A microtunneling through a mixed soil-and-rock face It is carried out by selecting a tunnel boring machine, a cutting head, and feed parameters capable of handling materials of varying strengths within the same section. The key is to control the stability of the cutting face, prevent deviations, manage sudden changes in torque and thrust, and reduce tool wear during excavation.
In projects of pipe ramming o microtunneling in terrestrial and subway applications, mixed ground conditions require a more precise geotechnical assessment than homogeneous ground, because the tunnel boring machine may encounter soft soil, rock, boulders, cemented layers, or irregular transitions all at once.
Why a mixed front is complex
In a mixed face, one part of the cutting head may be working on excavable soil while another part is working on rock or much harder material. This difference can result in:
- Sudden increases in cutting torque.
- Uneven tool wear.
- Risk of shaft misalignment.
- Vibrations or impacts to the head.
- Over-excavation in the soft soil area.
- Lower feed rate.
- Increased risk of blockages or stoppages.
- Difficulty maintaining stable pressure on the front.
Therefore, before proceeding, it is necessary to identify the location, thickness, continuity, and strength of each material within the layout.
What Is Reviewed Before Construction Begins
To determine the solution, the following are reviewed:
Geotechnical Engineering and Longitudinal Profile: borings, core samples, stratigraphic profile, continuity of layers, soil-rock transition, presence of boulders, fracturing, and soil variability.
Durability and abrasiveness: uniaxial compressive strength of rock, hardness, quartz content, abrasiveness, grain size distribution, and the behavior of cemented materials.
Water table and permeability: the presence of water, hydraulic pressure, and the risk of fine-grained material being carried away from the granular or soft portion of the front.
Diameter, length, and coverage: The larger the diameter or the smaller the coverage, the more important it is to control deformation, front pressure, and alignment.
Layout tolerances: at infrastructure crossings, in urban areas or near existing services, precision and settlement control are particularly critical.
How to Select a Tunnel Boring Machine and a Cutting Head
Mixed-ground operations may require a specific cutting head configuration, with tools designed for both soil and rock: discs, picks, scrapers, abrasion-resistant guards, or openings adapted to the material being excavated.
The choice of tunnel boring machine It depends on the stability of the slope, the presence of water, and the dominant type of material. In some cases, it may be worth considering a open shield if the ground is stable and accessible. If earth pressure control is required, a EPB tunnel boring machine. When there is water, permeability, or significant hydraulic pressure, a hydro-shield for water-logged terrain.
How Is Excavation Monitored During Construction?
During operation, certain parameters are monitored to detect changes in the terrain and adjust the operation accordingly:
- Cutting pair.
- Driving force.
- Feed rate.
- Frontal pressure.
- Excavated volume.
- Flow rates and sludge density, if applicable.
- Axis deviation and dimension.
- Vibrations or abnormal behavior.
- Tool wear.
- Incidents and stoppages.
In mixed ground conditions, interpreting this data as a whole is essential. An increase in torque may indicate that the drill bit has entered rock; an excavated volume greater than expected may indicate over-excavation in soft soil; and a progressive deviation may indicate uneven excavation in a transition zone.
Measures to Reduce Risks
To perform a mixed-face microtunneling operation more safely, several measures can be taken:
- Expand the geotechnical survey in transition zones.
- Adjust the route to avoid adverse ground-to-rock contacts.
- Select a mixed-cut head.
- Plan for replacement parts and a maintenance schedule.
- Monitor the face pressure and the volume excavated.
- Adjust the feed rate to the material encountered.
- Strengthen surveying and topographic control.
- Establish procedures for dealing with boulders, rock blocks, or unexpected rock.
- Plan for potential interventions or tool changes.
Risks of Failing to Properly Characterize the Mixed Front
If the mixed face is not correctly identified before construction begins, this can lead to cost overruns, delays, premature wear, increased thrusts, loss of alignment, settlements, over-excavation, jamming of the cutting head, or the need to modify the construction method.
Therefore, in areas with soil-rock transitions, heterogeneous backfill, or cemented materials, a preliminary technical review is crucial to confirm the feasibility of the trenchless solution.
Minimum checklist for studying a mixed front: surveys, boreholes, soil-rock profiles, rock strength, abrasiveness, grain size distribution, boulders, fracturing, water table, permeability, diameter, length, cover, cutting tools, tunnel boring machine type, face pressure, guidance, tolerances, and contingency plan.
Request a Technical review for microtunnels in mixed soil-and-rock excavation faces Before finalizing the route, selecting a tunnel boring machine, or preparing the bid.
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