At the inlet and outlet of a tunnel boring machine, the following can be applied soil improvement techniques to stabilize the face, control water, reduce material loss, and ensure a safe transition between the shaft and the natural ground. The most common methods include grouting, jet grouting, artificial freezing, lowering the water table, waterproofing treatments, diaphragm walls, bottom plugs, pre-cutting, or localized soil replacement.
In projects of pipe ramming, microtunneling in terrestrial and subway applications o vertical pits for driving and microtunneling, these measures are considered especially when there is a high water table, soft ground, permeable soil, fill, a risk of siphoning, low cover, nearby utilities, or a need to ensure watertightness at the start and end of the pipe.
Why Entry and Exit Are Critical Points
The entry from the launch shaft and the exit to the reception shaft are critical phases because the tunnel boring machine passes through a transition zone between an excavated structure—the shaft—and the natural ground. At this point, water ingress, loss of fine particles, face instability, deformations, settlement, or watertightness issues may occur around the tunnel boring machine’s path.
The risk increases when:
- High water table.
- Granular or permeable soils.
- Heterogeneous fillers.
- Soft clays or low-strength soils.
- High water pressure.
- Low coverage on the key.
- Nearby infrastructure or services.
- Deep wells or wells with a bottom subject to negative pressure.
- Connections to outfalls, intake structures, watercourses, or coastal areas.
In these cases, the choice of tunnel boring machine and land treatment must be analyzed together.
Common Land Improvement Techniques
| Technique | What is it used for? | When might it be appropriate? |
|---|---|---|
| Consolidation Injections | Improve strength and reduce permeability | Cracked soils, fill, sand, or transition zones |
| Jet grouting | Create treated blocks or soil-cement columns | Inlets/outlets with water, low resistance, or requiring a watertight seal |
| Artificial freezing | Temporarily Stabilize Saturated Soil | Wet soils and complex geometries where other techniques are insufficient |
| Lowering the Water Table | Reduce water pressure and facilitate excavation | Permeable soils with viable and controlled pumping |
| Bottom plug | Prevent hydraulic lift or siphoning | Deep wells with negative pressure or water entering from the bottom |
| Waterproof screens or enclosures | Seal the well and control leaks | Urban wells, areas with water, or nearby services |
| Localized Replacement | Remove the problematic soil and replace it | Surface areas with fill, soft ground, or obstacles |
| Sealing Treatments | Improve the seal around the opening | Start-up, commissioning, well joints, and connections |
How is the decision made about which technique to use?
The appropriate technique depends on the geotechnical conditions, the water table, the depth, the diameter, the water pressure, the permeability, the location of the wells, the available space, and safety considerations.
To define it, the following are analyzed:
- Geotechnical profile at the entrance and exit.
- Water table and water pressure.
- Permeability and the risk of fine-grained material being carried away.
- Diameter of the tunnel boring machine.
- Well depth and coverage.
- Distance to services, buildings, or infrastructure.
- Excavation method: open shield, EPB tunnel boring machine o hydro-shield for water-logged terrain.
- Settlement tolerances.
- Accessibility for performing the treatment.
- Timelines, Permits, and Environmental Monitoring.
It is not always necessary to stabilize the ground. In stable soils with low water content, proper design of the borehole, the inlet/outlet joint, and the drilling procedure may be sufficient. However, in saturated, permeable, or weak soils, ground stabilization can be critical to the safety of the project.
What risks do these treatments reduce?
Improvements to the entry and exit areas help reduce:
- Sudden influx of water into the well.
- Fine-grained material transport or soil loss.
- Front instability during startup.
- Surface settlements.
- Siphoning or bottom lifting.
- Leaks around the tunnel boring machine.
- Initial alignment deviations.
- Issues with the delivery of the shield.
- Damage to nearby utilities or structures.
- Stops and additional costs at the beginning or end of the leg.
Checks During Execution
During treatment and the startup or handover of the tunnel boring machine, parameters such as water pressure, pumping rates, settlements, shaft deformations, treatment quality, achieved resistance, residual permeability, exit alignment, joint tightness, face pressure, excavated volume, and incidents.
In construction projects with infrastructure crossings, urban areas, or the presence of critical services, these monitoring measures can be supplemented with topographic monitoring and alert thresholds.
Minimum checklist for evaluating site improvements: geotechnical conditions at the entrance and exit, water table, permeability, water pressure, diameter, well depth, overburden, affected utilities, risk of siphoning, stability of the excavation face, excavation method, access points for treatment, waterproofing requirements, monitoring, and contingency plan.
Request a Technical review of site improvements for the tunnel boring machine's entry and exit points before drilling wells, starting work, or accepting the microtunnel.
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