Working with open shields in rock massifs combines the most demanding cutting mechanics with the advantage of being able to evacuate debris dry. However, the hardness of the face, the presence of joints and shear torque peaks can cause micro-cracks that, if propagated, compromise tunnel stability and increase maintenance costs. In this guide-inspired by Eurohinca's experience and reinforced by the latest scientific literature-we review seven tactics that help minimize rock fracturing, while linking to internal resources that expand on each aspect.
1. Know the terrain before attacking
Every project should start with a robust geotechnical characterization: CERCHAR index tests, S-wave velocity and fracture profiles. Our area of Technical assistance and engineering usually complements the campaign with laboratory tests to estimate the specific cutting energy. These data feed into the rock-disc interaction models, anticipate wear and allow the selection of the ideal bit before the TBM - described in the Open shield- into the field.
2. Controlled pre-cutting: micro-fractures that save major fractures.
When the rock is extremely competent, adding pre-cutting discs ahead of the main cutting line generates micro-cracks that "soften" the face. The result is a decrease in torque and less vibration transmitted to the rock mass. This technique, documented in Pipe jacking The large-diameter crown gear can reduce the power requirement by up to 12 %. On site, the ring gear is equipped with narrower discs and the rotational speed is adjusted so that the kerfs do not overlap excessively.
3. Selective lubrication with foams and bentonite.
In rocky soils, lubrication is not intended to transport material, but to reduce friction between the liner and the wall. Applying foam or bentonite only on the roof and flanks - leaving the bottom dry - stabilizes the torque without creating unwanted hydraulic pressures at the base. This "180° lubrication" was successfully implemented in our microtunnel under Madrid's M-30, described in the section on Civil works. After 420 m of advance, the disks showed 15 % less wear than in previous campaigns without directed foam.
4. Real-time monitoring: IoT sensors and dashboards
Today it is possible to equip the shield platform with load cells, accelerometers and thermocouples whose reading is sent to the cloud every few seconds. When a sensor registers an abnormal peak in vibration or thrust, the system sends an alert to the Excavation systems to adjust parameters before fracture initiation. A study of Tunnelling and Underground Space Technology (vol. 148, 2024) concludes that tunnel boring machines with real-time feedback reduce overcutting events in hard massifs by 28 %. ScienceDirect.
5. Millimeter control of the alignment
Lateral deflections generate irregular stresses on the shield casing, increasing the likelihood of spiral breaks in the rock. The usual laser guidance is reinforced with high-frequency inertial systems to keep the trajectory within ±1 mm. This approach, which we detail in the category of Horizontal directional drillingThe DN2000 tunnel, with a final deflection of < 0.5 ‰, demonstrated its effectiveness at the crossing under the Besós tramway: 480 m of DN2000 tunnel with a final deflection of < 0.5 ‰.
6. Temporary reinforcement of the front by grouting
In areas with loose blocks or very marked fracturing, it is advisable to stabilize the face before passing the shield. Microcement is injected at low pressure, sealing the joints without creating pressure bulbs that could burst the rock. This technique, common in Vertical wells The tunnel was adapted to a marine intake tunnel in Gran Canaria: the TBM advanced under phreatic pressure, reducing rock falls to zero.
7. Deformation-absorbing liner design
Although open shielding often leaves exposed rock face, many rehabilitation projects require a segmented ring liner. Opting for tonic (concave-convex) and double O-rings increases tolerance without cracking the segments. You can view examples of this design in our Tunnel of voussoirs. In addition, reinforcing the concrete with metallic fibers increases the tensile strength, which-according to laboratory tests published in Journal of Rock Mechanics2025-reduces internal crack propagation by 40 %.
Case study: DN2000 open shield in granite, Barcelona, Spain.
In 2023 Eurohinca constructed a collector under B-20 combining the above seven tactics:
180° pre-cutting and lubrication lowered the average shear torque from 1.9 MN-m to 1.6 MN-m.
The IoT sensors identified two vibration peaks; the operator reduced speed and avoided damage to the crown.
The laser-inertial guidance maintained the alignment dimension within 4 mm.
The system of preventive grouting avoided landslides during the crossing of an inactive fault.
The dowels with fibers did not show micro-cracks in the ultrasonic post-processing tests.
The result: 27 days ahead of schedule and a reduction of 180 k€ in consumables.
Benefits of applying these strategies
High securityless fractures means less risk of blocks falling on personnel.
Savings on consumablesThe discs and augers last longer; the ring gear requires less maintenance.
Geometric qualityThe tunnel is kept within tolerances, facilitating the assembly of pipelines for Urban supply systems.
SustainabilityLubricating only where it is needed reduces bentonite consumption and its environmental footprint.
Credibility with the customerreal-time metrics -stored in the Civil works- are offered as verifiable KPIs.
Conclusions and next steps
Excavation with open shields in rocky soils requires a holistic approach that combines precision design, instrumentation and operation. By integrating precutting, selective lubrication, real-time sensing and adaptive reinforcement, you can dramatically reduce the occurrence of fractures and, with it, schedule and budget contingencies. If you are planning a crossing under critical infrastructure or a Infrastructure crossingsOur team can help you to size the optimal solution and implement the best practices described above.
Recommended readingThe study "On the rock-cutting mechanism and vibration characteristics of TBM cutters" (Elsevier, 2025) goes into detail on how to adjust torque and speed for each lithology. ScienceDirect.
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