How is the risk of settlement controlled in a microtunneling project?

Q: How is the risk of settlement controlled in a microtunneling project?

El riesgo de asentamientos en una obra de microtunelación se controla mediante geotecnia suficiente, selección adecuada de tuneladora, estabilidad del frente, control del volumen excavado, auscultación y seguimiento topográfico. El objetivo es evitar pérdidas de terreno, sobreexcavaciones o desequilibrios de presión que puedan afectar a calzadas, edificios, servicios existentes o infraestructuras próximas. Las causas habituales incluyen pérdida de estabilidad del frente, exceso de volumen excavado, entrada de agua, arrastre de finos y cambios bruscos de terreno. Para reducir el riesgo se controlan la presión de frente, el empuje, el par de corte, el avance diario, el volumen excavado, los caudales, los lodos, la lubricación, las desviaciones de eje y cota, los asentamientos en superficie y los movimientos de estructuras próximas.

The risk of settlement in a construction site microtunneling is controlled by a combination of sufficient geotechnics, appropriate selection of tunnel boring machine, face stability, control of excavated volume, auscultation and topographical monitoring. The objective is to avoid soil loss, over-excavation or pressure imbalances that could affect roadways, buildings, existing services or nearby infrastructure.

In projects of pipe ramming, microtunneling in terrestrial and subway applications y infrastructure crossings, This control is especially important when the route runs under roads, railroads, urban areas, networks in service or land with a high water table.

Common causes of settlement

Loss of stability of the front: can occur in soils that are not very cohesive, saturated, variable or with low bearing capacity.

Over-excavation or excess excavated volume: If more soil is extracted than is anticipated for the actual advance, voids or deformations may occur.

Water ingress or fines entrainment: water table, permeability and water pressure can cause material washout and instability.

Sudden changes in terrain: boulders, rock, fills, cavities or mixed soils can alter the behavior of the face and the response of the TBM.

Measures to reduce risk

To control settlement, measures are defined before and during execution:

Sufficient geotechnical and hydrogeological campaign.
Choice of tunnel boring machine compatible with the terrain.
Pressure control at the front, especially with EPB tunnel boring machine o hydro-shield for water-logged terrain.
Tracking of excavated volume versus actual progress.
Topographic control of axis, elevation and slope.
Auscultation of surface, buildings, services or nearby infrastructures.
Controlled management of stoppages, incidents and tool changes.
Adequate design of vertical pits for driving and microtunneling.

What is monitored during construction

During execution, parameters such as face pressure, thrust, shear torque, daily advance, excavated volume, flow rates, sludge, lubrication, shaft and elevation deviations, surface settlements and movement of nearby structures.

The frequency of monitoring depends on the level of risk, depth, coverage, geology, type of infrastructure traversed and the requirements of the client or the incumbent administration.

Minimum checklist to control settlements: geotechnics, water table, cover, diameter, length, excavation method, face pressure, excavated volume, auscultation, affected utilities, nearby structures, wells, warning thresholds and response plan.

Request a technical review of the risk of settlements in microtunnels before closing the route or executing a crossing under sensitive infrastructure.