The main bearing seal on a Herrenknecht EPB TBM failed 340 meters into a metro tunnel in Southeast Asia in 2019. The repair required stopping the entire project, excavating a rescue shaft from the surface, withdrawing the TBM partially, and rebuilding the seal system in a purpose-built intervention chamber. Total downtime: 11 months. Total additional project cost: estimated at $45 million USD. The failed component was a rubber seal system that, had it been inspected and replaced at the correct interval during the pre-launch maintenance window, would have cost less than $80,000.
This is not an extreme case. It is the scenario that every major tunneling contractor has witnessed or studied — because on a TBM operating in the ground, there is no such thing as a minor component failure. There is no pulling the machine into a workshop. There is no quick swap on site. Every failure is a project event, not a maintenance event.
If you work in tunneling procurement, equipment management, or project engineering, this guide gives you a complete picture of TBM rubber components: what they do, which are OEM-only items, which can be sourced from specialist manufacturers, and what inspection and replacement intervals apply to each category.
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The Scale and Complexity of a TBM
Before discussing individual components, the scale of a tunnel boring machine needs context. A 15-meter diameter Herrenknecht Slurry TBM for a road or rail tunnel has a cutter head weighing over 1,000 tonnes. That cutter head rotates at 1-4 RPM against rock or soil, driven by up to 20,000 kW of installed power. The machine body — the shield — is 80-120 meters long. The TBM advances by pushing hydraulic thrust jacks against the last installed tunnel lining ring, generating thrust forces of 50,000-100,000 kN on a large machine.
The rubber components on a machine of this scale are not maintenance consumables in the conventional sense. The main bearing seal is a customized, project-specific component. The segment gaskets are delivered in quantities of hundreds of thousands for a single tunnel project. The tail shield seal system operates under the combined pressure of groundwater, grout, and overburden — sometimes exceeding 6 bar — continuously, for years.
Understanding which rubber components are OEM project-critical items and which can be sourced from specialist manufacturers is the key supply chain decision every tunneling project must make.
Segment Gasket Seals: The Largest Volume Rubber Component on Any Tunnel Project
The tunnel lining of a bored tunnel is built from precast concrete segments — typically 6 to 8 segments per ring, plus a key segment. Each ring is assembled inside the TBM tail shield immediately behind the machine as it advances. The joints between segments are sealed by EPDM rubber gaskets to create a watertight tunnel.
The geometry is precise: each segment has a groove cast into its edges, and a profiled EPDM gasket is compressed into this groove. When the segment is bolted to its neighbors, the gaskets compress and seal the joint against groundwater infiltration. The seal must hold against the full groundwater head at the tunnel depth — which can exceed 4-5 bar on deep metro tunnels.
The scale of this supply is significant. A standard metro tunnel ring has 4 to 8 gasket elements per segment. With 7 segments per ring, a single ring uses 28 to 56 gasket elements. A 5-km metro tunnel at 1.5m advance per ring uses approximately 3,300 rings — meaning 90,000 to 180,000 individual gasket elements for one tunnel tube. Twin-tube metro lines double this. A major tunneling project is one of the largest single-project rubber supply contracts in the construction industry.
Segment gaskets are designed to project specifications — the gasket profile, compression ratio, hardness, and material must match the segment geometry and the groundwater pressure at the specific tunnel depth. Standard profiles exist from major gasket manufacturers, but project-specific customization is the norm for challenging groundwater conditions. EPDM is the preferred compound for segment gaskets due to its excellent water resistance, ozone resistance, and compression set behavior over the design life of the tunnel (typically 100-120 years for metro tunnels).
Segment gaskets are not a TBM maintenance item — they are a project supply item procured in the same supply chain as the precast segments themselves.
Main Drive Seal Systems: The Most Critical Rubber Component on a TBM
The cutter head is supported by a main bearing — a large-diameter slewing ring bearing — that allows the cutter head to rotate while the shield body remains stationary. Between the rotating cutter head and the stationary shield is a gap that must be sealed against the face pressure: the combined pressure of groundwater, soil, and slurry (on slurry TBMs) or conditioned soil (on EPB machines) acting on the cutter head.
The main drive seal system consists of multiple concentric lip seals and labyrinth seals arranged in several rows. The seals rotate against a polished stainless steel wear ring. Between the seal rows, pressurized grease is injected to lubricate the seals and provide a secondary pressure barrier. The combination of seal geometry, grease pressure, and wear ring surface finish determines the performance and life of the seal system.
Failure of the main bearing seal allows pressurized slurry or groundwater to enter the main bearing cavity. The bearing lubricant is contaminated, the bearing races corrode and are damaged by abrasive particles, and within a short period, the main bearing must be replaced. Replacing a main bearing in the middle of a tunnel requires one of the most complex interventions in the industry: excavating or constructing an intervention chamber, depressurizing the face, and physically rebuilding the front of the TBM in conditions of confined space, high humidity, and potential groundwater risk.
Main drive seal systems are custom-engineered to TBM specifications by the TBM manufacturer or their approved seal suppliers. They are not catalog items. The seal profiles, dimensions, and compound specifications are TBM-specific. Replacement seals must be sourced either directly from the TBM manufacturer (Herrenknecht, Robbins, NFM, Hitachi Zosen) or from a specialist seal manufacturer with the original technical specification documentation.
The correct replacement interval for main bearing seals depends on TBM diameter, advance rate, and ground conditions. For soft ground TBMs, main bearing seal inspection is typically scheduled at major maintenance windows, usually every 2,000-3,000 meters of advance. For hard rock TBMs operating in abrasive conditions, inspection intervals may be shorter.
Tail Shield Seal Systems: Protecting Against Grout and Groundwater
At the rear of the TBM shield, as the machine advances away from the last installed ring, a gap opens between the outer diameter of the lining rings and the surrounding ground. This gap — the annular void — is immediately filled with grout (a cement-based or two-component grout) to support the ground and consolidate the tunnel lining. The grout is injected under pressure through ports in the shield tail.
The tail shield seal system prevents the grout — injected at pressures up to 3-4 bar — and groundwater from entering the TBM body through the annular gap at the shield tail. The tail shield seal typically consists of three to four rows of steel wire brush seals with rubber lip seals, combined with grease injection between rows to maintain a pressure barrier.
The brush seals are wear items and are replaced at intervals determined by advance distance and ground conditions. The rubber lip seal elements are secondary sealing components. What makes tail shield seal replacement a planned project activity rather than an emergency repair is the accessibility: unlike the main bearing seal, tail shield brush seals can be replaced from inside the TBM, through access hatches, without stopping tunnel advance (in some configurations).
Vibration isolation within the TBM itself — at the conveyor systems, auxiliary machinery mounts, and thrust jack interfaces — uses standard rubber isolation principles, and these components can be sourced from specialist manufacturers.
Vibration Isolation Mounts Inside the TBM
While the main sealing components on a TBM are OEM or custom items, the vibration isolation components inside the TBM body are more accessible to aftermarket supply.
Thrust Jack Interface Pads
The hydraulic thrust jacks that advance the TBM push against the last installed lining ring through steel thrust shoes. The contact interface between the thrust shoe and the concrete lining ring typically includes a rubber or HDPE load-distribution pad. These pads prevent point loading on the concrete segment edges and distribute the thrust force more evenly.
For large TBMs generating 80,000-100,000 kN of total thrust, the load per individual thrust shoe is substantial — 2,000-5,000 kN per jack in a 20-jack system. The rubber pads must combine high compressive load capacity with sufficient flexibility to accommodate angular misalignment between the thrust shoe and the ring face. High-durometer natural rubber, 70-80 Shore A, is typical for these pads.
These are stock replacement items, specified by dimensions and load capacity, and can be sourced from specialist rubber manufacturers with the correct specification.
Conveyor and Auxiliary Machinery Mounts
Inside the TBM, the soil or rock removal conveyor system, the segment erector machinery, the grout mixing equipment, and all auxiliary systems are mounted to the TBM inner frame. Continuous vibration from the cutter head, thrust jacks, and conveyor drives creates a demanding environment for all rubber isolation mounts on these systems.
Standard rubber-metal anti-vibration mounts in the appropriate load range serve these applications. NBR compound is preferred given the oil and grease contamination typical inside TBM bodies. See our rubber mount product category for standard AVM specifications applicable to auxiliary TBM machinery.
Screw Conveyor Face Seals: High Wear in EPB Machines
Earth pressure balance TBMs use a screw conveyor to remove conditioned soil from the excavation chamber to the TBM body. The screw conveyor penetrates the bulk head — the pressure wall that maintains face pressure — through a rotating seal. This seal must contain the pressurized conditioned soil in the chamber while allowing the screw to rotate.
The EPB screw conveyor seal is a rubber lip seal arrangement, typically with multiple seal rows and grease injection, operating in highly abrasive conditions with soil particles, gravel, and construction debris. These are among the highest-wear rubber items on an EPB TBM. Replacement intervals depend heavily on ground conditions — in clean clay, seals may last thousands of meters; in sandy, gravelly ground, replacement may be required every 200-300 meters of advance.
EPB screw conveyor seals are typically OEM or custom items due to the specialized geometry and compound requirements for the specific machine and ground conditions.
Mini-Stories From TBM Projects
The Gasket Supply Chain Crisis
On a major highway tunnel project in northern Europe, the precast concrete manufacturer delivered 15,000 tunnel segments on schedule — but the gasket supplier experienced a production delay and delivered only 60% of the required EPDM gaskets. The segments could not be installed without gaskets — water infiltration in the 80-meter-depth groundwater table would have been immediate and severe. Tunnel advance halted for 6 weeks while an alternative gasket supplier was qualified and production caught up. The delay cost the project consortium approximately €8 million in standing time and contractual penalties. The lesson learned — a single-source gasket supply is unacceptable on a major tunnel project — is now reflected in most European tunneling contracts, which require at least one qualified backup supplier for all rubber supply items.
The Overlooked Isolation Mount
Defne Arslan, a mechanical engineer on a metro project in Istanbul, noticed during a routine inspection of the auxiliary machinery bay that three of the conveyor drive motor mounts had failed — the rubber element had separated from the steel plate, and the motor was sitting directly on the steel frame. The motors themselves were still operating. But the vibration from the conveyor drive was being transmitted directly through the TBM frame to the segment erector arm, creating position errors in the erector’s segment placement sequence. Segments were being placed 4-6mm out of specification — within tolerance, but accumulating over rings and creating geometry issues for the tunnel boring alignment. Replacement of the three failed mounts — standard NBR rubber-metal AVMs — took four hours and resolved the erector positioning issue immediately.
OEM vs. Aftermarket: What to Source Where
The distinction between OEM-only and aftermarket-sourceable TBM rubber components is clear when mapped against criticality and customization:
| Component | Source | Reason |
|---|---|---|
| Main bearing seal system | OEM or specialist seal manufacturer with TBM specs | Custom geometry, project-critical, no standard catalog |
| Tail shield brush seals | OEM or approved aftermarket with material certification | Custom diameter, pressure rating |
| Segment gaskets | Specialist manufacturer to project specification | Custom profile, EPDM compound, 100+ year design life |
| EPB screw conveyor seals | OEM or specialist manufacturer | Custom geometry, high-wear compound |
| Thrust jack pads | Specialist manufacturer to dimensions and load spec | Standard rubber, high load, sourceable |
| Conveyor and auxiliary mounts | Specialist manufacturer, standard catalog | Standard AVM specifications |
| Vibration isolation pads | Specialist manufacturer | Standard rubber isolation |
Babacan Group supplies rubber isolation mounts, vibration pads, and secondary rubber sealing elements for TBM projects. Main bearing seals, which require TBM-manufacturer-specific documentation and precision manufacturing, remain OEM items. For secondary components, specialist supply delivers equivalent performance with better availability and lead times than OEM channels.
For related rubber component guides in underground construction and mining, see our mining equipment vibration isolation guide and our hydraulic breaker rubber parts guide — hydraulic breakers are used in the same underground construction context for auxiliary rock breaking.
Compound Selection for TBM Applications
EPDM: Standard for segment gaskets. Excellent water resistance, ozone resistance, and long-term compression set behavior. Not suitable for petroleum-contaminated applications.
Natural rubber: Thrust jack pads and high-load compression elements where dynamic properties and high load capacity are required.
NBR (nitrile rubber): All auxiliary machinery mounts and vibration isolation inside the TBM body, where petroleum contamination from hydraulic systems and machinery lubrication is constant.
Polyurethane: Some TBM manufacturers use polyurethane for segment gaskets in high groundwater pressure applications due to its superior compressive load capacity. However, polyurethane gaskets require different groove geometry and are a project-specific design decision.
Shore hardness selection varies by application: segment gaskets are typically 55-65 Shore A for correct compression behavior in the segment groove; thrust jack pads 70-80 Shore A for high-load applications; auxiliary mounts 40-55 Shore A for effective vibration isolation.
TBM Applications in Railway Tunnels
Many of the world’s major rail tunnel projects — including metro systems, high-speed rail tunnels, and commuter rail tunnels — are constructed by TBM. The tunnel lining segments in these projects must comply with fire performance standards including EN 45545-2, which governs railway rolling stock and fixed railway infrastructure fire behavior. For tunnel segment gaskets installed in railway tunnels, compound selection must consider fire performance in addition to water sealing performance.
TBM-bored railway tunnels also typically use rail track systems mounted on isolated plinths or floating slab track to attenuate rail-generated vibration before it reaches the tunnel lining — a separate vibration isolation system that uses rubber-metal mounts or elastomeric bearings. See our chevron springs and railway bogie spare parts pages for related railway vibration isolation products, and our construction machinery suspension parts guide for broader context on vibration isolation in large construction projects.
Contact Babacan Group to discuss TBM rubber component supply for your project
Procurement Planning: Lead Times and Supply Strategy
TBM projects are planned years in advance — and rubber component procurement should be planned on the same schedule. Main bearing seals require 6-12 months lead time from specialist manufacturers. Segment gaskets must be qualified and production validated before TBM launch — a process requiring 3-6 months from final specification.
Secondary components — vibration isolation mounts, thrust jack pads, auxiliary seal elements — have shorter lead times but should be stocked on site in sufficient quantities for the project duration. A project running 5 km with a 500-meter annual advance rate will operate for 10 years. Sourcing rubber components from a single supplier without backup qualification creates unacceptable supply risk.
Babacan Group’s supply network spanning 84+ countries ensures geographic proximity to major tunneling projects in Europe, the Middle East, Asia, and Africa. With over 90,000 references and ISO 9001:2015 certified production, we are positioned as a qualified secondary supplier for all non-OEM-exclusive TBM rubber components.
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Browse our rubber parts product range for applicable standard components, or contact our engineering team for project-specific specifications.
Key Takeaways
- TBM rubber components are project-critical items, not maintenance consumables — a failed main bearing seal can halt a tunnel project for months and cost tens of millions of dollars.
- Segment gaskets are the largest volume rubber supply item on any tunnel project — a 5-km metro tunnel uses 90,000 to 180,000 EPDM gasket elements per tube, all project-specified.
- Main bearing seal systems and EPB screw conveyor seals are OEM or custom-manufactured items; vibration isolation mounts, thrust jack pads, and auxiliary machinery mounts can be sourced from specialist manufacturers.
- NBR compound is required for all auxiliary machinery mounts inside the TBM body due to constant petroleum contamination from hydraulic systems.
- Segment gasket supply requires a qualified backup supplier — single-source supply of this project-critical item is unacceptable risk on any major tunneling contract.
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