Introduction
Liquefiable and weak soils present significant challenges in geotechnical engineering. Under seismic loading or heavy construction, such soils may lose strength, leading to differential settlement, lateral spreading, and structural instability. Compaction grouting is a proven technique for improving the density and strength of these soils, making it a preferred solution in urban infrastructure, transportation, and high-rise construction projects. This article explores field applications of compaction grouting specifically in liquefiable and weak soil layers.
Understanding Liquefiable and Weak Soils
- Liquefiable soils: Typically loose, saturated sands or silts that lose shear strength during seismic activity.
- Weak soils: Soft clays, silts, or loose granular soils with low bearing capacity and high compressibility.
- Both types pose risks to foundations, retaining structures, tunnels, and roadways.
Compaction grouting densifies and stabilizes these soils, increasing load-bearing capacity and mitigating settlement or lateral displacement.
Mechanism of Compaction Grouting in Weak and Liquefiable Soils
- Grout is injected under pressure into the soil in a series of overlapping columns or bulb-shaped formations.
- The grout displaces and compacts surrounding soil particles, increasing density and stiffness.
- In liquefiable layers, densification reduces pore water pressure generation and improves resistance to seismic liquefaction.
- In weak soils, the process enhances bearing capacity, preventing excessive settlement under structural loads.
Field Applications
1. Seismic Mitigation of Liquefiable Sands
Project Example: Urban high-rise development on a site with loose, saturated sands.
Implementation:
- A grid of grout columns was injected beneath foundations to densify the liquefiable sand layer.
- Real-time monitoring ensured controlled uplift of nearby structures.
Outcome:
- Post-grouting densification reduced liquefaction potential during seismic simulations.
- Differential settlement was minimized, ensuring the stability of high-rise structures.
2. Stabilization of Weak Clay Layers for Foundation Support
Project Example: Soft clay layers beneath an airport runway.
Implementation:
- Compaction grouting was applied beneath the runway slab in targeted zones.
- Grout columns were spaced to achieve uniform load transfer and prevent uneven settlement.
Outcome:
- Bearing capacity of the weak clay increased significantly.
- The runway slab remained level under aircraft loading, reducing maintenance costs.
3. Underpinning Existing Structures on Compressible Soil
Project Example: Historic masonry building in a city center built on soft, compressible silts.
Implementation:
- Compaction grouting was used beneath the foundation in a staged approach.
- Grout injection pressures were carefully controlled to prevent uplift of the structure.
Outcome:
- Settlement was arrested immediately after grouting.
- The building remained stable, avoiding costly underpinning or relocation.
4. Ground Improvement for Transportation Infrastructure
Project Example: Railway embankment constructed over weak and potentially liquefiable soil.
Implementation:
- Grout columns were installed along the embankment footprint to densify underlying soils.
- Post-grouting instrumentation confirmed uniform settlement and increased lateral resistance.
Outcome:
- Enhanced soil stability prevented track deformation.
- Reduced the risk of service interruptions during heavy train operations.
Best Practices for Field Application
- Site Investigation: Conduct thorough soil and groundwater characterization to design grout volume, pressure, and column spacing.
- Grout Mix Design: Use low-mobility, high-viscosity grout suitable for the target soil and environmental conditions.
- Monitoring: Implement settlement markers, inclinometers, and piezometers to track soil behavior during and after grouting.
- Incremental Injection: Apply grout in stages, especially beneath sensitive structures, to control uplift and avoid soil fracturing.
- Post-Grouting Evaluation: Conduct in-situ testing (SPT, CPT, plate load) to verify improvement in soil properties.
Conclusion
Compaction grouting is a versatile and effective solution for liquefiable and weak soils, enhancing density, stiffness, and load-bearing capacity. Field applications across urban high-rises, transportation infrastructure, and historic structures demonstrate its ability to mitigate seismic risks, reduce differential settlement, and stabilize soft soils. With proper planning, monitoring, and execution, compaction grouting ensures safe, durable, and efficient ground improvement in challenging geotechnical conditions.
