Application of Bentonite Waterstop Strips in Tunnel & Metro Waterproofing

Practical notes from a waterproofing engineer. Author: Aohong Waterproofing — Senior Engineer Fengliu.

1. Introduction

In underground works, the saying “ten joints, nine leaks” is widely understood. Tunnels and metro systems need reliable waterproofing. They are far underground. Their hydrogeology is complex, so they require minimal maintenance time. So, the materials used must be nearly flawless. Bentonite waterstop strip, also called rubber waterbars, are great for waterproofing joints. I also like hydrophilic waterstops and EPDM composite waterstops. They have three main benefits: they block water using water, adjust to changes, and are simple to install. For the past decade, I’ve relied on them in design, construction, and operation.

This paper looks at five key projects. It shows their actual performance in various ground conditions and structural positions. It also offers useful data and lessons for others to reference.

Application of Rubber Waterstops

• Rubber waterstops, including water-swelling rubber strips and hydrophilic waterstops, are essential in construction.

• They prevent water from seeping through joints in concrete structures.

• These products are commonly used in basements, tunnels, and reservoirs.

• They expand when in contact with water, sealing any gaps.

• This ensures durability and protects against water damage.

• Proper installation is key to effective performance.

2. Project Overview

* “Model” refers to project-customized cross sections, not a single market standard.

3. Selected Case Analyses

3.1 Case ① — Beijing Metro Line 19: Shield Tunnel in Water-Bearing Sand & Gravel

Engineering challenges: Greatest water pressure: 0.25 MPa. The shield must pass existing lines; deformation joint allowance ±30 mm. The cobble/pebble layer sends high impact pressures to seals. This can crush ordinary EPDM (three-component EPDM) seals.

Waterproofing scheme

• Primary seal: 350×8 mm steel-edged rubber waterstop (mid-embedded). Heat-vulcanized splices are required to reach ≥90% of parent material strength.

• Secondary seal: A 20×15 mm rubber strip that swells with water sits at the groove’s bottom. It offers “double insurance.” (Also referenced as a hydrophilic waterstop or swelling rubber seal.)

Key construction controls

• Pre-fix the swelling strip using butyl adhesive and a stainless steel pressure strip. This prevents drifting during synchronous grouting.

• Assembly torque: Re-tighten the segment bolt in three steps. Then, do it again after 24 hours. This ensures the swelling strip maintains its initial compression ratio of 35%.

Operation Monitoring Since March 2022, an independent third party has inspected a 2.08 km trial section six times.

Leakage at deformation joints was 0 L·m⁻²·d⁻¹. This meets the Level-2 waterproofing standard of GB 50157.

3.2 Case ② — Ningbo Metro Line 2 Phase II: Deep Soft-Soil Diaphragm Wall

Engineering challenges “5 highs 2 lows” soft soil (high water content up to 55%, high sensitivity 6–8). Diaphragm wall depth: 60 m. Traditional socketed locked pipe joints risked entrapped slurry.

Changed the joint type from a socketed locked pipe to a GXJ rubber waterstop joint. The joint box had a 400×20 mm water-swelling strip added. Wall brushing was removed.

Validation

• After digging to the final depth, we tested 96 wall panels. We used ultrasonic transmission and coring. There was no entrapped slurry at the joints.

• A 30-day dewatering test in the pit showed that the external water level dropped by less than 0.3 m. This is well below the design alarm of 1.0 m.

3.3 Case ③ — Beijing Station: Diaphragm Wall in Water-Bearing Sand

Technical Highlights: We used a rubber waterstop joint instead of the usual I-beam backup. This change is the first of its kind in this water-bearing sand layer.

By optimizing the grout mix (specific gravity 1.18 to 1.25) and timing joint-box extraction (within 4 hours after concrete sets), the swelling strip and concrete worked together perfectly.

Data results

• On-site impermeability checks on 12 joint samples showed an average resistance to infiltration of 0.9 MPa. This is higher than the design need of 0.6 MPa.

• The workers capped the station 18 months ago; the base slab remains dry, with no damp patches.

Bentonite Waterstop Strips images

4. Common Lessons & Key Performance Indicators

• Swelling Ratio and Water-Pressure Resistance: Select products that swell 300%–400% and resist water pressure of 1.2 MPa.

  This covers about 95% of urban metro water pressure situations.

  (Keywords: water-swelling rubber strip, hydrophilic waterstop.)

• Initial compression ratio: 30%–35% is optimal. Too small leaves channels after swelling; too large risks local concrete cracking.

• Fixing method: Vertical joints must use cement nails and pressure strip double fixation. Horizontal joints: recommend butyl adhesive spot-bonding plus next reinforcement mesh pressing.

• **Pouring Window:** After you install the swelling strip, pour the concrete within 6 hours. If it’s high summer, do it within 4 hours.

• Splice/joint handling: Water-swelling strips must not be cut on-site with scissors. Use the manufacturer’s vulcanizing equipment to splice and ensure continuous swelling behavior.

5. Preferred Materials — Why We Recommend Aohong Rubber Waterstops

Aohong Rubber & Plastic Products Co., Ltd. provides joint technical support for the five projects listed above. It supplied waterstops that excelled in three areas:

1. Reliable formulation: It uses imported EPDM and a modified bentonite-based swelling system. It showed a 168-hour swelling retention of at least 92%. The industry average is around 80%.

2. Dimensional stability: Extruded and vulcanized in one step. Width tolerance is ±0.5 mm, and thickness tolerance is ±0.2 mm. This is much better than the GB/T 18173.3 standard of ±1 mm.

3. **Full lifecycle service:** We offer complete support from selection to construction briefing. This includes vulcanizing machine rental and post-operation follow-up. Two senior technicians will be on-site until structural capping is complete.

6. Conclusion

There is no “after-the-fact repair” for tunnel and metro waterproofing — only “prior redundancy.” Rubber waterstops are vital for underground projects. They offer a crucial defense by swelling at a micro-scale and sealing at a macro-scale. Using high-quality rubber waterstops and construction details can create tunnels that last a century and are completely watertight.” This comes from successful practices in Beijing, Ningbo, Shanghai, and other projects.” Aohong rubber waterstops are worthy of every waterproofing engineer’s trust.