Backfilling refers to the targeted placement of loose or bound materials to close voids, excavation pits, utility trenches, and backfills safely, durably, and with minimal settlement. In deconstruction, concrete demolition, rock excavation, and in tunnel construction and special foundation engineering, backfilling is a key step that restores structural stability, ensures drainage functions, and protects structures. In practice, backfill materials often arise directly in the process: deconstruction using concrete pulverizers, hydraulic rock and concrete splitters, or combination shears produces recyclable aggregates which—when quality-assured and used in accordance with standards—can serve as backfill material. efficient hydraulic power units provide the energy for the processing steps that create the basis for controlled, technically sound backfilling.
Definition: What is meant by backfilling
Backfilling is the planned installation of unbound or bound construction materials to fill ground and structural voids, trenches and demolition depressions, as well as to provide backfill behind support structures. Objectives include restoring load-bearing capacity, limiting settlements, protecting against water and frost damage, ensuring filter and drainage functions, and—depending on the application—fire protection. Backfilling is distinct from shoring: shoring provides temporary stabilization of excavation pits, whereas backfilling establishes the permanent final state. In the context of concrete demolition and special demolition, as well as rock excavation and tunnel construction, backfilling is often carried out immediately after dismantling or splitting to close open spaces and safely continue construction phases.
Fundamentals, objectives, and requirements of backfilling
Proper backfilling combines geotechnical, hydraulic, and construction operations aspects: suitable gradation and moisture content, adequate compaction (e.g., with reference to Proctor density), filter stability, frost resistance, and drainage capacity, tailored to loads, embedment depth, and subsoil. In the deconstruction environment, this is supported by source-separated processing of the arising materials. Tools such as concrete pulverizers and rock and concrete splitters produce defined aggregate with low foreign-material content; combination shears, Multi Cutters, steel shears, and tank cutters enable separation of reinforcement and embedded components. This creates material streams that—given suitability evidence and quality assurance—can be used as backfill material. Decisions on material selection, layer thicknesses, compaction energy, and drainage measures should ideally be based on a coordinated backfilling concept.
Backfilling in concrete demolition and special demolition
In concrete demolition, foundation pits, shaft heads, utility trenches, and component openings arise that must be specifically backfilled as part of special demolition. After removing foundations or slabs—often opened with concrete pulverizers, cut with combination shears, and stripped of reinforcement with steel shears—the depressions are filled in layers with suitable material. Unbound aggregates (e.g., recycled aggregate) carry loads and can be compacted economically; bound systems (e.g., cement-bound backfilling mixes) are used where compaction is technically impossible or not permitted.
Typical procedure
- Investigation and planning: capture geometry, load assumptions, groundwater, drainage, and connection details.
- Material concept: select based on gradation, filter criteria, frost protection, and compactability; consider suitability evidence.
- Selective deconstruction: release components with concrete pulverizers and rock and concrete splitters, cut reinforcement with steel shears, separate foreign materials.
- Processing: crushing, screening, interim storage; quality assurance of recyclates (e.g., foreign-material content, particle shape, moisture).
- Layered installation: define layer thickness and compaction energy, carefully interlock transitions to the existing structure.
- Drainage/filter: install drain pipes, filter geotextiles, or graded layers to control water removal.
- Control and documentation: record density tests, load-bearing capacity checks, and visual inspections.
Materials for backfilling
Material selection is based on function, accessibility, and construction condition. The decisive factors are gradation, moisture content, compactability, filter and drainage behavior, and chemical harmlessness. Relevant groups are:
Unbound aggregates
Natural sands and gravels as well as recycled aggregates from deconstruction are the standard solution for excavation pit and trench backfilling. They are highly compactable, can be used with frost and filter stability, and exhibit low settlement when installed correctly. Suitability increases when the grading curve is continuous (e.g., 0/32, 0/45) and the fines content is controlled.
Recyclates from deconstruction
In special demolition, concrete pulverizers, rock and concrete splitters, and combination shears produce recycled aggregates from concrete components. After removing reinforcement with steel shears and a tank cutter, these recycled materials—when quality-assured—can serve as backfill material. Advantages include short transport distances and resource conservation; note particle cleanliness, water absorption, and suitable grading to avoid settlements.
Bound backfilling systems
Cement-bound backfilling mixes, flowable fill, and foamed concrete are used under confined conditions, in utility corridors, or beneath sensitive existing structures. They are self-compacting and minimize vibrations. Binders and mix designs must be selected project-specifically; in tunnel construction, injection suspensions are also used for cavity backfilling.
Backfilling in rock excavation and tunnel construction
In rock excavation and tunnel construction, voids, fracture zones, and backspaces behind linings occur. After controlled rock release—often using rock and concrete splitters or rock splitting cylinders to minimize vibrations—voids are backfilled with drainable, filter-stable materials or with injection materials. The goals are to reduce subsequent breakouts, control the water regime, and ensure uniform load transfer to linings.
Cavity backfilling and sealing
Injections (e.g., cement-based suspensions) seal fracture systems and stabilize loosely deposited areas. Backfills behind segment or shotcrete linings improve bedding and reduce voids. Selection and use of injection materials are fundamentally project-specific; where applicable, regulatory requirements and recognized rules of practice must be observed.
Backfilling during building gutting and cutting
During building gutting and cutting, openings are created in slabs, walls, or shafts that are filled temporarily or permanently. Cut edges are often produced with concrete pulverizers to avoid vibrations; Multi Cutters and combination shears facilitate removal of embedded parts. Backfilling with lightweight, pumpable materials enables placement in hard-to-access interior areas without introducing high compaction energy. This helps minimize settlements and sound bridges.
Compaction, drainage, and quality assurance
Compaction largely determines durability. It is carried out in layers with specified layer thicknesses and compaction equipment, depending on material and accessibility. Tests (e.g., density checks, dynamic plate load tests) support quality assurance. Water must be drained in a targeted manner: drainage layers, filter geotextiles, and graded aggregates prevent fines migration and frost heave. Transitions to the existing structure must be executed carefully to balance differing stiffnesses.
Placement under confined conditions
In narrow shafts or beneath existing structures, compaction may be mechanically restricted. Self-compacting backfilling mixes, flowable fill, or foamed concrete reduce vibration and load effects on adjacent components. This is especially relevant in use areas that remain in operation during conversion.
Environmental and resource topics
Backfilling is a lever of the circular economy. Recyclates from deconstruction conserve primary raw materials and reduce transportation. Prerequisites are source-separated deconstruction, removal of disruptive materials (e.g., metals, organic fractions), and documented material quality. Tools such as concrete pulverizers, combination shears, steel shears, and tank cutters facilitate separation of concrete, steel, and embedded parts. Dust and water protection measures must be observed during processing and placement. Legal requirements for the use of recycled construction materials must be checked on a project-specific basis; the information provided is generally without guarantee and does not replace a case-by-case review.
Planning, interfaces, and documentation
A coordinated backfilling concept links deconstruction, processing, logistics, and placement. Key points include: subsoil and water conditions, load assumptions, material selection, drainage, compaction strategy, occupational safety, and documentation of evidence. Hydraulic power packs that drive concrete pulverizers, rock and concrete splitters, and other tools must be planned in terms of output and cycling so that processing and placement align in time and quality. Traceable documentation of installed layers, test values, and material provenance facilitates later inspections.
Typical sources of error and how to avoid them
- Inappropriate grading: leads to settlements or washout—select gradation project-specifically.
- Insufficient compaction: causes voids—adhere to layer thicknesses and compaction energy.
- Missing drainage: promotes frost heave and water pressure—provide drainage and filter systems.
- Mixing with foreign materials: reduces load-bearing capacity—consistent sorting during deconstruction (separation of concrete and steel).
- Excessive water addition: reduces density—control moisture content and plan placement to suit weather.
- Poor connection details: settlement edges at the existing structure—interlock transitions and, if necessary, grout.
- Insufficient documentation: makes evidence difficult—record installation and test data continuously.
Tools and processes related to backfilling
The connection between backfilling and deconstruction technology is direct: concrete pulverizers crush concrete in a controlled manner and produce defined aggregate for re-backfilling. Rock and concrete splitters and rock splitting cylinders separate components and rock with low vibration—an advantage in sensitive areas. Combination shears and Multi Cutters separate composite materials, steel shears prepare reinforcement for disposal or recycling, and tank cutters safely open thick-walled vessels. Hydraulic power packs supply these tools with the necessary energy. The result is clear material flows, short routes, and technically suitable materials for backfilling in concrete demolition and special demolition, during building gutting and cutting, in rock excavation and tunnel construction, as well as in special applications.
Occupational safety
Safety takes precedence: secure trenches, avoid edge loads, operate compaction equipment at a safe distance, and control material relocations. Dust extraction and wetting reduce emissions. Cavity backfilling within structures must be assessed structurally; the information provided here is general and does not replace project-specific planning.