A pilot tunnel is a small, advance tunnel used for exploration, drainage, and the safe development of a larger cavity. It reduces geotechnical risks, improves construction workflows, and creates access for surveying, utilities, and rescue. In sensitive environments—such as urban areas, near protected structures, or in heterogeneous rock—it enables controlled advance with low vibration levels. Where pinpoint work is required, particularly low-vibration methods such as drilling, splitting, and cutting come into play. Tools from Darda GmbH such as rock and concrete splitters, concrete demolition shears, and complementary hydraulic technology make a practical contribution here without making blasting techniques mandatory.
Definition: What is meant by pilot tunnel
A pilot tunnel is understood to be an exploration or guide tunnel that is significantly smaller than the final cross-section and precedes or accompanies the main tunnel in time. It serves to obtain geological information, to provide drainage, to reduce stresses in the rock mass, to improve ventilation, and to ensure safe logistics. The pilot tunnel often runs along the axis of the later tunnel but can be offset depending on the alignment. Through the early insight into rock mass and groundwater conditions, it enables adapted strategies for support, lining, and excavation advance.
Functions and benefits in rock and tunnel construction
The pilot tunnel fulfills several tasks that have proven their worth in planning and construction practice:
- Geotechnical investigation: identification of layer boundaries, fault zones, joints, and weathering; calibration of predictions.
- Water management and drainage: controlled discharge of inflows, reduction of hydrostatic pressures.
- Stress relief and pre-unloading: reduction of rock loads on the later final cross-section.
- Surveying and alignment control: axis definition, control of direction and gradient.
- Supply and rescue: slim transport routes for materials, utilities, and personnel; reserve structure for emergencies.
In projects involving rock excavation and tunnel construction, the pilot tunnel is often used as part of a staged excavation concept (e.g., following the principle of cyclic, conventional tunneling). Even when advancing with large tunnel boring machines, an exploration or drainage tunnel ahead can mitigate the risks of starting into unknown rock.
Typical geometries, alignment, and support stages
The geometry depends on purpose and geology. Common cross-sections range between 6 and 15 m². The pilot tunnel often runs centrally but can be shifted toward the edge to specifically intercept water-bearing zones. The support is usually temporary:
- Immediate support: shotcrete, lattice girders, rock bolts and anchors, and, where applicable, pipe canopy or spile/forepoling.
- Drainage: drains, perforated pipes, inspection shafts.
- Ventilation: supply of fresh air and extraction of exhaust gases and dust.
Temporary support and selective removal
When enlarging from the pilot tunnel to the final cross-section, local material is removed in a controlled manner. Concrete demolition shears are suitable for the selective removal of temporary shotcrete layers or smaller concrete components in confined spaces. Rock and concrete splitters help to reduce overbreak in the rock precisely and with low vibration without impairing surrounding support elements.
Construction methods: advancing the pilot tunnel
The advance can be by blasting, mechanical means, or a combination. The choice depends on rock mass class, water, vibration requirements, and accessibility. In urban areas or near sensitive facilities, low-noise, low-vibration methods are advantageous.
Conventional drill-and-blast
In conventional advance, boreholes are drilled, precisely charged, and blasted in short cycles. For pilot tunnels, finely tuned drilling and charging patterns with pre- and post-cuts are often used to protect the contour and limit vibrations. Blasting gases, dust, and ventilation management require powerful ventilation and clear safety processes.
Mechanical drilling, splitting, and cutting
Where blasting vibrations must be avoided, drilling with subsequent splitting is used: rock splitting cylinders or rock and concrete splitters are inserted into boreholes and generate controlled crack formation along the intended break line. For concrete components, installations, or lining elements in the pilot tunnel, concrete demolition shears support precise, piece-by-piece removal. Additionally, multi cutters can be used for sectioning mixed materials, and combination shears and steel shears for reinforcement, structural steel, and support arches. The energy supply is provided by compact hydraulic power units suitable for use in confined underground conditions.
Water, gas, and temperature: construction hydrogeology in the pilot tunnel
The pilot tunnel often serves as a drainage structure. Inflows are captured and diverted via probe holes, filter pipes, and open channels. This makes it possible to identify flow paths and protect the final cross-section from high inflows. In the presence of gas, measurements and controlled ventilation are essential. Temperature differences, icing, or swelling layers require adapted support strategies. Careful documentation of inflow volumes, turbidity, and chemical parameters improves forecasting for the main drive.
Surveying, monitoring, and documentation
In the pilot tunnel, proximity to the rock mass enables reliable classification according to common systems (e.g., RMR or the Q-system). Typical measures include:
- Geological mapping of the tunnel face and bench surfaces.
- Deformation measurements on the lining and in the rock mass.
- Exploratory drilling ahead to detect fault zones, water, or loose ground.
- Ongoing adaptation of the support concept to the encountered conditions.
The data obtained flow into the digital structure and geomodel and form the basis for decisions on support, enlargement, and the choice of excavation equipment.
Relation to products and areas of application
Pilot tunnels are a central element in rock excavation and tunnel construction. Depending on project phase and material, different tools are considered:
- Rock and concrete splitters: targeted opening of separation joints, removal of contour overbreak, low-vibration adjustments to the excavation profile.
- Concrete demolition shears: selective removal of temporary shotcrete shells, dismantling small concrete foundations, removing embedded parts during enlargement.
- Rock splitting cylinders: pinpoint crack propagation in competent rocks, helpful in sensitive areas without blasting.
- Combination shears and multi cutters: sectioning mixed materials in confined spaces, removal of lining components.
- Steel shears: cutting support arches, structural steel, and reinforcement.
- Hydraulic power units: supplying hydraulic tools in underground operations.
In concrete demolition and special deconstruction, pilot tunnel-like guide or exploration tunnels can also be created in existing structures to expose load paths or verify utility routing. In gutting and cutting, concrete demolition shears and multi cutters support low-damage creation of access openings. In natural stone extraction, small exploration drifts can help trace joints and plan extraction faces; splitting technology separates blocks along natural planes of weakness. For special deployment scenarios—such as rescue or utility galleries—low-vibration methods offer high control of construction boundaries.
Enlargement and interaction with the final cross-section
After exploration, the pilot tunnel is typically enlarged to the final cross-section. Contour fidelity, protection of the temporary support, and workforce safety are paramount. A proven approach is stepwise enlargement with immediate re-support. Where concrete components are in the way or temporary elements must be removed, concrete demolition shears enable controlled, low-splinter removal. Rock noses or local overbreak can be gently detached with rock and concrete splitters, improving the surface quality of the subsequent tunnel lining.
Special boundary conditions and special deployments
In heavily faulted rock, when intersecting water-bearing layers, or near sensitive structures, a pilot tunnel is often the decisive safety component. In existing facilities—such as dams, caverns, or underground industrial structures—a narrow guide tunnel can be installed for inspection and utility routing. In complex deconstruction scenarios, cutting and shearing tools help section installations such as pipelines or steel beams; for special hollow bodies the use of tank cutters is conceivable, provided the boundary conditions (material, contents, emissions) allow it. Such work always requires a careful, step-by-step approach and coordinated protective measures.
Planning, occupational safety, and environmental aspects
The planning of a pilot tunnel accounts for geology, hydrogeology, ventilation, logistics, emergency concepts, and later interaction with the final cross-section. Key principles are:
- Hazard analysis and construction-phase risk management with clear escalation pathways.
- Minimization of dust, noise, and vibrations; prefer low-vibration methods where required.
- Continuous ventilation management, gas monitoring, and reliable drainage.
- Qualified operation of hydraulic tools and regular maintenance of the power units.
- Proper handling and disposal of spoil, water, and auxiliaries in accordance with applicable regulations.
Legal and normative requirements may vary by region. The information provided is general and does not replace project-specific planning or binding regulations. In execution, always ensure clear interface coordination between excavation, support, surveying, and outfitting.