Introduction

Underground structures face the toughest waterproofing challenge of all: water under constant hydrostatic pressure, pressing in from every direction, with no easy access to the outside (positive) face. Tunnels, metro stations, deep basements, cellars, and underground tanks all share this problem. When water is pushing in from behind the wall, sealing the inside face alone often fails.

The solution is curtain injection (also called veil or area injection). Instead of just filling a crack, you inject grout through the structure to form a waterproof barrier in the soil behind the wall — stopping water before it ever reaches the concrete. This guide explains how it works and what it takes to do it well. For managed projects, see our Underground Waterproofing and Industrial Waterproofing services.

Crack Injection vs Curtain Injection

It helps to be clear about the difference:

  • Crack injection fills a specific crack or joint within the structure. It is targeted and covered in our injection grouting guide.
  • Curtain injection drills through the full wall thickness and injects grout into the ground behind it, forming a continuous waterproof “curtain” or “veil” across an area. It is used when water comes from behind over a broad surface, not from one identifiable crack.

Curtain injection is the technique of choice when the outside of the structure is inaccessible — exactly the situation in tunnels and deep basements.

How Curtain Injection Works

  1. A grid of holes is drilled completely through the wall on a regular pattern (typically 250–500mm spacing).
  2. Packers are set so resin can be injected through the wall into the soil behind.
  3. Grout is injected through each packer. As it enters the soil it spreads and links up with grout from neighbouring holes, forming an overlapping curtain.
  4. The curtain cuts off the water in the ground before it reaches the wall, while any grout that travels back into wall cracks seals those too.

The result is a waterproof zone in the ground that protects the entire injected area, not just individual cracks.

The Right Materials

Curtain injection materials must spread through soil and react reliably in wet ground.

  • Polygrout — hydrophobic PU that expands up to 3000% on contact with water, excellent for filling voids and soil behind the wall.
  • ChemShield Polyurea — a high-polymer PU grout engineered for demanding, continuously wet environments, with exceptional adhesion and elasticity.
  • Seal Injection PU-201 — two-component PU that foams rapidly even in fully water-filled ground, useful for an immediate cut-off.

For the underlying technique of injecting against flowing water, see PU Injection Grouting for Active Water Leaks.

The Right Packers and Equipment

Curtain injection uses packers designed for veil work and the pressure to drive grout into soil:

  • Lamella Impact Packer 18mm — purpose-designed for veil and curtain injection of gels, resins, and microemulsions to seal ground-contact structures such as cellars, tunnels, and channels. It even includes a gel-pipe holder for 10mm gel injection pipes.
  • Steel Injection Packers and Aluminium Injection Packers — for through-wall injection at high pressure.

Drive everything with a capable pump — the High Pressure Grouting Machine (up to 645 bar) for single-component grouts, or the Bosch Double Component Grouting Machine for two-component resins. Our machine selection guide helps you pick, and the packer guide covers port selection.

Step-by-Step Overview

  1. Survey the wet area and define the injection grid.
  2. Drill through-wall holes on the planned pattern.
  3. Set packers suited to veil injection.
  4. Inject systematically, usually bottom-to-top and across the grid, so adjacent injections overlap.
  5. Monitor grout take and pressure; areas that take a lot of grout indicate larger voids being filled.
  6. Seal the wall cracks themselves as a final pass, and finish the inside face.

Don’t Forget the Construction Joints

Underground structures are built in stages, so their construction joints are prime leak paths. Combine curtain injection with proper joint sealing — see Construction Joint Waterproofing and consider re-injectable hoses like the Injection Hose Packer system at every joint.

Safety and Practical Notes

  • Ventilation matters in confined underground spaces — PU resins contain isocyanates; ensure good airflow and proper PPE.
  • Expect high grout take in voided ground; budget material generously.
  • Work systematically — a missed hole leaves a gap in the curtain that water will find.

Conclusion

When water pushes in from behind an inaccessible wall — as it always does in tunnels and deep basements — curtain injection is the answer. By drilling through the wall and injecting expansive PU grouts like Polygrout or ChemShield Polyurea through purpose-built veil packers, you build a waterproof barrier in the ground itself. Combined with joint sealing and a final crack-injection pass, it delivers dry, durable underground structures.

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.

InjectionGroutResin — your trusted partner for injection grouting and waterproofing solutions across India, UAE, and Saudi Arabia.