Microtunneling is a highly controlled method, but its success depends on the correct evaluation and management of the geotechnical risks, especially in urban environments, heterogeneous terrain or areas with a high water table. Typical risks can affect both the stability of the face and the behavior of the pipe during jacking. Eurohinca, with more than 120 km executed in trenchless technologies, addresses these risks under strict engineering, operation and QA/QC procedures.
The most frequent risks and on-site mitigation strategies are described below:
Terrain variability and sudden changes in lithology.
Jumps between cohesive and granular soils, coarse gravels, boulders, or soil-rock transitions can lead to loss of lift, The use of the system is not recommended for the following reasons: pressure decompensation or accelerated wear of the head.
Mitigation:
Pre-selection of the shield type (EPB or Hydroshield) according to geotechnics.
Chamber pressure control to balance thrust and avoid voids.
Automatic readjustment of cutting torque and feed speed.
Continuous lubrication to reduce pipe overstress.
The importance of choosing the right method according to the terrain is explained in what is pipe jacking and how is it applied in subway projects?.
2. Water inflows and water table pressure.
Water can cause erosion, face instability, fines entrainment and risk of surface settlement.
Mitigation:
Precise adjustment of chamber pressure to balance the front.
Use of Hydroshield with pressurized muds for saturated soils.
Modification of the sludge or polymer mixture to improve rheology.
Continuous monitoring of sludge consumption, pumping and return.
The article management of pressures under high groundwater with hydroshields describes the principles of front-end stability and hydraulic control.
3. Loss of lift and surface settlements
In granular or poorly cohesive soils, excavation can generate voids, causing surface deformations.
Mitigation:
Simultaneous control of face pressure, cutting torque and speed.
Grout or bentonite injection to compensate voids.
Topographic monitoring of the terrain with robotic stations.
Continuous digital recording of feed rate and thrust.
4. Increased torque and risk of pipe blockage.
It may be due to excessive friction, lack of lubrication, demanding geometry or abrasive terrains.
Mitigation:
Repeated lubrication of the conduit (slurry/bentonite).
Intermediate thrust stations according to length and curvature.
Alignment control to avoid accumulated deviations.
Thermal management of the spindle and tool change if necessary.
In projects with curvature, the case microtunnel in curve at El Arenao shows how thrust and lubrication management allows tolerances to be maintained without exceeding limit loads.
5. Internal erosion and fines entrainment.
Loss of fines can compromise soil stability and lead to subsidence.
Mitigation:
Adjustment of sludge viscosity and density.
Monitoring of extracted vs. expected volume.
Exhaustive control of permeability and hydraulic gradients.
6. Risks due to the presence of nearby infrastructure
The proximity of pipelines, foundations or utility networks requires maximum precision.
Mitigation:
Layout with strict tolerances in plan and elevation.
Laser/gyro navigation with deviation alarms.
Continuous validation of the as-built during driving.
In all phases, Eurohinca applies its own protocols of monitoring, QA/QC and HSE management, based on their experience with microtunnels and subway and overland installations.
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