Underwater injection

Underwater injection is a specialized method in construction and repair in which flowable injection agents are introduced below the water level into cracks, joints, pores, or voids. The objective is to seal, strengthen, or backfill structural elements and soils, for example at quays, locks, bridge piers, tunnels, or foundations. In practice, underwater injection often interfaces with work steps where tools and equipment from the Darda GmbH portfolio can be used – for example when a concrete demolition shear exposes damaged concrete or rock and concrete splitters create controlled separations to make injection points accessible. This creates logical work interfaces between injection and the application areas concrete demolition and special deconstruction, strip-out and cutting, as well as rock excavation and tunnel construction. When executed with calibrated pressure control and suitable materials, underwater injection enables targeted remediation and service-life extension without large-scale dewatering.

Definition: What is meant by underwater injection?

Underwater injection, also referred to as underwater grouting, refers to the targeted introduction of cementitious or polymer injection agents in a water-bearing environment, without the need to completely dewater the work area. Injection is carried out through boreholes or joint connections using injection packers and pumps. Typical objectives include sealing against water ingress, void filling behind structures, contact injection between existing components and the subgrade, and ground improvement around waterfront structures. Materials used range from microcement suspensions to silicate and acrylate gels to water-reactive resins. Crucial factors are sufficient filtration stability, matching viscosity to the pore structure, and an injection pressure compatible with the structural element, complemented by underwater anti-washout behavior and low leaching potential.

• Success factors under water: controlled pumpability and gel time in line with water temperature and flow conditions
• Compatibility: mineral or polymer system selected for substrate chemistry and pore size
• Structural limits: injection pressure below uplift and tensile capacity, with allowance for buoyancy effects
• Environmental profile: low-emission formulations and measures to limit turbidity and dispersal

Application fields of underwater injection in construction and deconstruction

Underwater injection is used to stop leaks in concrete structures, fill voids behind sheet pile walls, stabilize undermining, or improve the contact between underwater concrete and rock. In tunnel heading with water ingress, a curtain injection can reduce inflow. In special deconstruction, injection-supported backfilling can limit settlements before selective removal of components with a concrete demolition shear or rock and concrete splitters. At quay facilities, injection enables repair of joints and bearing areas without large-scale drawdown. In natural stone extraction, injection can help stabilize water-bearing fractures before controlled separations are performed. Further use cases include culverts, weirs, and diaphragm walls where targeted sealing or contact improvement is required under operational constraints.

• Typical targets and effects: leak sealing, void backfilling, contact consolidation, permeability reduction, and load-transfer improvement
• Access strategies: drilling from the dry side where feasible or from barges and platforms with diver support

Methods and materials in underwater injection

The choice of injection system depends on the objective, water conditions, temperature, and the pore or crack structure of the ground or structural element.

Cementitious systems

Cement grouts, microcement suspensions, and ultra-fine graded mixes are suitable for contact and void filling as well as for ground improvement. Their advantages include volume stability, mineral compatibility, and cost control. Important aspects are homogeneous mixing, low sedimentation, and a controlled water-cement ratio. Under water, the risk of colmation should be addressed as filter clogging; dispersing and stabilizing additives can influence settlement behavior and penetration depth, while anti-washout agents help maintain cohesion in moving water.

Resin and gel injections

Water-reactive polyurethanes, epoxy resins, and acrylate gels are used when very fine cracks need to be injected or rapid sealing is required. Gels offer adjustable gel times and good penetration into very fine pore structures. Resins allow durable crack filling and increase watertightness. Under water, mixing and exothermic behavior must be controlled; attention must also be paid to environmental compatibility and low-emission formulations. System selection should consider hydrophilic vs. hydrophobic behavior, foaming vs. non-foaming reaction, and long-term stability under permanent water exposure.

• Selection notes: define target viscosity window, pot life or gel time, and post-expansion behavior
• Performance checks: verify adhesion to wet mineral surfaces and resistance to hydrolysis

Injection types

• Contact injection: Closes voids and settlement cavities between underwater concrete and subgrade.
• Curtain or sealing screen injection: Multi-row drilling patterns to reduce water ingress.
• Crack injection: Localized sealing of cracks and joints in structural elements.
• Ground improvement: Strengthening and reduction of permeability around structures.

The selection of injection type depends on inflow rate, expected permeability, structural sensitivity, and accessibility for drilling and packer placement.

Typical workflow under water

1. Expose and prepare: Remove damaged layers, shell and biofilm growth, and loose components. Depending on accessibility, a concrete demolition shear can be used for selective removal or rock and concrete splitters for controlled separations to establish injection points. Clean contact faces to saturated surface-dry where feasible and mark drill patterns for orientation under water.
2. Drilling and setting packers: Create boreholes according to a grid; install suitable injection packers with check valves. Prefer bottom-up sequencing and ensure reliable packer sealing on wet or irregular surfaces; verify pull-out resistance or collar tightness before pressurizing.
3. Mixing and delivery: Produce the injection suspension within the defined w/c ratio range or resin/gel per manufacturer specifications. Supply via hydraulic power pack at the surface and injection-capable pumps with pressure and flow control. Use continuous mixing where required and minimize residence time in hoses; for cementitious systems consider anti-washout admixtures.
4. Injection: Stepwise pressure increase, monitoring pressure, delivery rate, and outflow. Control gel times or setting time depending on water temperature. Inject from the lowest packer upward, relieve pressure gradually, and confirm uptake stabilization or defined refusal criteria before moving to the next point.
5. Closure and control: Remove packers and seal boreholes. Perform a leakage test, flow measurement, and visual inspection by divers or camera. Where possible, use tracer dyes or conductivity checks to assess spread under turbid conditions.
6. Documentation: Injection logs including pressure/flow history, material formulation, times, and batch numbers. Supplement with as-built drilling plans, temperature records, and notes on any re-injection.

Interfaces to products and application areas of Darda GmbH

Underwater injection is rarely an isolated step. It is often combined with mechanical pre- or post-works in which tools from the Darda GmbH program come into play. Low-vibration, precisely metered separation minimizes the risk of uncontrolled crack propagation in saturated concrete or rock and helps create reproducible injection faces.

• Concrete demolition shears: Selective removal of damaged edge zones on piers, quay walls, or foundation heads to create clean bonding surfaces for contact injections.
• Rock and concrete splitters as well as stone splitting cylinders: Low-stress opening of separation joints in rock or concrete to access injection channels or to enable pressure relief prior to sealing.
• Combination shears, steel shears, and multi cutters: Cutting free reinforcement, meshes, and attachments so that packers can be set and injection lines routed.
• Tank cutters: Dismantling and opening coated steel components in wet areas if voids must be made accessible prior to filling.
• Hydraulic power packs: Power supply for the above tools on pontoons, work platforms, or along the shore; the spatial separation allows wet and dry work areas to be organized safely.

These interfaces occur particularly in the application areas concrete demolition and special deconstruction, rock excavation and tunnel construction, strip-out and cutting, as well as in special operations in maritime or fluvial environments. Coordination of sequences and responsibilities should be defined in the method statement and risk assessment.

Planning, design, and quality control

Robust planning starts with ground and structural diagnostics: permeability, jointing, crack widths, water pressures, and currents determine material and pressure selection. For cementitious systems, particle size distribution, stabilization, and sedimentation behavior must be defined; for resins and gels, viscosity, reactivity, and gel time must be adjusted to water temperature and flow. Acceptance criteria for sealing and consolidation, such as residual inflow thresholds or target permeability, should be specified in advance.

• Design parameters: Target permeability (k-value), allowable injection pressure (relative to buoyancy and structural strength), grid and drilling depth, mix ratio, working time, and expected uptake per interval.
• Quality assurance: Pre-tests (penetration tests, filtration stability), monitoring of pressure/flow, sampling, leakage tests, and re-injection if required; on-site verification of gel time and temperature logging.
• Documentation: Complete injection logs, test records, photo documentation, and measured values serve acceptance and traceability.

Safety, environmental, and permitting aspects

Working under water imposes special requirements on occupational and environmental protection. Injection pressures must be selected to prevent inadmissible redistribution or uplift. Diving operations require coordinated procedures, clear communication, and emergency plans. On the materials side, low-emission formulations and the avoidance of turbidity are preferred; entries into bodies of water are to be minimized. Permits and notification procedures depend on the water body, the structure, and the materials used and must be coordinated with the competent authorities in advance. These notes are general in nature and do not constitute legal advice.

• Typical protective measures: silt curtains or temporary screens at injection fronts
• Pollution control: spill response kits and drip trays on floating platforms
• Monitoring: turbidity checks and visual control by divers or remote cameras
• Communication: clear hand signals and redundant comms between surface crew and divers

Typical challenges and practical solutions

• High current: Select shorter gel times and/or higher-viscosity systems; temporarily shield injection points.
• Very fine pore structure: Use microcements or gels with low viscosity; verify filtration stability.
• Unclear cavity geometry: Exploratory injections and stepwise pressure control, accompanied by measurement of outflows.
• Corroded reinforcement and delaminations: Expose beforehand with a concrete demolition shear, remove loose parts, then perform contact injection.
• Stress conditions in rock: Relieve using rock and concrete splitters, followed by targeted curtain injection.
• Low water temperature: adjust gel time and accelerator dosage; insulate lines to stabilize temperature.
• Limited visibility: use contrasting tracer dyes compatible with the system and structured logging to confirm spread.

Practical application examples

For the rehabilitation of a bridge pier in a riverbed, delaminated concrete layers are first removed with a concrete demolition shear and reinforcement areas are exposed. Drilling enables the setting of packers for a contact injection between the pier base and the foundation bedding. Finally, the area is backfilled with a cementitious suspension and watertightness is tested.

In tunnel heading with heavy water inflow, a curtain injection ahead of the tunnel face can reduce inflow. As part of the preparation, rock and concrete splitters open separation joints in a controlled manner to create suitable injection channels. After excavation, a crack injection follows for permanent sealing of the lining.

At a quay wall with undermining, cavities are identified through probe drilling. Using a grid of packers, a gel is first injected to quickly form a sealing layer, followed by a microcement suspension for volume stabilization. Steel shears and multi cutters previously created access by removing attached components.

In the area of lock sills and abutments, targeted sealing injections reduce infiltration beneath structural interfaces, followed by contact injections to improve load transfer prior to scheduled maintenance works.

Distinction from underwater concreting

Underwater injection is distinct from underwater concreting. While concreting creates large-volume components using tremie pipe or hopper methods, injection targets the penetration of existing cracks, pores, and voids. In repair practice, both methods are often combined: first injection for sealing or contact improvement, then concrete supplementation. The sequence depends on the condition of the structure, water conditions, and structural boundary conditions.

• When to prefer injection: fine cracks, localized leaks, void closure, and permeability reduction
• When to prefer concreting: new elements or large-volume replacements with defined geometry
• Combined strategies: staged sealing plus subsequent concrete placement to achieve both tightness and structural capacity