How does the water table influence the choice between EPB, hydroshield or open shield?

Q: How does the water table influence the choice between EPB, hydroshield or open shield?

The water table is influential because it determines the presence and pressure of water at the excavation face and thus the risk of water ingress, face instability, ground loss and settlement. With high water table or high pressures a closed shield is usually preferred: EPB is common in thin, low permeability soils (clays and silts) using the conditioned ground to balance pressure; hydro-shielding (slurry) is usually prioritized in permeable, saturated granular soils (sands and gravels) for its control of pressures and seepage. Open shield is considered when it can be worked without pressurization and with low risk of water ingress. The final choice also depends on the permeability, granulometry, coverage and sensitivity of the environment.

The water table (NF) influences because it determines the presence and pressure of water at the excavation face. When the tunnel is working below the NF, the risks of water inlet, front instability, land losses y seats; It is therefore often necessary to excavate with the front end. pressurized by means of a closed shield.

Practical selection criteria:

Open shield: usual option when it is possible to work without pressurization (stable front and low or controllable risk of water ingress).
EPB closed shield: usually fits with NF high at fine/cohesive soils (clays, silts) or mixed with low-medium permeability, where the control of the front is supported by the conditioned land and pressure equilibrium.
Closed shield Hydroshield (slurry): usually preferred with NF high at permeable granular soils (sands/gravels) and/or with high water pressures, The system is designed to keep the face pressurized by slurry and to better manage inflows and seepage.

In practice, it is not just the NF “in meters” that decides, but the combination of NF + permeability + granulometry + coverage + sensitivity of the environment (urban, under rivers or coast). For further criteria, see Closed shield tunnel boring machines y Choice of tunnel boring machine.

Minimum recommended data to decide EPB vs Hydroshield vs Open:

NF elevation and variations (seasonal/tidal if applicable).
Permeability and grain size (fines vs. sands/gravels).
Water pressure at front elevation (and coverage).
Admissible seating risk (services, buildings, infrastructure).
Environmental/logistical constraints (sludge management, pumping, discharge).

If you want us to validate it for a specific case (method, equipment and operating ranges), please contact us at Technical assistance and engineering.