Undermining refers to the washing out or infiltration of load-bearing soils beneath structures, traffic surfaces, and slope stabilizations by water. This creates voids, settlements, and fabric disturbances that impair structural stability. In practice, this particularly affects foundations, bank and retaining structures, bridge abutments, tunnel portals, sewers, impoundments, and concrete surfaces. Remediation often requires controlled exposure, selective removal, and the restoration of load-bearing layers. In these work steps—depending on the situation—concrete pulverizers, hydraulic rock and concrete splitters, hydraulic power units, and other attachments from Darda GmbH are used to work gently on the material, precisely, and with low vibration.
Definition: What is meant by undermining
Undermining is understood as the hydraulically induced removal or rearrangement of soil beneath structural elements or surface layers, resulting in the formation of voids, subsidence, and loss of bearing capacity. Causes include flowing or pressurized water, leaks, inadequate filter layers, or local flow concentrations. In the vicinity of waterways, the local deepening of the subsoil at component toes is often referred to as scour. In urban environments, undermining often appears on paver and concrete surfaces, utility trenches, basement exterior walls, foundation bases, and retaining walls. The transition to related processes such as suffosion (fine-grain removal), piping (erosion channels caused by underflow), and undercutting is gradual.
Causes and mechanisms of undermining
The development of undermining results from the hydraulic gradient, grain-size distribution, and lack of filter action. Once moving water loosens and transports fines from the ground, pore enlargement occurs up to the formation of voids. Local constrictions, edges, and joints increase flow velocities and promote erosion. Deficiencies in waterproofing and damaged drainage or sewer systems intensify the process. During flood events, dynamic pressure fluctuations additionally occur, deepening and widening scour laterally.
Typical causal chains at a glance
The mechanisms can usually be assigned to several recurring cause-and-effect chains. Understanding these chains is central to the selection and sequence of remediation steps.
Hydraulic drivers
- Concentrated flow at edges, foundation offsets, toe of slopes, and bridge abutments
- Groundwater uplift and hydraulic short-circuiting along utility trenches
- Leaks in pressure and combined water lines with continuous fines removal
- Floods, wave and ship traffic with alternating suction and pressure phases
Geotechnical and structural factors
- Missing or incorrectly designed filter and transition layers
- Poorly graded, washout-prone sands and silts
- Cracks and joints in concrete elements without backing protection
- Freeze-thaw cycles with loosening and washout paths
Construction operational influences
- Dewatering measures with uncontrolled flow paths
- Insufficient surface drainage and slope guidance
- Vibrations that cause loosened soil to settle
Damage patterns and risks
Undermining manifests over areas through settlements, slab cracking, and hollow-sounding coverings; on structures through tilting, joint widening, cracks, and local spalling. Typical are sinkholes at drainage points, washed-out joints, silted drain lines, and scour at the base of elements.
Early warning signs
- Damp spots with fine-grain discharge (sandy suspensions)
- Unusual depressions or hollows on surfaces
- Rattling or give of slabs and pavers
- Turbid water in shafts after rainfall events
Consequences for structural stability
- Reduced foundation base width and uneven settlements
- Loss of bearing areas, tendencies to overturn, and shear failure
- Damage to pipelines and shafts
- Increased risk during deconstruction due to unpredictable voids
Investigation, monitoring, and documentation
Careful investigation narrows down the extent of damage, location of voids, and inflow paths. Visual inspections, moisture and settlement mapping, soundings, density tests, and low-impact exposures are part of this. Geophysical methods and endoscopy can locate voids. Openings can often be created with low vibration using concrete pulverizers; massive or thick-walled concrete elements are selectively weakened with hydraulic splitters to then be removed in sections. All interventions must be secured and documented; level and crack monitoring accompany critical phases.
Prevention and structural measures
Prevention aims to guide flows, ensure filter stability, and protect structural joints against backflow. Near water bodies, scour protection, bed stabilization, and site-appropriate slope armoring are central. For buildings, intact drainage, functional drains, splash-water protection, and tight penetrations are the focus.
Proven approaches
- Filter and transition layers with graded grain sizes, if necessary geotextiles
- Scour protection mats, rock riprap, precast concrete elements with backing filter
- Sealing and targeted water routing at joints, penetrations, and edges
- Maintenance of drainage systems and regular flushing and leakage tests
Remediation of undermining: practical procedure
Remediation typically follows a structured sequence: secure, expose, remove damaged components, restore bearing capacity and surfaces.
1. Secure and control water
- Construction site safety, traffic diversions, load reduction
- Temporary seals, bypasses, pumping measures with controlled discharge
2. Expose and investigate
- Selective opening of coverings and concrete surfaces; in confined areas, concrete pulverizers prove effective for controlled, edge-preserving removal
- For massive elements, splitting with hydraulic splitters enables low-vibration access without additional crack formation
3. Remove damaged zones
- Removal of loose, hollowed, and delaminated concrete areas
- For strengthened sections and reinforced concrete, depending on conditions, concrete pulverizers, combination shears, or Multi Cutters are used
4. Backfill, grout, filter
- Backfill voids with filter-stable materials; coordinate injection and grouting carefully for erosion safety and water paths
- Compact subbase layers in lifts, minimize load redistribution
5. Restore and protect
- Construct surfaces, joints, and edges with protection against backflow
- Permanently upgrade scour protection and drainage
Throughout the process, a low-vibration working method is advantageous to avoid additional loosening. The combination of concrete pulverizers for precise removal and hydraulic splitters for controlled release of massive elements has proven particularly effective. Hydraulic power units supply the attachments with the required energy, even in hard-to-reach areas.
Application areas related to undermining
Concrete demolition and special deconstruction
Undermined foundation areas and retaining structures often require partial deconstruction to supplement filter and sealing elements. Sectional removal with concrete pulverizers limits crack propagation; splitting massive blocks with hydraulic splitters reduces vibration compared to percussive methods.
Strip-out and cutting
Creating inspection openings, relief openings, or drainage channels in areas affected by undermining requires precise, controlled cuts and removals. Low-noise, low-vibration methods protect adjacent components and sensitive installations.
Rock excavation and tunnel construction
At tunnel portals, retaining bodies, and in rock outcrops, undermined zones and scour can lead to voids. Targeted release of rock and concrete with splitting techniques creates safe access for grouting and backfilling without introducing additional shock excitation into the ground.
Natural stone extraction
Where water migrates along joints, loosened areas can develop. Controlled splitting limits unwanted breakouts and enables clean separation joints, especially in water-bearing zones.
Special deployment
After heavy rainfall and floods, fast, safe action is required: secure hazard areas, locate voids, reinforce traffic surfaces. In emergencies, mobile, hydraulic cutting and splitting methods prove their worth for exposing undermined areas and gently removing damaged elements. Where steel components must be removed, steel shears are an option; in complex plants, dismantling with a cutting torch can be part of the overall concept.
Material- and method-appropriate selection of tools
The choice of method depends on material, element thickness, reinforcement content, and accessibility. The goal is safe, controlled processing without creating additional erosion paths.
- Thin-walled to medium-thickness concrete elements: precise, sectional removal with concrete pulverizers
- Massive blocks and thick sections: low-vibration release using hydraulic splitters
- Reinforcement-intensive areas: combination with combination shears or Multi Cutters
- Steel structures in undermined zones: separation with steel shears under controlled load transfer
Terms and distinctions in context
Undermining is closely related to scour, backwashing, undercutting, suffosion, and piping. While scour usually describes local deepening at structural parts in flowing waters, undermining focuses on the formation of voids beneath components regardless of contact with a watercourse. Suffosion denotes the removal of fine grains from filter-unstable soils; piping stands for channel-like erosion tubes under a hydraulic gradient. In practice, terminology is less decisive than identifying inflow paths, filter failure, and the correct, step-by-step construction of a filter-stable remediation.
Safety, environment, and execution
Work on undermined structures requires special caution: unforeseen subsequent breakouts, void enlargement, and load redistribution are possible. Barriers, load reduction, and drainage tailored to the surroundings must be ensured before starting. Dust and noise protection, water and soil protection, as well as orderly construction logistics are part of professional execution. Permit issues and the requirements of water management authorities must be reviewed on a project-specific basis.