Compaction grouting is a specialized ground improvement technique used to densify loose or weak soils and control settlement beneath new or existing structures. Unlike other forms of grouting that permeate the soil matrix, compaction grouting works by displacing the soil and compacting it through controlled pressure injection of a stiff, low-mobility grout.
This article explores the principles, grout properties, installation methods, and applications of compaction grouting in geotechnical engineering.
1.What is Compaction Grouting?
Compaction grouting (also called low-mobility grouting) involves the injection of a very stiff cement-based grout into the ground under high pressure. Rather than permeating the soil, the grout forms bulbs or columns that push and compact surrounding soil particles, leading to improved ground strength and stiffness.
2.Objectives and Mechanism
- Densify Loose Soils: Increase relative density of granular soils to reduce liquefaction potential.
- Mitigate Settlement: Arrest ongoing or prevent future settlement under structures.
- Lift Structures: In controlled amounts, compaction grouting can raise settled foundations.
- Improve Bearing Capacity: Enhance the load-bearing ability of weak soils.
The key principle: Displacement and compaction of the soil mass, not permeation.
3.Grout Characteristics
- High viscosity / low mobility: Does not flow into pores.
- Stiff mix: Typically a sand-cement or neat cement grout with water-cement ratios as low as 0.4.
- Cohesive and non-erodible: Forms distinct bulbs that displace surrounding material.
- Set time and expansion: Can be modified using additives (e.g., bentonite, fly ash, plasticizers).
4.Installation Procedure
1.Drilling: A small-diameter hole is drilled to the target depth.
2.Grouting from the Bottom-Up: Grout is pumped into the ground in stages as the casing is gradually withdrawn.
3.Formation of Grout Bulbs: Grout displaces surrounding soil and creates compacted zones.
4.Monitoring: Pressure, volume, and ground movement are tracked to control outcomes and prevent damage.
5.Applications
- Underpinning Structures: Used beneath foundations of buildings, tanks, or bridges that are experiencing settlement.
- Liquefaction Mitigation: Stabilizes loose sandy soils in seismic zones.
- Void Filling: Useful for filling old utility lines, sinkholes, or abandoned mines.
- Stabilizing Slopes: Densifies subsurface soil layers and increases resistance to movement.
- Floor Slab Lifting: Relevels industrial slabs or warehouse floors without excavation.
6.Advantages of Compaction Grouting
- Highly Targeted: Allows treatment of specific problem zones.
- Minimal Disruption: No need for excavation or removal of soil.
- Structural Re-Leveling: Can gently lift foundations or slabs without damaging the structure.
- Effective in Granular Soils: Particularly useful for sands, silty sands, and loose fills.
- Quick Set-Up and Execution: Often faster than deep foundation retrofitting.
7.Limitations and Considerations
- Not Suitable for Cohesive Soils: Clays resist displacement, reducing effectiveness.
- Risk of Over-Lift or Heave: Requires skilled control of pressure and volume.
- Limited Reach: Grout bulb size is generally confined to a few feet per injection point.
- Requires Access and Clearance: Borehole drilling and grouting equipment need headroom and space.
8.Case Study Example
Project: Settlement remediation beneath a hospital wing in alluvial soil.
Solution: Compaction grouting injected from 12–18 feet depth, forming grout columns at a 1.5 m grid.
Result: Average building lift of 18 mm achieved, and post-grouting monitoring confirmed stabilization.
Conclusion
Compaction grouting is a proven, cost-effective, and non-invasive solution for densifying soil, controlling settlement, and restoring structural alignment. When applied with proper engineering design and execution, it delivers long-lasting improvement and protects assets in both new and existing construction environments.
