Safety tunnels are underground cavities, mostly running horizontally or with a slight incline, that structurally relieve, drain, and provide access to structures, slopes, or existing tunnels. They combine geotechnical stabilization with controlled water management and enable low-risk construction in sensitive environments. In planning, construction, maintenance, and deconstruction of such tunnels, hydraulic tools such as hydraulic rock and concrete splitters or concrete demolition shears are used as needed to separate material in a controlled, low-vibration, and precise manner.
Definition: What is meant by a safety tunnel
A safety tunnel is an auxiliary or side tunnel that primarily serves to stabilize and relieve the in-situ rock mass or a structure. It can reduce rock and pore water pressure, equalize the rock mass, provide access to the subsoil, and function as an access or inspection tunnel. Depending on purpose, geology, and location, support may be executed with shotcrete, anchors, steel ribs, or an inner lining; water management is regulated via drainage, and ventilation is provided as required.
Tasks and functions of a safety tunnel
Safety tunnels perform a range of technical functions that may overlap depending on the project:
- Relief of rock and water pressure to reduce deformations and to verify structural stability.
- Targeted dewatering and drainage to lower the groundwater level, to divert perched and joint water, and to minimize erosion and uplift forces.
- Investigation and monitoring of the subsoil (test boreholes, inclinometers, piezometers) to validate construction status and geotechnical parameters.
- Access, logistics, and rescue: temporary access for construction equipment, materials, inspection, and, where applicable, as an escape route.
- Construction preparation and support: pre-relief, pre-dewatering, and pilot headings for subsequent primary structures.
Geotechnical planning and design
The design of a safety tunnel follows the boundary conditions of geology, hydrogeology, topography, and the vicinity of sensitive structures. It is based on robust site investigation, a conservative safety approach, and phased construction planning.
Alignment, routing, and cross-section
The route is selected to keep critical shear zones, lenses of loose material, and water inflows manageable. Cross-section shapes (e.g., circular, horseshoe, rectangular) depend on the stress state, support concept, and required operational function.
Support and waterproofing
- Temporary support: shotcrete, fiber reinforcement, lattice girders, rock anchors for initial support.
- Permanent lining: inner concrete lining, possibly with a waterproofing membrane; drainable backfill for controlled water routing.
- Load transfer: structural connections with defined sliding and fixed bearings to avoid uncontrolled restraint forces.
Water management and drainage
Drain lines, cross passages, and sumps convey water in a controlled manner. Proof against backwater, icing, and sedimentation is part of the design, as is compliant discharge routing. Construction materials and embedded components must be selected for chemical resistance to waters and gases.
Construction methods and execution
The construction method is tailored to the environment (rock, loose ground, urban proximity). In addition to classical drill-and-blast methods, low-vibration mechanical techniques have become established where neighboring structures, operating facilities, or sensitive linings must be protected.
Excavation in rock
- Conventional excavation (NATM): staged excavation with interim shotcrete support and systematic anchoring.
- Low-vibration alternatives: rock splitting with hydraulic splitters or rock splitting cylinders for controlled crack initiation and minimized secondary breakage.
- Profile corrections: fine trimming at edges and disturbed zones without additional blasting to achieve lining quality.
Excavation in concrete or under the influence of existing structures
- Concrete removal: selective deconstruction of temporary cross-section closures, dams, or inner linings with concrete demolition shears for spark-reduced, dust-reduced separation.
- Reinforcement: cutting and removing embedded components with steel shears or combi shears; touch-up at transitions with multi cutters.
- Power supply: hydraulic power units dimension flow and pressure for consistent tool performance under restricted space conditions.
Cross-section enlargement, niches, and cross-passages
For pump sumps, passing bays, equipment niches, or cross passages, pinpoint, controlled removals are required. Mechanical splitting and shear methods allow short advance cycles, brief closures, and high dimensional accuracy, which is especially advantageous in service tunnels with running water.
Material separation in the safety tunnel: low-emission and controlled
The separation of rock and concrete in the tunnel must minimize vibration, noise, dust, and sparking. Concrete demolition shears enable load-path-oriented crushing of concrete with reduced edge damage; hydraulic splitters create defined separation joints in rock that are subsequently widened with low energy input. This reduces settlements in adjacent structures and allows water-bearing joints to be selectively opened or sealed.
Occupational safety, explosion protection, and legal framework
Safety tunnels are subject to stringent requirements for ventilation, dust and gas management, escape and rescue routes, and electrical safety. Measures for explosion protection, dewatering, and emergency power must be defined per project. Requirements from codes and official permits should be considered early, operating instructions documented in writing, and staff qualifications verified regularly. The information in this text is general in nature and does not replace project- or site-specific assessments.
Maintenance and deconstruction of safety tunnels
In operation, condition inspections of the lining, drainage, and monitoring points are essential. In rehabilitation or decommissioning, the focus is often on selective deconstruction of individual components, sealing drains, or installing dams. Controlled separation techniques prove effective here: concrete demolition shears for inner linings and component edges, steel shears for reinforcement and embedded parts, hydraulic splitters for rock removal with limited overbreak. Hydraulic power packs ensure the supply even in confined tunnel cross-sections.
Interfaces to application areas of Darda GmbH
- Rock demolition and tunnel construction: excavation, profile corrections, and niches with low-vibration splitting methods; complement to conventional lining.
- Concrete demolition and specialized deconstruction: deconstruction of temporary construction stages, inner linings, or cross-section constrictions during ongoing operations with concrete demolition shears.
- Strip-out and cutting: selective removal of inserts, anchors, cable troughs, and manhole covers with steel shears, combi shears, or multi cutters.
- Natural stone extraction: applied splitting technology transferable to underground block extraction where material flow and protection of the surroundings are prioritized.
- Special deployment: work in water-bearing tunnels, under confined space conditions, or near sensitive infrastructure using low-spark, low-emission methods.
Practice-oriented workflows in building safety tunnels
- Site investigation and planning: define site investigation, monitoring concept, construction states, support stages, and water management.
- Excavation preparation: set up ventilation, power supply via hydraulic power packs, dewatering, and rescue routes.
- Excavation and support: stepwise removal, initial support, installation of drainage; mechanical splitting and shear work as needed.
- Lining: shotcrete, anchors, lattice girders, inner lining, waterproofing, and permanent drainage.
- Commissioning and monitoring: check deformation, flow, and lining condition; scheduled maintenance.
Environmental compatibility and neighborhood protection
In urban or environmentally sensitive locations, low vibrations, minimized noise, and limited dust are crucial. Mechanical splitting and concrete demolition shears reduce emissions, limit overbreak, and facilitate the protection of adjacent utilities or structures. Targeted water management in the safety tunnel prevents uncontrolled discharges and helps protect the surroundings.
Terminology and delineation
Safety tunnels are to be distinguished from pure drainage or rescue tunnels, although functions may overlap. In contrast to installation or logistics tunnels, the focus of safety tunnels is on geotechnical relief. Depending on the project, they may also be referred to as relief, pressure-relief, or drainage tunnels; what is decisive is the intended effect on stresses, water levels, and accessibility.