A high water table can have a decisive effect on a microtunneling project because it increases the water pressure on the excavation face, the shafts, the pipe joints and the excavated material extraction systems. When the ground is saturated or highly permeable, it is necessary to control the stability of the face, prevent water ingress, limit ground losses and ensure watertightness throughout the execution.
In this type of scenario, Eurohinca analyzes the geotechnics, hydrogeology, depth, expected pressure and permeability of the ground to define if the microtunnel should be executed with EPB closed shield TBM, hydro-shield for water-logged terrain or other solution within the trenchless technologies.
Main effects of high water table
Pressure on the excavation face: groundwater can destabilize the front if not properly balanced. Therefore, in saturated soils, the use of closed shields capable of controlling the working pressure is valued.
Water inlet in wells and tunnels: The design of the attack and reception wells must consider hydraulic thrusts, waterproofing, pumping, joints, ground treatments and access safety. The design of the vertical pits for driving and microtunneling is especially critical in construction sites with a high water table.
Risk of soil loss or subsidence: in permeable or poorly cohesive soils, uncontrolled water ingress can entrain fines, generate voids and cause surface settlement. This risk is especially relevant in infrastructure crossings, urban areas, roads, railroads or utilities in operation.
Conditioning the choice of tunnel boring machine: the water table directly influences the choice of tunnel boring machine. Depending on the terrain, it may be necessary to work with earth pressure -EPB- or mud pressure -hydroshield- to keep the face stable.
Management of sludge, water and excavated material: the presence of water can modify the system of extraction, separation, treatment and management of excavated material. In hydroshields, the sludge circuit and the separation plant are an essential part of operational control.
Sealing requirements: pipe joints, well connections and inlet and outlet points must be designed to withstand the anticipated hydraulic pressure and prevent leaks during construction and in service.
How to mitigate the risk associated with water
Mitigation begins with a sufficient geotechnical and hydrogeological campaign: boreholes, permeability tests, water table measurement, water pressure analysis and soil characterization. With this information, the excavation system, working pressure, well design, pumping systems, auxiliary treatments and monitoring controls are defined.
During execution, parameters such as face pressure, thrust, torque, flow rates, excavated volume, sludge density and pressure, settlements, seepage and well behavior are monitored. The objective is to maintain face stability, control water and reduce the risk of surface incidents.
In practical terms, a high water table alone does not make a microtunneling job unfeasible, but it does require proper TBM selection, careful borehole design and more stringent operational control.
Minimum checklist for assessing a microtunnel with a high water table: depth of the layout, measured water table, expected water pressure, permeability, granulometry, geotechnics, diameter, length, type of pipeline, location of wells, foreseeable flow rates, nearby infrastructure and settlement tolerances.
Request a technical review for microtunnels with high groundwater table and attach available geotechnical and hydrogeological data to assess the safest solution.
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