Sealing tunnel and drainage joints under water pressure

A tunnel joint is asked to do something a façade joint never is: hold back water that is actively pushing against it. That changes both the product and the detail.

Pressure changes the requirement

A sealant bead resists water by adhesion and by its own strength. Under hydrostatic pressure, water finds the weakest interface and works at it continuously. A bead with high elongation but low modulus deforms under pressure; a stiffer, water-pressure-rated product holds shape.

That is why KA340B — higher modulus, 550% elongation, 80% recovery — is the grade for below-grade and drainage work, while the softer KA340A is for movement joints that are not under pressure.

The waterstop is part of the system

A surface bead alone is rarely a watertight detail in a tunnel. The typical system includes:

1. A waterstop at the construction joint, embedded in the concrete.
2. A prepared joint recess with primer.
3. A pressure-rated sealant bead, at the specified depth.
4. Drainage or a secondary barrier behind the lining.

Specifying the sealant without the waterstop is asking one component to do the job of the system.

Movement in tunnels

Tunnels move: settlement, thermal cycles, traffic load. The joint detail has to accommodate that movement while staying watertight, which is why the recess geometry and the backer rod matter as much as the product.

Pressure, in numbers

Hydrostatic pressure is easy to calculate and rarely written down:

10 m of water head ≈ 100 kPa ≈ 0.1 MPa
30 m of water head ≈ 300 kPa ≈ 0.3 MPa

Against a sealant with 2 MPa tensile strength, 0.3 MPa sounds small. The number that matters is not tensile strength but sustained adhesion under pressure, which is a different property and is not on most data sheets. Water does not need to break the bead; it needs to find one point of adhesion loss and work at it for months.

That is why below-grade work uses a stiffer, higher-modulus grade (KA340B rather than KA340A) and why the waterstop carries the waterproofing duty.

Injection versus surface sealing

A leaking construction joint in a tunnel is usually repaired by injection, not by a surface bead:

• Injection fills the crack or the joint void with a resin that cures in place, sealing from inside the structure.
• Surface sealing protects the joint faces and handles movement, but cannot stop water that is already behind the lining.

The two are complementary. A repair that only re-seals the surface of a leaking joint will fail at the next wet season, because the water path behind it is unchanged.

What to test before handover

1. Flood test the joint, or the compartment, before the lining is closed.
2. Inspect the waterstop at every construction joint — the most common defect is a waterstop that was displaced during pouring.
3. Record installation temperature and joint width, as for any movement joint.
4. Photograph the recess before sealing; it is the only record of the geometry that was actually built.

Waterstop types and where each belongs

Type	Position	Handles movement?
PVC waterstop	Embedded across the joint, both sides	Yes, within its profile
Hydrophilic strip	Surface of the joint, swells on contact	Limited; good for construction joints
Bentonite	Behind the lining, as a blanket	No — it is a backfill seal
Injection hose	Embedded, grouted after curing	Yes, and re-groutable

A hydrophilic strip alone is not a waterstop for a moving joint; it swells, seals, and then loses volume when the joint opens. The reliable detail pairs an embedded waterstop with a sealant bead and, where the joint may leak later, an injection hose that allows a repair without demolition.

What this means for buyers

1. Specify the waterstop and the sealant together.
2. Use a water-pressure-rated grade for below-grade joints.
3. Inspect after the first wet cycle, not at handover.
4. Keep batch records; a tunnel repair is expensive enough to justify traceability.