The bumps, blocks, and buried obstacles These are detected prior to microtunneling by combining geotechnical investigations, site surveys, in-situ tests, a review of documentation, and, when applicable, geophysical methods or localized test pits. The objective is to identify hard features, heterogeneities, or interferences that could affect the tunnel boring machine’s progress, tool wear, the stability of the tunnel face, or the risk of jamming.
In projects of pipe ramming, microtunneling in terrestrial and subway applications y infrastructure crossings, this early detection is critical because an unforeseen obstacle can lead to work stoppages, detours, cost overruns, tool changes, or even the need to modify the construction method.
What we are trying to detect
Before beginning the project, it is advisable to identify:
- Natural bowling pins and blocks, commonly found in alluvial deposits, glaciers, colluvial deposits, or heterogeneous soils.
- Sound or Fractured Rock, especially if it occurs sporadically within softer soils.
- Anthropic landfills, with remnants of concrete, masonry, breakwaters, old foundations, or debris.
- Undocumented underground utilities, such as old pipes, tunnels, drains, cables, or off-plan collectors.
- Nearby structural elements, such as piles, diaphragm walls, foundations, slabs, walls, or anchors.
- Cavities or holes, which can affect the stability of the slope and the behavior of the ground.
Common Detection Methods
Survey Campaign:
Boring surveys allow for the assessment of the ground along the route and in the areas where wells are to be drilled. They provide core samples, a stratigraphic description, the depth of strata, and information on the presence of rock, boulders, fill material, or abrupt changes in material. Their density and depth must be tailored to the project’s risk profile.
Field and laboratory tests:
SPT, CPT, penetrometers, strength tests, particle size analysis, abrasiveness tests, and simple compression tests help assess the excavability of the soil and the possible presence of hard or heterogeneous materials.
Historical and Documentary Review:
Old maps, geological surveys, information on previous construction projects, existing utility networks, orthophotos, urban development plans, or documentation of nearby infrastructure can reveal fill material, foundations, old riverbeds, tunnels, or utilities that are not visible.
Applied Geophysics:
In certain cases, ground-penetrating radar, seismic surveys, electrical tomography, magnetometry, or other indirect methods may be used to locate anomalies. Their effectiveness depends on the terrain, depth, moisture content, urban interference, and the size of the obstacle.
Localized test drilling or exploratory drilling:
When there is suspicion of utilities, fill material, foundations, or shallow obstructions, test pits allow for physical verification of the information before beginning the pile driving.
Inspection of affected services:
The location of existing networks must combine company maps, field surveys, manhole inspections, test pits, and coordination with utility providers.
How Detection Affects the Choice of Tunnel Boring Machine
The presence of boulders, rock formations, or mixed terrain affects the choice of tunnel boring machine, the design of the cutting head, the tools, the chip removal system, the feed parameters, and the maintenance strategy.
If the ground is stable and suitable for excavation, you may want to consider a open shield. If ground pressure control is required, one option to consider is a EPB tunnel boring machine. If there is also water, permeability, or significant hydraulic pressure, a hydro-shield for water-logged terrain.
Risks of Not Detecting Them in Time
Failure to identify bumps, blocks, or buried obstacles can result in:
- Jamming or seizing of the cutting head.
- Accelerated tool wear.
- Increase in thrust and shear torque.
- Alignment or dimensional deviations.
- Over-excavation or loss of soil.
- Extended stoppages for maintenance.
- Damage to pipes, joints, or equipment.
- Increased time and cost.
- Need for adjunctive treatments or a change in treatment method.
What to Do If the Risk Is High
When the likelihood of encountering obstacles is high, it is possible to expand the geotechnical investigation, increase the density of boreholes, conduct test pits in critical areas, combine geophysical methods, revise the alignment, adjust the depth, select a more suitable cutting head, or establish contingency procedures.
In some projects, it may also be necessary to adjust the location of the vertical pits for driving and microtunneling to facilitate access, maintenance, tool changes, or equipment retrieval.
Minimum checklist for detecting buried obstacles: surveys, boreholes, geological profiles, SPT/CPT tests, grain size analysis, abrasiveness, water table, historical maps, utility plans, ground-penetrating radar or geophysical surveys (if applicable), targeted test pits, manhole inspections, review of nearby foundations, and analysis of previous construction work.
Request a Technical review of the risk posed by boulders, rock blocks, and buried obstacles before finalizing the route or selecting the tunnel boring machine.
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