The minimum coverage for a microtunnel It is determined by analyzing the vertical distance between the tunnel or pipe bench and the surface or infrastructure being crossed. There is no single standard applicable to all cases: it depends on the geotechnical conditions, the diameter, the excavation method, the water table, surface loads, the type of infrastructure, the owner’s requirements, and the acceptable risk of settlement or damage.
At underpasses roads, railways, or other infrastructure, the covering must allow the work to be carried out while ensuring stability of the excavation face, control of deformations, structural safety, and sufficient separation from road surfaces, ballast, foundations, drainage systems, existing utilities, or watercourses. At hydraulic crossings, it may also apply to river and watercourse crossings, where erosion, scouring, the water table, and channel dynamics all play a role.
Factors Affecting Minimum Coverage
Geotechnical Properties of the Soil:
The strength, cohesion, grain size, presence of rock, boulders, fill material, soft soil, cavities, or mixed ground all affect the stability around the microtunnel. Competent ground may allow for solutions with lower risk; soft or heterogeneous ground typically requires more extensive testing.
Microtunnel diameter:
The larger the diameter, the greater the excavated volume and the greater the potential impact on the ground. The relationship between diameter, depth, and overburden is key to assessing deformations, stability, and compatibility with the infrastructure being crossed.
Water table and water pressure:
When groundwater or hydraulic pressure is present, the overburden must be analyzed in conjunction with the stability of the excavation face, permeability, and the excavation system. In these cases, it may be necessary to assess EPB tunnel boring machine o hydro-shield for water-logged terrain.
Surface loads:
For roads and railways, factors such as traffic loads, trains, embankments, platforms, vibrations, nearby structures, and potential operational limitations are taken into account. The pavement must be compatible with these loads and meet the required deformation limits.
Type of infrastructure crossed:
Crossing a local road is not the same as crossing a highway, a railroad track, a waterway, a port area, or critical infrastructure. Each contracting authority may establish specific requirements regarding depth, clearance, monitoring, construction procedures, and tolerances.
Existing utilities and drainage systems:
The cover must be inspected along with buried utility lines, collectors, longitudinal drains, ditches, foundations, piles, electrical conduits, telecommunications lines, and active pipelines.
Risk of subsidence:
When the cover is thin, sensitivity to ground loss, over-excavation, or pressure imbalances increases. For this reason, the cover is analyzed in conjunction with the monitoring plan, topographic control, and excavation parameters.
How coverage is calculated or verified
Minimum coverage is verified through a technical inspection that combines:
- Plan and longitudinal profile of the crossing.
- Outer diameter of the pipe or sleeve.
- Vertical distance to the surface, roadbed, ballast, riverbed, or foundations.
- Geotechnical Engineering and Hydrogeology.
- Water table and permeability.
- Traffic, rail, or hydraulic loads.
- Separation from existing services.
- Construction method and type of tunnel boring machine.
- Risk of subsidence.
- Requirements for the infrastructure operator.
- Monitoring and Control Plan.
The choice of tunnel boring machine has a direct impact: an open shield, an EPB, or a hydroshield have different front-control capabilities and, therefore, different stability and coverage requirements depending on the terrain.
Coverage under a road, railroad, or waterway
Under the highway:
The subgrade, embankment, traffic, drainage, existing utilities, impact on traffic lanes, inspection, and settlement tolerances are reviewed. The pavement must prevent deformations that could affect the roadbed or traffic safety.
Under the railroad tracks:
The analysis is typically more rigorous due to the track’s sensitivity to differential movements. Factors considered include ballast, track bed, dynamic loads, clearance limits, operating restrictions, operating speed, monitoring, and alert thresholds defined by the operator.
Low water level:
The following factors are analyzed: depth below the riverbed, scouring, erosion, hydraulic regime, water table, permeability, risk of water ingress, environmental impacts, and permits from the competent authority.
Why It Shouldn't Be Set as a General Rule
Although there may be guidelines or requirements set by the owner, the cover should not be determined solely by a generic rule. A cover that appears sufficient may be critical in soft or saturated soil or where utilities are nearby. And a reduced cover may be feasible if the soil is stable, the excavation method is appropriate, and site supervision is well defined.
Therefore, the minimum cover must be justified on a case-by-case basis based on geotechnical analysis, longitudinal profile, construction method, risk analysis, and the constraints of the infrastructure being crossed.
Minimum checklist for defining coverage: plan and profile, outer diameter, depth, keystone coverage, geotechnical analysis, water table, permeability, traffic or rail loads, channel and scouring, affected utilities, drainage systems, excavation method, settlement tolerances, owner’s requirements, and monitoring plan.
Request a Technical review of minimum coverage requirements for microtunneling or pipe jacking under infrastructure before closing the route or processing permits.
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