
Driving PU into a leaking crack needs pressure and the right ports.
- Pump: the High Pressure Grouting Machine handles single-component PU at up to 645 bar. For two-component resins that mix at the nozzle, use the Bosch Double Component Grouting Machine. See Choosing an Injection Grouting Machine for the full comparison.
- Packers: Steel Injection Packers or Aluminium Injection Packers for high-pressure work. Pick yours with our packer selection guide.
Step-by-Step: Sealing an Active Leak
1. Assess and Plan
Find where the water enters, not just where it appears. Water often travels along the crack before emerging, so the visible wet patch may be far from the actual entry point. Mark the full crack and plan packer positions.
2. Drill at an Angle
Drill packer holes at roughly 45° so each borehole intersects the crack inside the structure, typically at half the wall thickness. Alternate sides of the crack and space holes 150–300mm apart depending on width.
3. Set the Packers
Insert and tighten a steel packer in each hole until the rubber sleeve grips firmly. A loose packer will blow out under PU pressure.
4. Inject Bottom-Up
Connect the pump to the lowest packer and begin injecting. Working upward is essential — the expanding foam pushes the water ahead of it and out through the higher, still-open packers. When resin or foam appears at the next packer up, cap the current one and move up.
5. Watch the Reaction
With hydrophobic PU you will see foam expand and the water flow drop, then stop. Hold pressure briefly to let the foam pack the crack fully, then move on.
6. Re-Inject if Needed
Once the first pass has cured, re-check every weep point. A second, lighter injection pass closes any residual paths. This is normal and expected on difficult leaks.
7. Finish
Cut packers flush, fill the holes with fast-set mortar or epoxy paste, and — for a complete job — apply a surface waterproofing layer over the repaired area.
Pro Tips for Tough Leaks
- Pre-wet hydrophobic PU cracks by flushing with water; the resin needs moisture to react.
- Don’t over-pressurise a wide crack — let the foam do the work rather than blasting resin straight through.
- Have a fast resin ready (Seal Injection PU-201) for sudden high-flow gushers, then follow with a permanent hydrophobic seal.
- Keep the pump primed — air in the line wastes resin and slows the response when you need it most.
Where PU Injection Shines
PU injection is the method of choice for basements (see our Basement Waterproofing guide), tunnels and underground structures (see Tunnel & Underground Curtain Injection), lift pits, water-retaining structures, and any construction joint that has started to weep.
Conclusion
When water is actively flowing, polyurethane injection grouting is unmatched. Choose a hydrophobic foam like Polygrout or Seal Injection PU-101 for most leaks, keep a fast two-component PU on hand for gushers, pair them with steel packers and a high-pressure pump, and always inject bottom-up. Done properly, the leak stops for good.
Get Started
Have a waterproofing or crack-repair project? Request a free quote and our technical team will recommend the right products and method for your site.
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