Tunnel injection

Tunnel injection is a central technique to seal the rock mass around underground structures, to consolidate it, and to reliably fill voids. When correctly planned and executed, it reduces water inflows, improves load-bearing capacity, and stabilizes the tunnel heading. In practice, it is closely interlinked with preparatory and accompanying works in the demolition and fit-out environment: structural openings, local exposures, and the precise deconstruction of shotcrete shells or concrete components often form the prerequisite for borehole drilling, packer installation, and injection lines. Depending on project scope, this phase can involve, among other tools, concrete demolition shears as well as hydraulic rock and concrete splitters from Darda GmbH—not as a replacement for the injection technology, but as a precise complement in execution.

Definition: What Is Meant by Tunnel Injection

Tunnel injection refers to the targeted introduction of injection materials (e.g., cement suspensions, silicate or resin systems) into rock, soil, or voids around a tunnel. The objective is sealing against water, the consolidation of loose zones, as well as contact and void filling between the lining and surrounding medium. The injection is performed via boreholes with packer systems and is monitored with pressure- and flow-controlled procedures. Typical applications are pre-excavation injections ahead of the tunnel face, sealing under water inflow conditions, contact injections behind the tunnel lining, and consolidation injections in settlement-sensitive areas.

Planning and Objectives of Tunnel Injection

Planning starts with the geological–hydrogeological assessment: stratigraphy, fracturing, permeability, groundwater levels, and expected water pressures determine the injection concept. From this follow the grid, borehole lengths, drilling inclinations, packer spacing, target pressures, mix ratios, and the sequence of primary, secondary, and tertiary injections. Key objectives are: reduction of permeability to a defined limit value, creation of a closed sealing curtain, improvement of shear and compressive strength in disturbed zones, and permanent contact between the shotcrete or tunnel lining and the rock mass. For work preparation, accesses, niches, tie-in points, and safe work platforms must be created. Where required, components are locally and with low vibration levels deconstructed—often using concrete demolition shears or hydraulic splitters from Darda GmbH—in order to protect the adjacent structure and to create controlled edges for drilling and injection work.

Methods and Materials of Injection Technology

Depending on the task, a distinction is made between sealing, consolidation, and contact injections. The method is selected so that the injection material reaches the governing pores and fractures without causing unwanted bleed-out or soil displacement. Decisive factors are grain size distribution, viscosity, reaction or setting time, chemical compatibility, durability, and the controllability of the pressure–volume ratio.

Sealing Injection

Sealing aims at a significant reduction of permeability ahead of or around the excavation. In fractured rock, fine-grained cement suspensions with graded water–cement ratios are injected. In very fine pore spaces, reaction-controlled systems such as silicate or acrylate gels are used to create a tight gel curtain. Pressure control must be chosen to prevent hydraulic loosening and to avoid inadmissible loading of existing shotcrete shells or inner linings.

Consolidation Injection

Consolidation injection increases the load-bearing capacity of loose zones, slide masses, or faulted areas. Cement suspensions with tailored additives and low sedimentation tendencies penetrate fractured and porous areas and, after hardening, form a load-transferring matrix. In organic soils or very fine pore spaces, alternative low-viscosity systems are used, with attention to environmental compatibility and durability.

Contact and Void Injection

Contact injections close gaps between lining (e.g., shotcrete, segment rings) and rock mass. Void injections fill larger cavities in the ground. For segmental tunnels, ring-shaped injection sections with controllable pressure holding are executed to minimize settlements and to uniformly backfill the lining.

Material Selection

• Cement suspensions (micro- and ultrafine): for fractured rock, contact injection, consolidation; adjustable via water–cement ratio and admixtures.
• Silicate systems: for fine pore spaces; gel-forming with adjustable gel time; suitable for sealing under moderate water inflows.
• Polyurethane resins: for rapid, local sealing under pressing water; foaming or solid; suitable for emergency sealing.
• Acrylate gels: very low viscosity, defined gel times; for curtains in fine-grained soils; careful handling required.
• Bentonite and blended systems: volume-stable, sealing in larger voids or as a supplement in curtain areas.

Drilling Technology/Methods, Packer Systems, and Pump Control

Injection relies on a clean drilling grid and reproducible technique. Boreholes are created with precise position and depth, flushed, cleaned, and equipped with mechanical or inflatable packers. Pump control maintains defined pressures and flow rates and documents curves for quality assurance. In confined conditions, structural elements are selectively removed to create drilling accesses and niches—among other means with concrete demolition shears from Darda GmbH—to work with minimal dust and low vibration levels. In rock, hydraulic splitters from Darda GmbH can be used to carefully widen small windows without impairing surrounding stability.

1. Define the drilling grid, secure axis and elevation references.
2. Create the borehole, monitor for washout and stability.
3. Clean the borehole (flushing, blowing), remove sediments.
4. Set packers, perform a leakage test, and define injection sections.
5. Mix the injection material (control homogeneity, temperature, viscosity).
6. Grout/inject with the specified pressure–volume regime, observe staged control.
7. Close sections, switch to secondary and tertiary boreholes as specified.
8. Log (pressure, volume, time, material, temperature) and evaluate characteristic curves.

Quality Assurance and Success Control

The effectiveness of an injection is assessed by pre- and post-measurements. Water pressure tests, permeability determinations, and the comparison of pressure–volume curves indicate whether target values have been achieved. Visual checks at bleed-out points and controlled core extraction can provide additional insight. Complete documentation ensures traceability and supports later structure assessment.

• Reference measurements before injection (e.g., permeability, water inflow).
• Ongoing monitoring of process parameters during grouting.
• Post-measurements and, if necessary, re-injections in defined sections.
• Assessment of material quantities in relation to the injected void volume.

Interlinking with Demolition and Fit-Out Works

In practice, injection work is seldom executed in isolation. It is often necessary to locally open shotcrete, expose reinforcement, supplement joints, or remove built-ins. Controlled, low-vibration methods are required to avoid destabilizing the surroundings and falsifying measurements. In such situations, tools from Darda GmbH are used as complements, without replacing the injection process.

• Concrete demolition shears: selective removal of shotcrete shells, creating clean openings for drilling and packer niches, removing defective concrete in the area of injection joints.
• Hydraulic splitters: low-vibration widening of narrow accesses in rock, creating controlled crack joints to receive grouting lances or to relieve areas prior to sealing works.
• Hydraulic power pack: supplies hydraulic tools with the required drive power under confined and ventilation-sensitive conditions, for example via hydraulic power units.
• Multi cutters and steel shear: cutting reinforcement steel and sections to safely route injection lines or distribution systems.
• Hydraulic demolition shear: flexible cutting of mixed cross-sections (concrete/rebar) in tight spaces.
• Rock wedge splitter: pinpoint opening in massive areas where conventional drilling reaches its limits.
• Tank cutter: for special operations, e.g., safely cutting thick-walled pipeline components away from ignition sources, if project-specific requirements call for it.

Areas of Application and Typical Use Cases

Tunnel injection appears in various construction and deconstruction phases. Depending on the context, objectives and methods vary, often in combination with preparatory works from concrete demolition and building gutting within deconstruction.

• Rock excavation and tunnel construction: pre-excavation sealing to reduce water inflows, consolidation of disturbed zones, canopy measures in the crown area; selectively creating openings with concrete demolition shears for injection manifolds during construction.
• Concrete demolition and special demolition: contact injections behind existing linings, void filling under backwash conditions; creating accesses and niches with hydraulic splitters for safe packer installation.
• Building gutting and cutting: local exposure of reinforcement and built-ins, cutting pipes and sections to route injection lines; subsequent sealing and consolidation injections for stabilization.
• Natural stone extraction: in special cases, sealing measures at quarry access tunnels; controlled splitting to prepare drilling patterns.
• Special operations: emergency sealing under sudden water inflow with fast-reacting systems; rapid, controlled opening of areas with concrete demolition shears for counter-injections and relief boreholes.

Occupational and Environmental Protection

Safe workflows and environmentally compatible procedures are integral to injection technology. Pressure control, material handling, ventilation, and monitoring must match the construction site situation. During deconstruction, attention must be paid to sparks, vibrations, and noise as well as dust exposure and aerosols.

• Define pressure limits and isolation concepts; avoid blow-outs (check valves, secure packers).
• Material storage and mixing according to manufacturer specifications; avoid carryover into sensitive zones.
• Supplementary dust extraction and watering when removing with concrete demolition shears; consistently use personal protective equipment.
• Handle reaction resins and gels only in well-ventilated areas; keep emergency kits ready.
• Compliance with generally accepted rules of the art and regulatory requirements lies with the project participants.

Typical Sources of Error and How to Avoid Them

Quality deficiencies often arise from inadequate preparation, incorrect material selection, or unsuitable pressure control. A structured execution with clear checkpoints reduces risks and rework.

• Insufficient borehole cleaning: leads to incrustation and poor penetration—consistent flushing and tightness checks.
• Unsuitable viscosity or gel time: material does not penetrate or gels too early—adjust material parameters to temperature and host rock.
• Excessive pressures: hydraulic loosening, creation of new flow paths—adhere to pressure stages and limits.
• Gappy drilling grid: incomplete sealing curtain—complete the primary/secondary/tertiary grid in full.
• Insufficient documentation: missing evidence—continuous logging of pressure, volume, time, and material.
• Lack of coordination with deconstruction works: clashes and delays—early coordination, e.g., for creating niches with concrete demolition shears.

Project Flow: From Preliminary Investigation to Post-Check

A clearly structured flow connects investigation, planning, execution, and testing. This reduces technical risks and enables reproducible results.

1. Preliminary investigation and assessment of permeability, strength, and water pressures.
2. Define the injection concept with grid, pressures, materials, and sequence.
3. Structural preparation: create safe accesses; locally open areas where necessary with hydraulic splitters or concrete demolition shears.
4. Create, clean, and pack off the boreholes; tightness check.
5. Mix and inject according to specifications; continuous monitoring.
6. Intermediate checks, parameter adjustments, re-injections as needed.
7. Final proof tests and documentation of results.

Notes on Integration into Different Excavation Methods

In drill-and-blast excavation, pre-excavation injection is arranged in a grid ahead of the tunnel face to control water inflows and consolidate loose zones. In the New Austrian Tunneling Method, coordination is tight with the shotcrete shell; local openings for packer niches are selected to preserve load paths. In mechanized excavation with a tunnel boring machine, sealing of the annular space and contact injection are the focus. In all methods, the pressure control must match the lining, measurement and verification procedures are to be defined early, and preparatory deconstruction steps—such as with concrete demolition shears from Darda GmbH—are to be planned so that they do not impair injection and monitoring works.