Introduction
Urban construction projects often face challenges related to weak, loose, or unstable soils. Such conditions can lead to differential settlement, structural instability, and safety hazards. Compaction grouting, also known as “pressure grouting” or “jet grouting” in certain contexts, is a ground improvement technique used to densify soils, lift settled structures, and enhance foundation performance. This article explores several case studies demonstrating the effective application of compaction grouting in urban construction projects, highlighting practical lessons, methods, and outcomes.
Overview of Compaction Grouting
Compaction grouting involves injecting a low-mobility, high-viscosity grout into the ground under pressure. The injected grout displaces and compacts surrounding soil, increasing density and improving load-bearing capacity. Key advantages include:
- Mitigation of differential settlement
- Stabilization of loose or sandy soils
- Minimal disruption to urban infrastructure
- Suitability for underpinning existing structures
The technique is particularly valuable in densely built-up areas where conventional excavation or deep foundation methods are constrained by space and existing utilities.
Case Study 1: Underpinning a Historic Building
Project Background
A 19th-century masonry building located in the city center exhibited uneven settlement due to soft, compressible soil beneath the foundations. Traditional underpinning methods were deemed risky due to the building’s fragile structure.
Implementation
Compaction grouting was employed to stabilize the soil beneath the foundation. Grout columns were injected in a grid pattern, with careful monitoring to prevent excessive uplift of the structure.
Results
- Differential settlement was arrested within days.
- Structural monitoring confirmed that vertical movement remained within safe limits.
- The building was preserved without major excavation or disruption.
Key Lesson: Compaction grouting can provide targeted soil improvement, enabling preservation of historic urban structures without intrusive construction.
Case Study 2: High-Rise Foundation Settlement Control
Project Background
A 30-story residential tower in a metropolitan area experienced settlement concerns due to underlying loose sands. Space constraints and nearby traffic limited the use of conventional deep foundations or soil replacement.
Implementation
Engineers designed a program of deep compaction grouting beneath the building footprint. High-pressure grout injections were used to densify the loose soil layers and reduce compressibility.
Results
- Settlement rates were reduced by 80% compared to predictions.
- Construction proceeded without disruption to adjacent roads or structures.
- Long-term monitoring indicated sustained performance with negligible post-construction settlement.
Key Lesson: Compaction grouting is highly effective in high-rise construction where soil densification is required beneath large loads, especially in urban areas with limited space.
Case Study 3: Metro Tunnel Ground Improvement
Project Background
A metro construction project required excavation in soft, water-saturated soil beneath existing urban infrastructure. The risk of ground loss and settlement threatened nearby utilities and roadways.
Implementation
Pre-excavation compaction grouting was performed to densify the soil along the tunnel alignment. Grout was injected in a series of overlapping columns to create a stable soil mass.
Results
- Settlement along the tunnel alignment was significantly reduced.
- Excavation proceeded safely with minimal impact on surrounding structures.
- Project delays due to ground instability were avoided.
Key Lesson: Compaction grouting can provide proactive ground stabilization in urban tunneling projects, mitigating risks to adjacent infrastructure.
Best Practices and Considerations
- Pre-Construction Soil Investigation: Accurate soil profiling is essential for determining grout volume, pressure, and injection spacing.
- Real-Time Monitoring: Settlement sensors and inclinometers ensure controlled compaction without causing uplift or structural damage.
- Urban Constraints: Planning must account for traffic, underground utilities, and neighboring buildings to prevent unintended impacts.
- Environmental Management: Proper handling of grout materials prevents contamination of groundwater or nearby surfaces.
Conclusion
Compaction grouting has proven to be a versatile and effective solution for soil stabilization, foundation support, and settlement mitigation in urban construction projects. The case studies presented demonstrate its adaptability—from historic building preservation to high-rise foundations and metro tunnel excavation. By carefully planning and monitoring grouting operations, engineers can achieve safe, efficient, and minimally disruptive ground improvement, ensuring the long-term stability of urban infrastructure.
