Underwater water jet cutting refers to the separating machining of concrete, steel, and composite components using a high-pressure water jet guided under water, frequently with the addition of an abrasive. The method is used wherever conventional separation techniques reach their limits due to flying sparks, heat generation, vibration, or restricted access—for example at bridge piers, locks, quay facilities, underwater foundation elements, or in flooded excavation pits. Thanks to the cold, spark-free cutting action, the method is particularly suitable for sensitive environments and demanding deconstruction tasks.
Definition: What is meant by underwater water jet cutting
Underwater water jet cutting is understood as chipless cold separation using a bundled water jet at very high pressure (typically several thousand bar), which is guided under water—either in full immersion or within a flooded working chamber. To increase material removal performance, a mineral abrasive, such as garnet sand, is often added to the water jet. A distinction is made between a pure water jet (for soft materials) and an abrasive water jet (for hard materials such as steel, concrete, or rock). The kerf is narrow, the heat-affected impact on the material is minimal, and the process is conducted with diver guidance or remote control.
Fields of application and typical projects
Typical tasks range from separating reinforced concrete components in flowing waters to cutting through sheet pile walls, driven piles, and steel beams, and opening tanks or shafts that are flooded for safety or environmental reasons. In complex deconstruction projects, underwater water jet cutting is frequently combined with hydraulic demolition tools—for example, when large concrete segments must be released in a controlled manner and recovered after cutting.
Process engineering and process parameters
The performance of underwater water jet cutting depends on several parameters that together determine cutting quality and productivity.
Types of water jet
A pure water jet is suitable for soft sealing or coating layers. For concrete, steel, and composite cross-sections, an abrasive water jet is used. The abrasive (with defined grain sizes) is metered near the nozzle so that a focused, high-energy jet forms under water.
Pressure, nozzles, and feed
Ultra-high-pressure pump units generate the process pressure (for example, compact hydraulic power units), and the nozzle shapes the jet. Decisive parameters include pressure level, nozzle orifice, abrasive feed, standoff distance, and feed rate. Under water, the jet cross-section expands faster than in air, which is why guidance systems and distance control are important to achieve a consistent kerf.
Underwater-specific aspects
Currents, limited visibility, and cavitation influence the process. Shields, capture hoods, and suction systems reduce turbidity and capture abrasive. Cut guides can be fixed magnetically, mechanically, or using templates to ensure geometry even with changing water levels.
Areas of use in the construction and deconstruction context
The method is used in various scenarios of component separation. In the context of the tools and applications of Darda GmbH, the following typical references arise:
- Concrete demolition and special demolition: Separating underwater foundation elements, pier projections, or cantilever deck arms in flooded zones. After the cut separation, concrete pulverizers can expose segment edges or release residual bonds. This corresponds with established concrete demolition and deconstruction practices.
- Strip-out and cutting: Creating openings in flooded shafts, basins, or pump chambers; precise notching of recesses. In exposed areas, rock wedge splitters and concrete splitters are used to selectively break up massive cross-sections (for example, rock and concrete splitters).
- Rock removal and tunnel construction: Local removal of shotcrete or cutting of embedded parts below the groundwater table. In water-bearing sections, reinforcement can be exposed with the jet technique before mechanical tools continue.
- Natural stone extraction: Less common under water, but relevant in impounded quarries or shoreline failures when dry working is not possible. The jet technique minimizes vibration.
- Special operations: Cold cutting in sensitive zones with explosion risk (ATEX zone) or contaminated sites that are flooded for emission control. The spark-free separation supports a controlled approach.
Combination with hydraulic demolition tools of Darda GmbH
In many projects, underwater water jet cutting is used as a precise separating cut, while hydraulic tools take over structural release and segmentation. Sensible process chains include, for example:
- Pre-cutting under water: The water jet separates the outer contour of a concrete cross-section. Subsequently, rock wedge splitters and concrete splitters reduce the inner bond through controlled crack formation to define blocks to be removed.
- Exposing and separating the reinforcement: After a jet cut along the concrete surface, concrete pulverizers engage to release residual bonds and make reinforcement accessible. In addition, hydraulic demolition shears or multi cutters—depending on accessibility—segment steel parts.
- Segmentation for recovery: After the cut, components are divided into manageable elements so that cranes or lifting devices can recover them safely. The combination of a precise cut and mechanical splitting action reduces constraint points.
Which sequence is appropriate depends on component build-up, water level, static boundary conditions, and hoisting logistics. Early coordination between cutting technology and hydraulic tools is important to align cut lines, splitting openings, and lifting points.
Quality, dimensional accuracy, and cut appearance
The kerf is typically narrow and—depending on parameter selection and guidance—shows low taper. In steel, clean separating surfaces result without thermal zones; in concrete, the aggregate is exposed, and the reinforcement is either cut with the jet or selectively freed. For precisely fitting follow-up work (such as comprehensive refurbishment or new connections), guidance systems and constant standoff distances are crucial.
Special features with reinforced concrete
Varying hardnesses (mortar, aggregate, steel) require stable process parameters. It is often efficient to cut the concrete shell first and then cut the reinforcement in a second step mechanically. Here, steel can be segmented by hydraulic demolition shears or—on waterside interface areas—concrete pulverizers help remove residual concrete to achieve clear cut edges.
Safety and environmental aspects
Under water, hazards from high pressure, jet impact, limited visibility, and currents are added. Remote-controlled carrier systems or diver-assisted methods with clear responsibility and exclusion concepts are standard. For emission control, capture hoods and filtration systems retain abrasive and removed material. Noise emission and low vibration levels are lower than with percussive processes, which supports the protection of adjacent structures.
Legal and organizational notes
Permitting and protection requirements vary by water body, structure, and project. In general, water protection, occupational safety, and coordination with navigation or water management must be considered. Project- and safety-specific requirements must always be coordinated with the responsible authorities.
Project planning, sequence, and logistics
Careful planning improves quality and cost-effectiveness. Proven steps include:
- As-built survey: Material build-up (concrete compressive strength class, reinforcement ratio, steel grades), wall thicknesses, embedded parts, utility lines, water conditions.
- Cutting strategy: Defining cut path, access, guidance systems, start and run-out zones, and segment sizes for recovery.
- Resource planning: High-pressure unit, abrasive logistics, water management, power supply, work platforms, chain hoist.
- Emission management: Collecting and filtering abrasive and fines, turbidity control, protection of adjacent uses.
- Downstream processing: Mechanical segmentation with rock wedge splitters and concrete splitters or concrete pulverizers for edge cleanup and controlled release of residual bonds.
Cost-effectiveness and performance factors
The cost structure is determined by setup effort, access (pontoons, divers, robotics), performance parameters (pressure, abrasive consumption), material thickness, and the number of cuts. Advantages arise from minimal rework on cut surfaces, minimized edge damage, and the ability to work at all under difficult boundary conditions. A smart combination with hydraulic tools reduces cutting scope and improves recovery logistics.
Limits, alternatives, and complements
Very large wall thicknesses, unfavorable access, or strong currents can limit productivity. Alternatives include wire saw systems, saw chains, or purely mechanical removal. A hybrid approach is often sensible: pilot boreholes, targeted use of rock wedge splitters and concrete splitters to reduce cross-section, subsequent jet cut, and mechanical finishing with concrete pulverizers or hydraulic demolition shears.
Typical scenarios from practice
Examples of proven applications include cutting off bridge pier projections in the riverbed, opening lock sections for installing new components, cutting through sheet pile walls in ports, or creating access openings in flooded shafts. In all these tasks, the combination of precise, spark-free cutting and subsequent mechanical segmentation enables the safe recovery of components.
Key technical parameters at a glance
- Pressure level: High pressure in the multi-thousand-bar range, adapted to material and thickness.
- Abrasive: Defined grain sizes for concrete and steel; dosing depends on the desired cutting speed.
- Kerf: Narrow, with low taper under stable guidance.
- Process control: Diver-guided or remote-controlled; fixation by rails, templates, or magnetic guides.
- Rework: Edge cleanup and segment release with concrete pulverizers and rock wedge splitters and concrete splitters, depending on the component and access.