Tunnel load refers to all actions acting on a tunnel during construction, operation, maintenance, or deconstruction. It encompasses the interplay of rock mass, ground and mine water, construction stages, traffic, and temperature. A precise understanding of these loads is crucial for planning, execution, and the safe handling of components and materials—from rock breakout to controlled concrete demolition. In practice, tunnel load directly influences the choice of excavation method and equipment, for example in rock breakout and tunnel construction, in concrete demolition and special demolition, or in building gutting and cutting. Tools such as concrete demolition shears or rock and concrete splitters are preferred where a low-vibration approach helps reduce load redistribution and protect tunnel support.
Definition: What is meant by tunnel load
Tunnel load is understood as the totality of static and dynamic actions acting on the lining, support, and fittings of a tunnel. These include permanent loads from rock pressure and overburden, variable loads from traffic, water levels and temperature, as well as exceptional actions, for example from construction stages, vibrations, or fire. A typical distinction is made between loads in the construction stage (e.g., during excavation and support) and loads in the final stage (operation). The load level depends on the geological environment, the overburden, the construction method (e.g., cyclic blasting works, excavator breakout, hydraulic splitting) and the lining (shotcrete, segment rings, inner lining).
Types of loads in tunnel construction: permanent, variable and exceptional
Tunnel loads can be classified by duration, cause, and direction. For design, they are combined into load combinations; for construction execution, their temporal evolution and redistributions are decisive. Typical categories are:
- Permanent loads: self-weight of the lining, rock pressure, overburden, long-term deformations (creep/shrinkage), settlements.
- Hydraulic actions: ground and mine water pressure, pore water pressure, uplift, pressure fluctuations during excavation backfilling and re-filling.
- Variable loads: traffic actions (road, rail), aerodynamic pressure surges, temperature actions, maintenance and inspection loads.
- Exceptional actions: construction stage (partial excavation, partial ring), vibrations, earthquakes, fire exposure, local impact loads.
- Internal component effects: restraint from temperature and moisture gradients, shrinkage and heat of hydration, assembly tolerances.
Depending on the construction method, load paths can vary significantly. In cyclic excavation, the early load-bearing effect of the shotcrete shell is decisive; in segmental tunnels, ring-by-ring closure and the gasketed joint govern behavior. A shear-resistant ring joint reduces redistributions, while delayed ring formation can allow higher temporary deformations.
Design and load case combinations
For structural checks, actions are combined into governing combinations. In the ultimate limit state, unfavorable superpositions (e.g., high rock pressure and water pressure) are of interest; in the serviceability limit state, deformations, cracking, and watertightness. It is important to distinguish between short-term peak loads and long-term effects. The verifications must be performed project- and site-specific; the following points serve for general orientation only:
- Determination of geotechnical parameters: rock mass classification, jointing, overburden, water levels, permeability.
- Definition of construction phases and load stages: excavation, temporary support, temporary props, ring or shell formation.
- Combinations for construction and final stage: permanent, frequent, quasi-permanent, and exceptional (e.g., fire, earthquake).
- Assessment of restraint and redistribution: temperature, shrinkage, settlements, construction joints.
- Monitoring and adaptation: measurement data to validate assumptions, with adjustment of support as needed.
Tunnel load in the construction stage
The construction stage is central to safety: partial cross-sections, open rings, and incomplete systems lead to redistributions and peak loads. Stability depends on the sequence of work steps, from crown support to closure of the load-bearing cross-section.
Excavation, crown support and shotcrete
Shotcrete, lattice girders, anchors, and steel ribs form early-acting supports. The time window between excavation and support should remain short to limit deformations and convergences. Tunnel load during this phase is highly time-dependent.
Influence of the excavation method
Blasting, excavator excavation, or hydraulic splitting influence vibrations and load redistribution. Rock and concrete splitters are used when vibrations must be minimized or when sensitive structures are nearby. They generate controlled cracking in rock or concrete, which favors low-load removal in the construction stage.
Hydraulic power packs and system tuning
Hydraulic power packs provide the necessary pressure for split cylinders, concrete demolition shears, and other tools. Proper tuning of pressure, flow rate, and stroke speed supports controlled load paths and prevents shock loads on temporary supports.
Tunnel load in operation
In operation, rock and water predominate, supplemented by traffic loads and temperature. Aerodynamic effects (e.g., pressure waves in rail tunnels) can be locally governing. Durability and watertightness are in focus, particularly on water-bearing sections and in aggressive environments.
Temperature, creep and shrinkage
Temperature gradients and moisture changes generate restraint that can cause cracking. In inner linings, creep and shrinkage act over the long term; joint design and curing are therefore important.
Fire actions
Fire exposure constitutes exceptional actions that can trigger high temperature peaks, spalling, and strength losses. Material selection, protective layers, and maintenance strategies generally take such scenarios into account at the conceptual level.
Influence of rock mass, overburden and water
Geology and hydrology determine the nature and magnitude of tunnel load. Fracture water can generate additional pressure, karst voids lead to uneven support, and weak rock to settlements. High overburden increases rock pressure; horizontal stresses are relevant in tectonically pre-stressed zones.
Hard rock, soft rock and unconsolidated soil
In hard rock, the stability of the jointed blocks and their orientation dominates; in soft rock, time- and creep effects are pronounced. Unconsolidated soil often requires closed or frozen cross-sections and dewatering to control hydraulic loads.
Monitoring and the observational method
Measurements accompany excavation, lining, and operation. The aim is to verify assumptions, understand load paths, and adapt measures. Typical measured quantities are:
- Convergence and displacements of the tunnel cross-section.
- Anchor forces, strains and stresses in lining components.
- Groundwater and pore water pressure, flow rates.
- Temperature and moisture in the inner lining.
- Vibrations during demolition and cutting operations.
Tunnel load and controlled demolition
During deconstruction or cross-section adjustments, components often carry residual load. Local interventions change the load path; accordingly, the work sequence and choice of tools are decisive. The goal is removal with minimal vibration and low risk of spalling to avoid endangering adjacent construction stages and operational equipment.
Concrete demolition shears in tunnels
Concrete demolition shears enable gripping and shearing removal of inner linings, haunches, and webs in manageable cross-sections. They are helpful when reinforcement must be considered and when localized loads on temporary shoring are to be limited. In building gutting and cutting, service platforms, cable ducts, or thin-walled installations are removed in advance to reroute loads in a controlled manner.
Rock and concrete splitters for cross-section enlargements
For cross-section enlargements or niches, split cylinders are used to open rock or concrete linearly. This results in lower vibration levels and reduces the risk of undesirable load redistributions. In rock breakout and tunnel construction, this approach helps preserve existing supports, such as shotcrete arches or anchor heads.
Combination shears, multi cutters and steel shears
For steel profiles, lattice girders, reinforcements, and cable trays, combination shears, multi cutters, and steel shears are used. They cut metallic installations in a controlled manner, thereby minimizing unintended impulse loads. In industrially used tunnel facilities, a tank cutter may be required for dismantling vessels and pipelines in niches and cross-passages.
Work sequences that favorably influence tunnel loads
A low-impact construction or deconstruction sequence reduces risks. The following have proven effective:
- Step-by-step work with early closure of load-bearing cross-sections.
- Pre-relief by sawing, drilling, or splitting along planned separation cuts.
- Install temporary shoring before separating load-bearing components.
- Low-impact tool selection (e.g., hydraulic splitting, gripping separation with concrete demolition shears).
- Continuous monitoring and adaptation of measures.
Planning and execution aspects
Planning for tunnel load includes forecasting actions, selecting the construction method, and defining supports. In execution, logistics, ventilation, water management, and emergency concepts must be considered. For repair and special demolition, the rule is: interventions in load-bearing structures only with a coordinated sequence and documented intermediate states. Notes of this kind are of a general nature and do not replace any object-specific calculation or release.
Term distinctions and common misconceptions
“Tunnel load” is not synonymous with “rock pressure”: it also includes water, traffic, temperature, and exceptional actions. Likewise, “low-vibration” is not the same as “load-free”: even gentle methods generate local stress changes that must be considered in planning. This applies in particular to partial demolition and adaptations in existing structures.