How do I choose between open shield, EPB and hydroshield?

Q: How do I choose between open shield, EPB and hydroshield?

The choice between open shield, EPB and hydroshield depends on the geology, presence of water, face stability, ground pressure, grain size, abrasiveness and tolerances required by the project. The open shield is usually valued in stable ground with low presence of water. The EPB is used when it is necessary to balance the earth pressure at the face, especially in cohesive or conditionable soils. The hydroshield is considered in saturated, permeable soils or with significant water pressure, where it is necessary to stabilize the face by slurry pressure. The decision is made by analyzing the geotechnics, water table, permeability, diameter, length, depth, environment, construction risks, ancillary civil works and execution constraints.

The choice between open shield, EPB and hydroshield depends mainly on the geology, the presence of water, face stability, ground pressure, granulometry, abrasiveness and the tolerances required by the project. There is no universal TBM: each excavation system responds best to specific ground conditions, safety and face control.

In an initial phase, Eurohinca analyzes the available geotechnical and hydrogeological information to define whether the project can be implemented with open shield, EPB closed shield TBM o hydro shield. This decision is part of the process of choice of tunnel boring machine and conditions safety, precision, performance and risk control during execution.

Main selection criteria

Open shield: is usually valued in stable soils, with low water presence and conditions where the face can be kept under control during excavation. It is a suitable option when the geology allows working with direct or semi-direct access to the face, provided that the risk of instability, water ingress or loss of material is compatible with the method.

EPB -Earth Pressure Balance-: is used when it is necessary to balance ground pressure at the face, especially in cohesive soils or soils where the excavated material can be conditioned to maintain stability. The EPB tunnel boring machine allows to control the excavation pressure and reduce the risk of subsidence, over-excavation or loss of face.

Hydroshield: is considered when there are saturated soils, high permeability, significant presence of water or the need to stabilize the face by mud pressure. The hydro-shield for water-logged terrain is especially relevant in crossings under rivers, coastal areas, outfalls, catchments or routes with high water table.

Data needed to decide

To make the right choice between open shield, EPB and hydroshield it is necessary to study:

Geology of the layout: type of soil or rock, presence of boulders, mixed soils, fracturing, abrasiveness and geotechnical variability.

Hydrogeological conditions: water table, permeability, water pressure, risk of water inflow and need for face control.

Geometry of the work: diameter, length, depth, slope, radius of curvature and alignment tolerances.

Environment crossed: roads, railroads, watercourses, buildings, existing services, urban areas or environmentally sensitive areas.

Construction risks: subsidence, over-excavation, loss of stability, tool wear, head blockage, water ingress or deviations in routing.

Logistics and auxiliary civil works: space available for vertical pits for driving and microtunneling, The project includes the management of sludge or excavated material, accesses, stockpiles and auxiliary equipment.

In practical terms, the open shield is associated with more stable and controllable terrain; the EPB is used when earth pressure equilibrium is required; and the hydro shield is selected when water, permeability or hydraulic pressure are determining factors. The final validation must be done with sufficient geotechnical information and a specific technical review of the layout.

Minimum checklist for selecting the type of shield: geotechnical, water table, permeability, water pressure, diameter, length, depth, tolerances, infrastructure traversed, well spacing and site restrictions.

Request a technical review to choose the most suitable tunnel boring machine according to the geology, the water table and the constraints of your project.