In mechanized tunnelling, backfilling is an essential process for ensuring stability, safety, and operational continuity during tunnel construction. Filling the annular gap — the space created between the estrados of the precast segments and the excavation profile — plays a crucial role in preventing surface subsidence and providing uniform support for the segmental lining.
As the TBM (Tunnel Boring Machine) advances, two-component grout is continuously injected, saturating the available space and immediately stabilizing the surrounding soil.
Properly executed backfilling allows the tunnel alignment to be maintained within design limitations and ensures the long-term structural behavior of the lining. But the quality of the process depends not only on the injected grout: the sealing systems located at the end of the shield, namely the tail brushes (BrushX) and the sealing grease, play an equally important role.
After introducing the operation of the injection system and the two-component grout, this article delves into these key elements, with particular attention to AB Tunnelling products and our technical partners.
This is why TBM backfilling plays a central role not only in soil stabilization but also in maintaining controlled pressure conditions throughout the excavation cycle, ensuring both safety and the structural integrity of the tunnel.
TBM backfilling: the operating principle of the injection system
Backfilling occurs at the end of the TBM shield, in direct contact with the estrados of the precast segments. The backfill is conveyed through pumping lines to nozzles located in the TBM’s tail shield and pumped into the annular space formed behind the TBM’s tail shield between the extrados of the precast segments and the excavated soil. Brushes and sealing grease create a sealing chamber that allows the backfill to properly fill the annular space without leaks, maintaining the necessary pressure during injection.
The system uses two distinct components:
– Liquid A: cement + bentonite + water + retarders
– Liquid B: sodium silicate
A few seconds after mixing, the mortar changes from fluid to gel; after about an hour, it hardens, reaching the strength required by the specifications. The final strength depends on the cement dosage and the type of silicate used.
The next phase: the importance of the tail shield seal
After the injection phase, the quality of the backfilling depends largely on the TBM’s ability to ensure a perfect seal in the tail shield. This result is achieved thanks to two fundamental elements:
– the tail brushes (BrushX)
– the tail sealing grease
AB Tunnelling develops high-performance brushes, called BrushX, designed to ensure the best possible seal and minimize the risk of leaks, soil ingress, and concrete lining instability.
BrushX brushes: designed to ensure maximum sealing
Tail brushes are a strategic element for the success of backfilling. Their effectiveness depends on a series of design factors.
✔️ Material quality and thickness of spring plates
The spring plates that support the brush body must guarantee elasticity, resistance, and geometrical maintenance even under pressure, avoiding deformations that could compromise the seal.
✔️ Dimensions and distribution of crimped wires
The crimped wires are the heart of the brush. BrushX tail skin brushes are characterized by:
– high-strength wires
– calibrated crimping
– optimized quantity in each brush
– configurations designed to minimize leaks and consumption.
These elements provide an effective barrier against water, soil, and mortar, maintaining the operating pressures required by the TBM.
✔️ Additional sealing systems
To further increase sealing, BrushX tail skin brushes can be integrated with:
– neoprene sheets
– teflon sheets
These materials increase sealing and improve the ability to withstand high pressures, reducing leaks and sealing grease consumption.
Globalchimica sealing grease
The sealing grease is another key component of the tail seal system. Its purpose is to prevent soil and water from entering the shield during excavation, maintaining the tightness of the entire system.
AB Tunnelling, through its partner Globalchimica, is able to supply high-performance sealing greases with specific characteristics for mechanized tunneling.
✔️ Sealing range: up to 10 bar
The grease must withstand very high pressures. The products offered by AB Tunnelling cover an operating range of up to 10 bar, depending on the geotechnical requirements and the type of TBM used.
✔️ Water wash-out resistance
The grease guarantees:
– high adhesion to metals
– volumetric stability
– water wash-out resistance
– durability in terms of resistance to high pressures without decomposing
✔️ Consistent pumpability even at low temperatures
To ensure operational continuity, the grease must be easily pumped, avoiding overloads or blockages.
✔️ Biodegradability
The products are formulated to reduce environmental impact, an essential aspect on modern construction sites.
✔️ Cellophane-lined drums to avoid hazardous waste
The cellophane lining allows the drums to be disposed of as ordinary waste, simplifying construction site logistics and reducing costs.
Why brushes and sealing grease are as crucial as the mortar
In many applications, the quality of the backfill is primarily attributed to the injected cement mix. In reality, proper execution also depends (and above all) on the TBM’s ability to maintain a perfect seal around the tail shield.
A TBM with inefficient brushes or inadequate sealing grease can:
– disperse the grout before gelation
– reduce support pressures
– cause soil ingress
– trigger surface settlements
– drastically increase grease consumption and operating costs
– compromise the concrete lining geometry
BrushX tail skin brushes and Globalchimica grease are designed to prevent precisely these risks, ensuring a stable and controlled grouting environment.
TBM system integration: a comprehensive engineering approach
AB Tunnelling offers a coordinated ecosystem of solutions for Tunnel Boring Equipment, including:
– tail brushes (BrushX)
– sealing grease (together with Globalchimica)
– bentonite
– support materials
Each component is designed or selected to ensure:
– compatibility with TBM specifications
– customizable configurations
– reduction in maintenance
– long life
– wear resistance
🔗 Learn more about AB Tunnelling’s Tunnel Boring Equipment — engineered for precision, durability, and efficiency.
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