Backfilling methods secure, stabilize, and seal structures, excavations, voids, and utility corridors. They form a central link between demolition, strip-out, cutting, and the subsequent reconstruction or decommissioning. Especially after selective interventions with concrete demolition shears, hydraulic rock and concrete splitters, or after core drilling, openings, joints, and voids are created that must be backfilled in a controlled and permanent manner to avoid deformations, water ingress, and settlement. In all areas of application—from concrete demolition and special deconstruction to rock excavation and tunnel construction through to natural stone extraction and special operations—backfilling requires reliable material- and process-safe planning as well as clean execution.
Definition: What is meant by backfilling methods
Backfilling methods refer to the targeted placement and compaction of bulk materials, mortars, suspensions, or lightweight materials into voids, trenches, annular gaps, joints, or deconstruction areas. Objectives include restoring load-bearing capacity, limiting deformations, improving the subsoil, sealing against water or media, and closing openings for fire protection and acoustics. Backfilling is carried out as gravity backfilling, layered granular fill placement, injection or grouting methods, or casting with self-compacting materials. The choice of method is guided by geometry, accessibility, environmental conditions, and structural requirements.
Techniques and materials for backfilling
Backfilling can be broadly divided into bulk-fill, casting, and injection methods. In addition, special materials such as flowable backfill material or foam concrete are used when compaction energy is to be limited or low bulk densities are required. After demolition activities with concrete demolition shears or controlled blasting/splitting using rock and concrete splitters, defined voids and separation joints are created that allow for targeted material selection and installation technique.
Mineral bulk materials and layered placement
For trenches, utility corridors, and larger voids, stable, compactable bulk materials are used. Installation quality is decisive for load-bearing capacity and low settlement.
- Aggregates: gravel, sand, crushed stone, recycled construction material (quality assured), if applicable frost protection material
- Placement: in layers, defined layer thicknesses, compaction with suitable equipment (low vibration levels near existing structures)
- Controls: density testing, visual check of particle-size distribution, moisture control
Grout mortars and non-shrink systems
For joints, sleeves, foundation undergrouts, and component connections after separation with concrete demolition shears, cement-based grouts with low shrinkage tendency and high early strength development are suitable.
- Requirements: flowability, volume stability, bond strength, where applicable sulfate and frost resistance
- Applications: undergrouting of machine foundations, annular gaps at embedded parts, fire stop (sealing) systems (always in accordance with manufacturer and standards)
Injection and grouting methods
Injection methods fill fine cracks, fissures, and annular gaps or stabilize the ground. They are executed with pressure control and are suitable for hard-to-access areas, for example behind existing components after selective deconstruction.
- Media: cement grout/microcement, cement suspensions with bentonite, silicate or resin systems (depending on subsoil and approvals)
- Methods: packer injection via boreholes, sleeve pipes, annular gap grouting, compensation grouting
- Control: pressure and volume logging, staged filling from bottom to top, flushing and cleaning cycles
Flowable backfill materials: flowable backfill and foam concrete
When compaction processes are to be avoided or complex geometries are present, self-compacting, re-excavatable systems are used.
- Flowable backfill material: binder-modified soil with defined consistency and subsequent load-bearing capacity, ideal for utility corridors
- Foam concrete: very lightweight, pumpable backfill for large-volume voids, shafts, or adit closures
Planning and design of backfilling
Robust planning starts with an inventory: geometry, access, groundwater, existing structure, and load transfer. After demolition or cutting work—such as with concrete demolition shears, multi cutters, or combination shears—edges, joints, and substrates must be prepared so that backfill materials adhere or can be installed to be stable.
Volume, pressure, and sequence
The backfill volume is determined from measurements and void allowances. Injection work is monitored for pressure and quantity to avoid uplift and heave. As a rule, filling proceeds from the lowest point to avoid air entrapment, and in sections so that settlements occur in a controlled manner.
Material compatibility
Backfill materials must be compatible with surrounding construction materials (e.g., alkali, sulfate, and chloride compatibility, temperature behavior, fire requirements). Near reinforcement, low-chloride and cement-bound systems are typically used.
Backfilling in concrete demolition and special deconstruction
Selective deconstruction with hydraulic concrete demolition shears creates defined fracture edges and enables targeted joint or void backfilling without additional cracking in the existing structure. After separating components or coring openings, penetrations, sleeves, and connection joints are closed with grouts or injection grouts to restore structural action, tightness, and fire protection.
- Follow-up work: chamfering edges, cleaning, pre-wetting, bonding bridge depending on the system
- Grouting: non-shrink, flowable, with defined minimum layer thickness
- Injection: install packers in drill channels, staged grouting, documentation of pressure and volume
Backfilling in rock excavation and tunnel construction
In rock excavation, rock and concrete splitters as well as rock splitting cylinders create controlled separation joints. These are then backfilled to close fissures, block water paths, and stabilize slopes. In tunnel construction, annular gap grouting between segmental linings and ground plays a central role; anchor boreholes, cable ducts, and niches are also closed with suitable suspensions or grouts.
- Annular gap: pumpable, low-settlement suspension, pressure-controlled
- Fissures: microcement injection with fine particle-size distribution
- Boreholes: cement grout or encapsulating systems depending on exposure
Strip-out and cutting: safely closing openings
During strip-out and separation of components, chases, chases for utilities, core drill openings, and saw cuts are created. After cutting, voids, annular gaps, and penetrations must be backfilled. Highly flowable, non-shrink grouts prove effective here—or, for complex geometries, flowable backfill materials that completely fill voids.
Natural stone extraction and re-backfilling
In natural stone extraction, temporary voids and trenches are created that must be backfilled again for stability or recultivation. After loosening with rock and concrete splitters, stable granular fills are placed in layers; where settlements are critical, foam concrete or flowable backfill material can be used.
Special applications: void backfilling and compensation grouting
Unexpected voids (e.g., old lines, chimneys, gravel lenses) are often pressure-supported filled with suspensions or foam concrete. Where settlement is a risk, compensation grouting can raise the ground in a targeted manner. After removal of tank systems or large-volume components, shafts and pits are backfilled for safety reasons to prevent subsequent subsidence.
Execution: sequence of steps and quality assurance
- Preparation: clear out, clean, prepare edges, define drilling or injection points
- Material selection: suitability per structural, building physics, and environmental requirements
- Installation: from bottom to top, free of air, pressure- or gravity-controlled
- Aftercare: smooth surfaces, protect curing, control moisture balance
- Documentation: delivery notes, test protocols, quantity and pressure curves
- Quality: density and consistency tests, temperature control, visual inspection for voids
- Acceptance: if applicable, plate load test or load tests at the agreed depth/zone
Occupational safety and environmental protection
Backfilling works are planning- and monitoring-intensive. Pressurized lines, uplift, and media leakage must be avoided. Emissions and leaching are reduced through appropriate material selection and construction sequence. In water-sensitive areas, only approved systems are used; disposal of residual quantities is carried out in accordance with the applicable requirements. The statements here are always general and do not replace project-specific approval or legal review.
Typical interfaces to Darda GmbH equipment
Hydraulic tools from Darda GmbH create the prerequisites for precise backfilling: they open component joints with low vibration, produce defined voids, and reduce cracking in the existing structure—key foundations for controlled backfilling and safe grouting.
- Concrete demolition shears: selective separation of concrete components, followed by joint and annular gap grouting
- Rock and concrete splitters as well as rock splitting cylinders: controlled loosening in rock/concrete, then fissure and borehole grouting
- Combination shears and multi cutters: exposing utilities/reinforcement, preparation of backfilling areas
- Steel shears: deconstruction of metallic installations, subsequent void backfilling
- Tank cutters: after removal of large-volume tanks, safe void backfilling remains central
- Hydraulic power units: energy-efficient supply of the tools for low-vibration preparatory work
Common sources of error and how to avoid them
Insufficient compaction, unsuitable material selection, or lack of venting lead to settlements, leaks, or load redistributions. This is countered with a coordinated mix design, defined installation technique, sufficient trial areas, and seamless documentation. Especially close to existing structures, low-vibration deconstruction with concrete demolition shears or rock and concrete splitters is advisable so that the subsequent backfilling can take place in a low-damage environment.