Underwater foundations support structures in and adjacent to bodies of water. They secure bridge piers, quay facilities, locks, offshore structures, and shoreline protections. Planning and execution differ significantly from land-based foundations: water, currents, limited visibility, corrosion, and scour act as additional boundary conditions. Special methods and tools are used for construction, repair, and deconstruction. In practice, underwater foundations touch many application areas—from concrete demolition and special demolition to rock excavation and tunnel construction. Where controlled deconstruction is required, concrete demolition shear as well as hydraulic rock and concrete splitters from Darda GmbH play a technical role because they operate with low vibration and are suitable for sensitive aquatic zones.
Definition: What is meant by underwater foundation
An underwater foundation is a foundation that is constructed, operated permanently, or dismantled wholly or partly below a free water surface. This includes pile foundations, caissons, sheet pile cofferdams, and shallow-founded foundation bodies located in tidal zones, rivers, lakes, or the sea. Underwater foundations transfer structural loads into competent soil layers or rock and resist additional actions from currents, waves, buoyancy, abrasion, and corrosion.
Types and construction methods of underwater foundations
The choice of foundation type depends on subsoil, water level, loads, and site logistics. Common variants are:
- Pile foundations: driven piles or bored piles in reinforced concrete or steel, singly or as pile groups with pile caps. Suitable for soft sediments and large water depths.
- Caissons: prefabricated or in-situ fabricated reinforced concrete boxes that are lowered, aligned, and filled with underwater concrete.
- Sheet pile cofferdams: closed sheet pile geometries, filled internally or used as a dewatered excavation with bracing.
- Shallow foundations: massive foundation slabs or blocks on compacted subgrade or rock with scour and erosion protection.
- Rock anchors and micropiles: for securing on bedrock or in confined conditions.
Construction methods
Underwater, concrete is often placed as tremie concrete. In this process, a continuous column of concrete flows through a pipe down to the subgrade to avoid washout. Alternatively, it is pumped or cast in dewatered boxes. For foundations on rock, drilling followed by grouting and anchor technology is common.
Materials and material concepts
Underwater foundations require durable construction materials. Important aspects are:
- Underwater concrete with anti-washout protection, dense matrix, and adjusted consistency. Admixtures reduce segregation, ensure workability, and provide early strength.
- Reinforcement protection through sufficient concrete cover, appropriate exposure classes, and supplementary measures such as coatings or cathodic systems where specified for the project.
- Steel components with corrosion protection systems, for example multilayer coatings and constructive water drainage.
- Scour and erosion protection using riprap, mattresses, geotextiles, or concrete blocks.
Design and actions
In addition to permanent and variable loads, hydrodynamic forces act. Designers consider:
- Currents, waves, ship traffic, and ice loads
- Buoyancy and hydrodynamic suction effects
- Scour formation, abrasion due to suspended solids and debris
- Fatigue and dynamic actions
- Soil parameters under saturation, settlements, and creep
The design follows the technical rules applicable to the project. Safety concepts, partial safety factors, and verifications are to be defined on an object-specific basis.
Execution: construction under water
The construction sequence depends on water depth, tidal windows, and logistics. Typical steps are:
- Surveying, subsoil investigation, and definition of temporary works
- Construction of sheet pile cofferdams or installation of caissons
- Foundation preparation: soil removal, formation level, drilling and grouting
- Reinforcement installation and underwater concreting (tremie or pump method)
- Curing, quality assurance, scour and shoreline protection
Quality assurance
Underwater, control is performed using diving inspections, sonar, concrete records, and, where applicable, test cores. Documentation and monitoring are central components of construction supervision.
Maintenance and typical damage patterns
Underwater foundations age due to water chemistry, currents, and mechanical loading. Frequent findings are:
- Washouts and scour at the foundation edge
- Cracks, spalling, and exposed reinforcement in the splash zone
- Abrasion from sediment transport and cavitation in flow-adjacent zones
- Chloride-induced corrosion on reinforced concrete and steel parts
Rehabilitation uses concrete technology measures, injections, jacketing, and strengthening. For local deconstruction under water, controlled, low-vibration methods are advantageous.
Deconstruction, rehabilitation, and special demolition
During deconstruction, loads are removed segment by segment to limit turbidity, noise, and vibration. In sensitive areas, the concrete demolition shear from Darda GmbH is frequently used. It crushes concrete on the member, separates reinforcement visibly, and enables transport of individual segments. Where drilling is possible, the hydraulic splitter (wedge) allows a controlled splitting technique—even in the tidal or splash zone. This approach fits within the application areas of concrete demolition and deconstruction as well as special assignments with tight environmental windows.
Tool selection and hydraulics
Hydraulic tools are powered via a hydraulic power pack on a pontoon or from shore (Power units). Hose routing and couplings must be planned to be diver-friendly, well secured, and protected from damage. For cutting steel sections, sheet piles, or reinforcement, steel shear or Multi Cutters from Darda GmbH are suitable where material-selective deconstruction is specified.
Rock exposure and removal
If the foundation meets rock, rock wedge splitter can open the rock in a controlled manner. This is particularly useful when adjusting bearing areas, remediating scour, or removing protruding rock ribs. The use intersects the fields of rock excavation and tunnel construction as well as natural stone extraction when blocks are to be released in a targeted manner.
Environmental and occupational safety
Work on the underwater foundation requires strict safety and environmental management. Key aspects are:
- Diver safety, permits, communication, and rescue concepts
- Hose and line management for hydraulics in a wet environment
- Minimization of turbidity and sediment load, e.g., through careful segmentation
- Reduction of noise and vibration, for example using splitting methods instead of percussive methods
- Separate collection of concrete, steel, and coatings for recycling
Permits and ecological requirements must be observed on a project-specific basis. Any information provided here is general and non-binding; the applicable codes and regulatory determinations in the respective project are authoritative.
Planning and logistics
Underwater construction sites are logistically demanding. Success factors are:
- Make optimal use of tidal and weather windows as well as current velocities
- Pre-assembly on land, modular components, and short installation times
- Redundant lifting and safety equipment on pontoons and workboats
- Clear interfaces between dive team, crane, concrete logistics, and quality assurance
Application areas related to Darda GmbH tools
Underwater foundations touch several application areas in which tools from Darda GmbH typically appear:
- Concrete demolition and special demolition: segment-by-segment removal with concrete demolition shears, splitting massive cross-sections with hydraulic splitter (wedge).
- Rock excavation and tunnel construction: exposing and profiling rock bearing surfaces with rock wedge splitters, controlled rock removal without blasting pressure waves.
- Gutting and cutting: separation of reinforcement and steel components with steel shear or Multi Cutters, supported by hydraulic power pack.
- Special assignment: work in ecologically sensitive zones, in the tidal area or with restricted accessibility, where low-vibration methods offer advantages.
Practical notes for tendering and execution
Precise specifications are crucial for economical and safe implementation. Recommended are:
- Clear indication of water levels, currents, and visibility ranges
- Definition of permissible limits for turbidity, noise, and vibration
- Specifications for segment size during deconstruction and for material separation
- Proof requirements for concreting procedures, sampling, and monitoring
- Documentation of scour protection measures and their control
Diagnostics and monitoring
For condition assessment of underwater foundations, specialists combine diving inspections, ROV footage, sonar measurements, and concrete technology testing. Additionally, non-destructive methods are used to assess thickness, delaminations, or voids. Continuous monitoring supports the early detection of scour and erosion processes.