Underpinning grouting is a proven injection method used in construction, refurbishment, and special demolition to fill voids, improve bearings, compensate settlements, or purposefully lift structural elements. In projects where hydraulic demolition tools such as concrete demolition shear or hydraulic rock and concrete splitters by Darda GmbH are used, underpinning grouting often serves for preparation, stabilization, or follow-up securing so that components can be processed in a controlled manner with low vibration levels.
Definition: What is meant by underpinning grouting
Underpinning grouting refers to the targeted injection of injection materials (for example cement suspensions, microcements, fine grout, or reactive injection resin) from below or from a lower level into voids, contact joints, and loose zones beneath slabs, foundations, walls, or bases. The aim is cavity filling, the restoration of load transfer, the increase of contact stiffness, the limited lifting of settled structural elements, as well as sealing against water or air flows. The method differs from underpinning (load-bearing auxiliary foundations) and from pure crack injection (local crack sealing), but it can be combined with both.
Practical relevance and typical applications
Underpinning grouting is used wherever load transfer is uncertain, voids are suspected, or components need to be corrected in a controlled way. Typical situations in the context of demolition, refurbishment, and tunnel construction:
- Compensation of settlements beneath floor slabs, machine foundations, or stair bearings before precise cutting and deconstruction work.
- Contact and consolidation grouting below base slabs to improve overturning and shear strength prior to the use of concrete demolition shear.
- Filling voids after removing load-bearing or bracing elements before hydraulic wedge splitter introduce splitting forces in a controlled manner.
- Sealing and backfilling in tunnel construction (for example under the invert or at connection joints) to reduce water paths and stabilize the lining.
- Stabilizing masonry bearings in existing buildings when openings are cut or components are deconstructed with concrete demolition shear.
Process and technique of underpinning grouting
Planning and execution follow a systematic approach tailored to subsoil, structural element, and objectives. Crucial are the preliminary investigation, the selection of the injection material, the arrangement of boreholes, and the control of pressure.
Pre-investigation and objective definition
Before starting, subsoil and structural data are reviewed, voids are located (for example by probing or hammer sounding), and measurement points for settlement and lift control are installed. It is defined whether underpinning grouting is primarily intended to fill, densify, seal, or lift.
Borehole pattern and packer arrangement
Boreholes are arranged in a grid or along lines. Packers are used to seal the injection path. Depth is based on the location of voids or the contact zone (for example underside of slabs, foundation base). For thin components, short, closely spaced boreholes are preferred to minimize bypass paths.
Injection materials
- Cement suspensions and fine grout: well-suited for cavity filling, load-bearing contact zones, economical and compatible with mineral building materials.
- Microcements: for fine pores and dense materials; long pumpability, good penetration.
- Injection resins: low viscosity, fast reaction, suitable for sealing or targeted lifting; use depends on environmental and fire protection requirements.
Pressure control and process monitoring
Injection is carried out in stages. Injection pressure and injected volume are continuously monitored. Criteria for switching or stopping include, for example, refusal (no further intake), reaching the pressure limit, the desired lift, or emergence at defined control points. Lifts are monitored geodetically in real time to avoid inadmissible deformations.
Documentation
For verification and quality assurance, drilling logs, injected volumes, pressures, times, material batches, and measurement data are documented continuously. This facilitates parameter adjustments during execution.
Interfaces with demolition methods and hydraulic tools
Underpinning grouting and hydraulic demolition tools interlock. A coordinated sequence reduces risks, prevents uncontrolled cracking, and improves occupational safety.
- Before deploying concrete demolition shear: underpinning grouting stabilizes bearings and contact joints so that crushing and cutting forces are introduced in a controlled manner.
- Accompanying the splitting technique with hydraulic wedge splitter: In areas with suspected voids, grouting is carried out in advance or in sections to secure load paths and avoid collateral breakage.
- After deconstruction: Filling resulting voids prevents settlements of adjacent components and prepares follow-up measures (for example new bearings).
- Hydraulic power pack supplies demolition tools efficiently and with low vibration; separate injection pumps are used for underpinning grouting. Workflows must be kept separate but coordinated in time and space.
Particularities in application areas
Concrete demolition and special demolition
In selective deconstruction, underpinning grouting beneath slabs, platforms, and foundations reduces the risk of punching shear failures or unintended lifts. In combination with concrete demolition shear, components can be released section by section while underpinning grouting strengthens the remaining bearings.
Building gutting and cutting
When creating openings (for example door and ceiling openings), underpinning grouting stabilizes bearing edges. This keeps cut joints clean and reduces unwanted crack propagation. Especially in confined conditions where hand-held tools and concrete demolition shear are used, this increases execution safety.
Rock excavation and tunnel construction
In drifts and tunnel bores, underpinning grouting supports the invert, fills voids, and reduces water paths. Before using splitters in rock, consolidating loosely deposited zones may be required so that splitting forces act where planned. These measures are common in rock demolition and tunnel construction.
Natural stone extraction
In the extraction and setting of massive natural stone, underpinning grouting in the bedding joint improves contact faces and closes local voids. This enables more precise splitting and protects edges from spalling.
Special application
In damage cases (undercut voids, unexpected settlements), emergency grouting can mobilize structural reserves until further measures are implemented. Combining with quiet, low-vibration tools such as concrete demolition shear is particularly advantageous in vibration-sensitive environments.
Risks, limits, and quality assurance
The effectiveness of underpinning grouting depends on subsoil, pore structure, injection material, and process control. Potential risks are uncontrolled bypass paths, unintended lifting, material washout, or insufficient bonds. Prevention:
- Careful preliminary investigation and test boreholes.
- Stepwise pressure increase with limit values and measurement control.
- Graded particle-size or viscosity strategy (from low-viscosity to coarser systems).
- Geodetic monitoring of sensitive points during injection.
- Regrouting only after evaluating the first results.
Material selection and equipment
The choice of injection material depends on the objective (filling, sealing, lifting), temperature, moisture, and environmental requirements. Mineral systems are often the first choice for concrete and masonry. Reactive systems are used when very fine cracks, rapid curing, or high tightness are required. Execution requires injection pumps with adjustable delivery rate, packers, check valves, and measuring devices. Hydraulic power pack supplies concurrently used demolition tools such as concrete demolition shear or hydraulic wedge splitter; logistics must be planned so that trades do not impede each other.
Planning, structural analysis, and cautions
Underpinning grouting affects load distribution and requires a structural analysis. Lifting measures must be small-dosed and closely monitored. Requirements from applicable standards and guidelines—especially regarding injection materials, occupational and environmental protection, and documentation—must be observed. Binding statements on load-bearing capacity may only be made as part of individual planning and assessment.
Measurement and monitoring methods
For quality assurance, measuring staffs, leveling, crack monitoring, and pressure logs have proven effective. For sensitive components, additional sensors (for example displacement measurement at bearing points) are used. The goal is to relate injected volumes, pressures, and deformations to demonstrate the success of underpinning grouting.
Sustainability and environmental aspects
Underpinning grouting is a low vibration levels method and can preserve components or enable their targeted deconstruction. In combination with quiet hydraulic tools (such as concrete demolition shear), emissions in existing buildings are reduced. The selection of injection materials should be based on environmental compatibility, recyclability, and the deconstruction concept; escaping material must be retained and disposed of professionally.