The cutting head of a tunnel boring machine It is selected based on the type of soil, strength, grain size, abrasiveness, presence of water, stability of the excavation face, expected boulders or blocks, diameter, driving length, and the system for removing the excavated material. There is no such thing as a universal drill head: cohesive clay, saturated sand, permeable gravel, competent rock, or a mixed face each require different tools, bore diameters, protective measures, and excavation parameters.
In projects of pipe ramming, microtunneling o infrastructure crossings, the selection of the head must be coordinated with the choice of tunnel boring machine, the type of shield, face control, available torque, expected wear, and the ability to remove excavated material.
Primary Selection Criteria
Before configuring the print head, check the following:
| Criterion | Why does the head cause this? |
|---|---|
| Type of land | Specify whether picks, teeth, scrapers, discs, or combination tools are needed |
| Material Strength | It affects power, torque, penetration, and wear |
| Particle Size Distribution | It affects opening, extraction, the risk of blockage, and material transport |
| Abrasiveness | Determine protective measures, wear-resistant materials, and maintenance frequency |
| Water table | Requires monitoring of pressure, stability, and extraction under saturated conditions |
| Skittles or blocks | They may require robust tools, adequate openings, and a contingency strategy |
| Front Stability | Set open shield, EPB o hydro shield |
| Diameter and Length | They affect balance, thrust, maintenance, and accessibility |
| Construction Site Environment | In urban areas and critical infrastructure, seat and pressure monitoring is prioritized |
Clay Cutting Head
At clays and cohesive soils, the cutting head must cut, break up, and convey the material without causing excessive sticking, blockages, or buildup in the chamber.
The following is reviewed in particular:
- Plasticity of clay.
- Soft, firm, or hard consistency.
- Stickiness and tendency to clog.
- Presence of sandy or silty layers.
- Water table.
- Need for site preparation.
- Risk of overpressure or loss of forward motion.
In clay soils, picks, scrapers, blades, or configurations with specially designed openings can be used to prevent clogging. If the clay is highly plastic or sticky, the design must provide for face cleaning, torque control, and possible conditioning.
Cutting head for sand
At sands, the priority is usually to maintain the stability of the face and control the flow of material, especially if they are saturated or under water pressure.
The following are analyzed:
- Sand density.
- Particle size distribution and fines content.
- Permeability.
- Water pressure.
- Risk of fine material carryover.
- Tendency toward over-excavation.
- Front Stability.
In saturated or permeable soils, it may be necessary to work with controlled pressure using hydro-shield for water-logged terrain or, as appropriate, with EPB if the material can be properly conditioned. The cutterhead must allow for controlled intake of the soil and prevent volume loss that could lead to settlement.
Gravel Cutting Head
At gravel and coarse granular soils, the head must be sturdy, wear-resistant, and capable of handling larger particles without jamming.
The following is being reviewed:
- Maximum gravel size.
- Particle size distribution.
- Shape of the edges.
- Abrasiveness.
- Presence of boluses.
- Permeability.
- Water table.
- Extraction system capacity.
In gravel, the cutterhead openings, scrapers, protective guards, and material conveyance system are critical. If boulders or blocks are present, it must be assessed whether the tunnel boring machine can break them up, accommodate them, or if preliminary investigation and contingency measures are required.
Rock-cutting head
At rock, the selection depends on the strength, fracture behavior, abrasiveness, hardness, diameter, and continuity of the rock mass.
The following are analyzed:
- Uniaxial compressive strength.
- Fracturing and RQD.
- Abrasiveness.
- Mineralogical hardness.
- Presence of quartz.
- Discontinuities.
- Risk of clumping.
- Need for cutting discs.
- The machine's torque and thrust capacity.
In competent rock, cutting discs, specialized cutters, abrasion-resistant guards, and configurations designed to fracture the material can be used. In highly abrasive rock, tool wear and the need for intervention or maintenance must be evaluated as early as the bidding phase.
Cutting head for mixed terrain
At mixed-use properties, the face combines materials of varying strength, such as soil and rock, clay and gravel, backfill and blocks, or irregular transitions. This situation is one of the most challenging when it comes to selecting the cutterhead.
The design must provide for:
- Combination tools.
- Protection against uneven wear.
- Shear torque control.
- Risk of deviation due to uneven excavation.
- Over-excavation in soft soil areas.
- Impacts on hard surfaces.
- Front Stability.
- Sudden changes in terrain are possible.
In these cases, the selection of the head must be coordinated with the evaluation of mixed soil-rock faces, abrasiveness, the presence of clumps, and the guidance system.
Relationship Between the Cutterhead, Shield, and Extraction System
The cutterhead is not selected in isolation. It must be compatible with the type of tunnel boring machine and with the method used to control and remove the excavated material.
| System | Connection to the head |
|---|---|
| Open shield | Requires stable ground or auxiliary measures; the cutterhead is suitable for direct excavation |
| EPB | The head must allow for control of material feed and maintain soil pressure |
| Hydroshield | The head works with a slurry circuit and must handle excavated material in suspension |
| Abrasive surfaces | They demand protective gear, durable tools, and provisions for wear and tear |
| Land with boulders | Further study is needed on acceptable size, fragmentation, and the risk of blockage |
Compatibility between the cutting head, chamber, openings, crushing system, debris or slurry conveyance, and available power is critical to preventing downtime.
Parameters Monitored During Excavation
Once driving begins, the behavior of the pile driver head is controlled by:
- Cutting pair.
- Driving force.
- Feed rate.
- Frontal pressure.
- Excavated volume.
- Sludge flow rates and density, if applicable.
- Energy consumption.
- Vibrations or impacts.
- Tool wear.
- Presence of clumps or unanticipated material.
- Axis and dimension deviations.
- Issues involving system freezes or performance degradation.
This data makes it possible to adjust the feed rate, identify changes in terrain, and confirm whether the selected cutterhead is suitable for the actual conditions.
Risks of Selecting an Inappropriate Cutting Head
An incorrect selection may result in:
- Premature tool wear.
- Increased torque and thrust.
- Blockages in the head or chamber.
- Loss of performance.
- Over-excavation.
- Settlements.
- Alignment deviations.
- Difficulty passing stool.
- Extended stops.
- Damage to the tunnel boring machine or pipeline.
- Unforeseen need for intervention.
- Cost overruns and delays.
At underpasses, rivers, urban areas, or critical infrastructure, these risks must be analyzed before finalizing the construction method.
Information Required for Design or RFQ
To select or review the cutting head, it is advisable to provide the following information: geotechnical profile, boreholes, core samples, soil type, grain size distribution, maximum size of gravel or boulders, rock strength, abrasiveness, quartz content, water table, permeability, water pressure, borehole diameter, drive length, depth, overburden, alignment, radius of curvature, tolerances, type of tunnel boring machine (TBM) planned, muck removal system, available torque, expected thrust, accessibility for maintenance, environmental sensitivity, monitoring, and production requirements.
Request a Technical inspection of the cutting head and selection of a tunnel boring machine for microtunneling before finalizing the geotechnical analysis, construction method, or RFQ documentation.
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