The risk of cavities, karst features, or holes The route of a microtunnel is evaluated by combining regional geology, boreholes, core samples, geotechnical tests, hydrogeology, historical reviews, and, when warranted by the context, geophysical methods. The objective is to anticipate voids, dissolved zones, fill material, open fractures, or ground loss that could affect the stability of the tunnel face, the alignment, settlements, and the safety of the boring operation.
In projects of pipe ramming, microtunneling in terrestrial and subway applications o infrastructure crossings, this analysis is particularly important when the route passes through limestone, gypsum, soluble soils, anthropogenic fill, former quarries, mining areas, riverbeds, or areas with a history of subsidence.
What signs indicate a risk of cavities or karst?
Before deciding on a construction method, it is advisable to check whether the following exist:
- Carbonate, gypsum, or soluble soils.
- Documented sinkholes, subsidence, or ground collapse.
- Anthropogenic fill, old tunnels, or excavations.
- Water losses in boreholes or test wells.
- Open fractures or severely damaged areas.
- Abrupt changes in resistance with depth.
- Voids detected in core samples or through geophysical surveys.
- Variable water table or underground water flow.
- Upcoming projects with similar geotechnical issues.
These signs do not automatically mean that the microtunnel is unfeasible, but they do require a higher level of investigation and monitoring.
Methods for Assessing Risk
1. Geological and Historical Study
The regional geology, geological mapping, history of karst formation, mining, previous excavations, old riverbeds, backfills, sinkholes, subsidence, and documentation of nearby construction projects are reviewed. This phase helps identify areas where risks may be concentrated.
2. Boreholes and Core Samples
Drilling allows for direct assessment of the ground along the route and in wellbore areas. In ground where cavities may be present, it is important to record fluid losses, sections with no recovery, open fractures, soft fill, voids, altered rock, or abrupt changes in lithology.
3. Geotechnical and hydrogeological tests
SPT/CPT tests, pressure meters, permeability tests, pumping tests, or piezometric measurements can help identify areas of low resistance, high permeability, water flow, or anomalous soil behavior.
4. Complementary geophysical methods
When geological data or borehole surveys indicate a risk, ground-penetrating radar, electrical tomography, seismic surveys, microgravimetry, or other indirect methods may be used to identify anomalies. The choice depends on depth, soil type, moisture content, urban interference, and the expected size of the voids.
5. Localized sampling, inspections, or surveys
In shallow areas or where fill, old utilities, tunnels, or buried structures are suspected, test pits or spot inspections may be conducted to verify the information before driving the piles.
How the Trenchless Solution Works
The risk of cavities affects the choice of tunnel boring machine, the face control system, working pressure, monitoring plan, and contingency strategy. In areas where pressure control is required, solutions such as the following may be considered: EPB tunnel boring machine o hydro-shield for water-logged terrain, depending on stability, permeability, and the water table.
It may also be necessary to adjust the route, increase coverage, or change the location of vertical pits for driving and microtunneling, carry out preliminary site preparations, establish warning thresholds, or step up surface monitoring.
Risks if not properly assessed
Failure to identify cavities, karst features, or voids can result in:
- Loss of stability at the front.
- Over-excavation or loss of soil.
- Surface settlement or subsidence.
- Sudden inflow of water or entrainment of fine particles.
- Alignment or dimensional deviations.
- Unexpected increase in thrust or cutting torque.
- Blockage, partial collapse, or shutdown of the tunnel boring machine.
- Damage to existing services or nearby infrastructure.
- Extension of the deadline, cost, and corrective measures.
Common Mitigation Measures
When the risk of cavities is confirmed or suspected, measures such as expanding the geotechnical investigation, increasing the density of boreholes, combining geophysical surveys with boreholes, adjusting the alignment, injecting or treating critical areas, modifying the tunnel boring machine, controlling face pressure, limiting advance speeds, enhancing monitoring, and preparing response procedures in the event of ground loss or water ingress.
Minimum checklist for evaluating caves or karst features: geological mapping, history of sinkholes or subsidence, core drilling, sample retrieval, water loss, fracturing, water table, permeability, geophysical surveys (if applicable), localized test pits, longitudinal profile, coverage, diameter, affected utilities, nearby infrastructure, excavation method, monitoring plan and contingencies.
Request a Technical review of the risk of cavities, karst, or voids in microtunneling before finalizing the route or selecting the tunnel boring machine.
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