The entry and exit seal for a tunnel boring machine It is designed to allow the controlled passage of the shield between the shaft and the ground, preventing water ingress, fine-grained material carryover, loss of pressure at the cutting face, and seepage around the machine or the pipe. When water pressure is present, the seal must be verified in conjunction with geotechnical data, the water table, hydraulic pressure, the well’s watertightness, and the start-up or acceptance procedure.
In projects of pipe ramming, microtunneling in terrestrial and subway applications o vertical pits for driving and microtunneling, this point is critical because the transition between the well and the surrounding ground often poses risks of seepage, instability, and material loss.
Why the seal is critical under water pressure
When the tunnel boring machine enters or exits the shaft, a gap opens up in the support structure. If the ground is saturated or there is hydraulic pressure, water can find its way through the weakest point among the ground, the shield, the pipe, and the shaft wall.
A poorly designed stamp can cause:
- Sudden influx of water into the well.
- Sediment transport and soil loss.
- Front instability during the takeoff or landing.
- Surface settlements.
- Leaks around the pipe.
- Pressure loss in closed shields.
- Difficulty maintaining the initial course.
- Incidents during the installation, removal, or recovery of the tunnel boring machine.
In construction projects with a high water table, waterproofing must be coordinated with the choice of tunnel boring machine, especially when working with EPB tunnel boring machine o hydro-shield for water-logged terrain.
Common elements of an entry or exit stamp
| Element | Main function | When is it relevant? |
|---|---|---|
| Seal or gasket | Seal the perimeter of the shield or pipe | Wells with a water table or water pressure |
| Entrance sign or frame | Secure and compress the sealing system | Starts with strict tolerances |
| Wall penetration or reinforced opening | Allow the tunnel boring machine to pass through while maintaining the stability of the shaft | Retaining walls, piles, rings, or well structures |
| Dual seal or redundant system | Improve security against data breaches | High water pressure or a sensitive environment |
| Injections or sealing grout | Fill gaps and improve ground-to-structure contact | Permeable soils, fill, or irregular contacts |
| Site Preparation | Reduce permeability and improve strength | Sand, gravel, soft ground, or entering/exiting underwater |
| Pumping and Piezometric Control | Managing Pressure and Flow Rates | Deep wells or active aquifers |
Information needed to design it
To define the seal, the following are reviewed:
- Water table and expected water pressure.
- Soil permeability and grain size distribution.
- Outer diameter of the tunnel boring machine and the pipe.
- Tolerances between the flange, gasket, and opening.
- Well type and support system.
- Thickness of the wall, screen, piles, or rings.
- Digging or receiving depth.
- Stability of the front during the transition.
- Excavation method: open shield, EPB, or hydro-shield.
- Working pressure of the tunnel boring machine.
- Possible treatments to improve the soil.
- Space available for assembly, inspection, and maintenance.
- Emergency Procedure for Leaks.
How the solution is defined
The logo design should not be treated as an isolated detail. It must be coordinated with three design decisions:
1. Well Design
The borehole must be stable and watertight in the passage area. The entry or exit opening must withstand thrusts, seepage, installation tolerances, and possible deformations. In water-saturated soils, it may be necessary to supplement the support with grouting, jet grouting, a bottom plug, or localized treatments.
2. Site preparation in the launch or landing area
When the ground is permeable, soft, or heterogeneous, prior ground improvement may be required to create a more stable and less permeable block around the tunnel boring machine’s path.
3. Operating Procedure
Start-up and commissioning must be carried out in a controlled sequence: seal installation, leak test, pressure check, initial excavation, flow monitoring, gradual advancement, and response to leaks.
Additional Measures for High Water Pressure
When water pressure is significant, more robust solutions can be used:
- Double seal.
- Chambers or intermediate spaces for injection.
- Sealed with grease, mortar, or grout.
- Injections applied around the wall penetration.
- Jet grouting or treated block at the outlet.
- Artificial freezing in special cases.
- Controlled pumping and piezometric monitoring.
- Preliminary leak tests.
- Contingency Plan for Water Ingress.
The selection depends on the hydraulic risk, depth, soil type, diameter, surroundings, and the consequences of potential seepage.
What is checked during startup or reception
As the tunnel boring machine enters or exits, water flow rates, pressure, turbidity, fine-grained material carryover, shaft deformations, seal performance, initial alignment, face pressure, excavated volume, settlements, and any abnormal seepage are monitored.
At infrastructure crossings, urban areas, waterways, or sensitive environments, this monitoring can be supplemented with auscultation and alert thresholds.
Minimum checklist for designing the seal: water table, water pressure, permeability, grain size, shield diameter, pipe diameter, tolerances, well type, wall thickness, support system, working pressure, ground treatment, pumping, installation space, leak testing, and emergency plan.
Request a Technical inspection of the water-pressurized entry and exit seals of the tunnel boring machine Before drilling the well, starting up the rig, or preparing to receive the shield.
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