How is the optimal layout of a microtunnel defined prior to construction?

Q: How is the optimal layout of a microtunnel defined prior to construction?

El trazado óptimo de un microtúnel se define combinando criterios de funcionalidad hidráulica o técnica, geotecnia, profundidad, pendiente, radio de curvatura, servicios afectados, ubicación de pozos, método constructivo, tolerancias y riesgos de ejecución. No se trata solo de unir un punto de entrada y salida: el trazado debe ser construible, seguro, controlable y compatible con la tecnología SIN zanja seleccionada. Para optimizarlo se analizan planta, perfil longitudinal, datos geotécnicos, nivel freático, interferencias, infraestructuras atravesadas, ubicación de pozos, radios de curvatura, sistemas de guiado y restricciones de obra. La revisión puede llevar a ajustar profundidad, desplazar el eje, cambiar la ubicación de pozos, dividir el cruce en tramos, ampliar la geotecnia o comparar alternativas como hinca de tubería, perforación horizontal dirigida o Direct Pipe.

The optimal layout of a microtunnel is defined by combining criteria of hydraulic or technical functionality, geotechnics, depth, slope, radius of curvature, affected services, location of wells, construction method, tolerances and execution risks. It is not just a question of joining an entry and exit point: the layout must be constructible, safe, controllable and compatible with selected trenchless technology.

In a previous phase, Eurohinca analyzes the ground plan, longitudinal profile, geotechnical data, interferences and site constraints to assess whether the route can be executed by pipe ramming, microtunneling in terrestrial and subway applications or another solution such as horizontal directional drilling o Direct Pipe.

Criteria for defining the layout

Driving function: the layout must meet the purpose of the project: sewerage, supply, drainage, gas, outfall, catchment, infrastructure crossing or other conduction. In gravity networks, slope and elevations are especially critical; in pressurized pipelines, depth, radii, connections and installation constraints may be more important.

Plan and longitudinal profile: alignment, length, entry and exit elevations, minimum depth of cover, slope, radii of curvature and connection points are studied. The layout must allow maintaining the axis and elevation tolerances during the execution.

Geotechnics and hydrogeology: the layout must be contrasted with the terrain profile, water table, presence of rock, boulders, mixed terrain, abrasive soils, permeability and possible water pressures. This information conditions the choice of tunnel boring machine and the need for open shield, EPB or hydroshield.

Interference and existing services: the position of water, sewage, gas, electricity, telecommunications, foundations, tunnels, roads, railroads, watercourses or critical infrastructures is reviewed. The layout must maintain sufficient clearances and reduce risks in the following areas infrastructure crossings.

Location of wells: the vertical pits for driving and microtunneling condition the beginning and end of the section. They should be located where there is space for equipment, push frames, pipe stockpiles, cranes, access, pumping, ventilation and safety. Sometimes, the optimal layout depends more on where the shafts can be executed than on the shortest line.

Depth and coverage: the pipeline must have sufficient coverage to avoid interference, protect existing infrastructure, maintain ground stability and meet hydraulic or structural requirements. Excessive depth, however, can increase the cost of wells, pumping, lifting and safety.

Radius of curvature and guidance: If the route includes curves, radii compatible with the pipe, the TBM, the guidance system, the length of the section and the admissible tolerances must be verified. In curved routes, the topographic control and the navigation system acquire greater relevance.

Execution risks: risks of settlement, loss of face, deflection, water ingress, tool wear, excessive thrust, head blockage, interference with utilities or access limitations are analyzed.

How the layout is optimized prior to the construction site

The layout is optimized by comparing alternatives in plan and profile. In this review, depth adjustments, axis displacements, change of well location, division into several sections, modification of slopes, extension of geotechnics or selection of another trenchless technology may be proposed.

In urban environments, crossings under roads or railroads, watercourses, outfalls, catchments or areas with critical services, this early review reduces uncertainty and avoids costly changes during execution.

Expected result of the layout revision

Once the route has been analyzed, a technically feasible solution must be defined with axis, profile, wells, recommended technology, main risks, necessary controls and execution conditions. This analysis facilitates the preparation of bids, tenders, permits and work planning.

Minimum checklist to review the layout of a microtunnel: ground plan, longitudinal profile, length, diameter, inlet and outlet elevations, slope, depth, bending radii, geotechnics, water table, services affected, infrastructure crossed, location of wells, accesses, work restrictions and required tolerances.

Request a technical review of your microtunnel layout before closing the design or preparing the bid.