Backfilling operations describe the planned filling, backfilling and sealing of voids in structures and in the ground. They combine geotechnical engineering, deconstruction, material processing and logistics. In the context of concrete demolition, special demolition, rock excavation, tunnel construction and natural stone extraction, they ensure structural stability, improve load transfer and enable a circular economy through the reuse of mineral construction materials. Tools and systems from Darda GmbH such as concrete demolition shear or hydraulic wedge splitter are not used for the backfill itself, but for upstream steps of selective dismantling, block splitting and processing.
Definition: What is meant by backfilling operations
Backfilling operations refer to the targeted placement of suitable material in voids, trenches, galleries, caverns or behind components (backfilling) to ensure functions such as load-bearing capacity, deformation control, erosion protection, fire protection, noise insulation or water conveyance. This includes layered backfilling (placement and compaction in lifts), flowable backfill material (pumpable, self-levelling mixes) and injection method (pressure-assisted grouting of cracks, joints and hard-to-reach voids). In deconstruction and demolition scenarios, backfilling demand arises, for example, after foundation demolition, during decommissioning of shafts, in the annular space behind segment rings in tunnel construction or to seal abandoned voids in rock.
Background and terminology of backfilling operations
The term “Versatz” originates in mining, where extracted voids are filled with waste rock, sandy–gravelly mixes or hydraulically bound materials to ensure roof stability and the integrity of mine workings. In structural engineering a differentiated terminology has become established: backfilling (e.g., behind a retaining wall), backfilling of excavation pits and pipeline trench, flowable backfill material (self-compacting), injection and grouted backfill (crack and void filling). In deconstruction the term is used functionally when, after removing concrete and masonry structures, voids must be closed to ensure operational and structural safety. Material processing of demolition concrete using concrete demolition shear as well as low vibration levels rock separation with hydraulic wedge splitter from Darda GmbH produces suitable aggregates for later installation as backfill or as recycled construction material.
Distinction and fields of application
Backfilling operations differ from general earthworks in that they are function- and safety-critical and often take place in confined, sensitive or underground areas. Typical application fields are:
- Concrete demolition and special demolition: backfilling of foundation pits, shafts, channels and plant rooms; sealing of voids after partial demolition.
- Building gutting and cutting: backfilling behind newly installed walls or foundation widenings; compensating fills inside buildings.
- Rock excavation and tunnel construction: annulus grouting, gallery and niche backfilling, support and sealing measures in the rock mass.
- Natural stone extraction: orderly backfilling of overburden and broken blocks for stabilization and re-use, temporary slope protection.
- Special use: sealing to contain fire gases, emergency backfilling in cases of daylight collapse, securing decommissioned utility corridors.
Planning, material selection and qualities
Material selection is based on function, place of installation, accessibility and environmental requirements. The goal is a suitable grading curve, sufficient compactability and/or flowability, durability, and compatibility with adjacent components.
- Unbound fills: soil, sand, gravel, crushed aggregates and recycled concrete; suitable under dry conditions, with sufficient space for compaction and low settlement requirements.
- Hydraulically bound backfill: cement- or hydraulically bound mixes for higher load-bearing capacity, lower settlements and defined early and final strengths.
- Flowable backfill material (pumpable): self-levelling, for hard-to-reach voids, annular spaces and complex geometries.
- Injection/grouted backfill: fine-grained, possibly swelling systems for crack and joint filling as well as sealing.
When processing demolition concrete into recycled aggregates, concrete demolition shear contribute to a clean separation of reinforcement and mineral fraction. Hydraulic wedge splitter enable low vibration levels block splitting and the production of defined piece sizes. This creates aggregates that can be used in backfilling as recycled concrete provided the applicable requirements for cleanliness, absence of contaminants, grading and frost/de-icing salt resistance are met.
Process flow on site
- Investigation and planning: recording of void geometry, boundary conditions (water, vibrations, reserve capacity), accessibility and logistics.
- Material provision: selection and qualification of backfill materials, where applicable production of recycled concrete aggregates from deconstruction using concrete demolition shear and hydraulic wedge splitter followed by screening.
- Transport and conveying: conveyor belt, buckets, hose line and pump conveying depending on aggregate size and flow properties.
- Placement: placement in layers with compaction or continuous pumping/grouting with controlled placement rate.
- Quality assurance: checks of moisture, density, compaction (e.g., via suitability and control tests), slump/flow and strength development.
- Documentation: material origin, placed quantities, test values, acceptance of lifts and, where applicable, grouting logs.
Material processing from deconstruction
Selective deconstruction facilitates the recovery of high-quality recycled aggregates. Concrete demolition shear separate concrete and steel, providing single-grade mineral fractions for backfill. Hydraulic wedge splitters from Darda GmbH generate defined fragments with low vibration levels, which is advantageous in sensitive areas such as hospitals or existing buildings in operation. The materials thus prepared can, after appropriate testing, be used in backfilling.
Equipment technology surrounding backfilling operations
Backfilling itself is driven by materials and process. Nevertheless, equipment technology upstream is crucial:
- Concrete demolition shear: for selective removal and size reduction of concrete elements, exposing reinforcement and reducing to backfill-suitable aggregate sizes.
- Hydraulic wedge splitter as well as rock wedge splitter: low-vibration block splitting in rock, concrete foundations or massive components; helpful in densely built areas and in tunnel construction, often realized with hydraulic rock and concrete splitters.
- Hydraulic shear and multi cutters: universal cutting and separation tasks in mixed constructions; contribution to single-grade separation.
- Steel shear: removal and downsizing of reinforcement and sections to ensure trouble-free backfill placement.
- Cutting torch: safe dismantling of tanks prior to backfilling surrounding voids, e.g., in industrial deconstruction.
- Hydraulic power pack: energy supply to the tools via matched hydraulic power units for controlled, reproducible workflows.
Placement methods: backfilling, flowable backfill material and injection method
The choice of method depends on geometry, water conditions and required load-bearing capacity.
- Backfilling: placement in layers with limited lift thickness and compaction energy. Advantage: simple logistics, robust quality assurance. Suitable for accessible excavation pits and shafts.
- Flowable backfill material: pumpable mixes with defined consistency and limited heat development. Advantage: homogeneous void filling, minimal compaction work, well-suited for annular spaces and hard-to-reach areas.
- Injection method: pressure-groutable, fine-grained systems for crack and joint filling. Advantage: targeted closure of very small voids, even over long distances.
Quality assurance, compaction and control
Key target values are grading distribution, water content, degree of compaction or flowability and—on bound systems—strength development. Proven in practice are:
- Preliminary suitability tests to define mix composition and lift thickness.
- Ongoing site controls (e.g., density and moisture measurements, where applicable load plate test or dynamic test procedures).
- Logging of placement rates and pump pressures for flowable and injection backfill.
- Monitoring of settlement and recompaction, especially with recycled concrete.
Applicable technical rules and guidelines apply to recycled construction materials and bound mixes. They must be considered project-specifically, particularly regarding environmental compatibility, structural behavior and durability.
Occupational safety and environmental protection
Safety and environmental aspects are integral parts of backfill planning:
- Hazard analysis for confined spaces, fall edges and possible gas accumulation.
- Dust suppression and noise reduction measures, e.g., by coordinated work methods and appropriate water spraying.
- Control of water ingress and drainage; avoidance of washouts.
- Gentle methods with low vibration levels, e.g., rock separation with hydraulic wedge splitter in existing surroundings.
- Material certificates for environmental compatibility of recycled aggregates.
Sustainability and circular economy
Backfilling operations enable the high-value recycling of mineral residuals. Through selective dismantling with concrete demolition shear and low-vibration block splitting with hydraulic wedge splitter from Darda GmbH, clean aggregates are produced. Where suitable, these can substitute primary raw materials. In addition, low vibration levels reduce risks for neighboring buildings and utility line infrastructure—an important aspect in inner-city projects and in tunnel construction.
Typical challenges and practical solutions
- Water ingress: drainage, temporary sealing, selection of water-insensitive mixes; controlled placement rates.
- Confined access: use of pumpable flowable backfill material, modular conveying equipment, short mixing times.
- Heterogeneous aggregates: targeted processing and screening; pre-crushing with concrete demolition shear for more uniform grading.
- Settlement requirements: smaller lifts, matched compaction energy, if necessary hydraulically bound systems.
- Neighbor vibrations: rock separation with splitting technology instead of percussive or blasting works.
- Rebar remnants and foreign matter: upstream separation with steel shear and hydraulic shear, magnetic separation.
Examples from application areas
Concrete demolition and special demolition
After the demolition of a foundation block, the excavation is filled in layers with recycled concrete aggregates. Concrete demolition shear first process the demolition concrete, steel shear remove reinforcement. Compaction is carried out lift by lift, and sensitive utility lines are protected by placement in the correct position.
Rock excavation and tunnel construction
In gallery construction an annular space forms behind the lining. Flowable backfill material is introduced evenly at low pressure. In areas requiring neighbor protection, the excavation is previously carried out with hydraulic wedge splitter to minimize impacts.
Natural stone extraction
Overburden and by-products are orderly placed as backfill to stabilize slopes and control water paths. Rock wedge splitters produce shape-adapted blocks for layered placement, reducing settlements.
Key parameters and practical notes
- Grading distribution: a uniform grading reduces voids and increases density.
- Water content: optimal moisture level for compactability and bond in bound systems.
- Degree of compaction: layer-by-layer control to ensure load-bearing capacity.
- Flowability: for pumpable backfill maintain consistent consistency to avoid segregation.
- Strength and early stability: adapt to the construction phase schedule, especially for traffic loads and subsequent trades.