Introduction
Urban infrastructure faces unique geotechnical challenges, including restricted access, sensitivity to ground movement, and the presence of adjacent structures. Compaction grouting has emerged as a reliable ground improvement solution in these environments. By injecting a stiff, low-mobility grout into loose or weak soils, this technique densifies the ground without excavation or vibration—making it ideal for urban settings. This article reviews the methods, advantages, and case studies demonstrating the effectiveness of compaction grouting in urban infrastructure projects.
What is Compaction Grouting?
Compaction grouting involves the injection of a cementitious, low-slump grout that remains in a near-solid state as it is pumped into the soil. The grout forms discrete bulbs that displace and compact the surrounding soil. This technique improves the bearing capacity of the ground, reduces settlements, and enhances resistance to liquefaction.
Key Objectives in Urban Projects
- Mitigate foundation settlement
- Increase bearing capacity
- Stabilize voids or abandoned utilities
- Prevent damage during adjacent construction (e.g., tunneling, excavation)
- Minimize disruption to surrounding infrastructure and operations
Compaction Grouting Methods
1.Bottom-Up Grouting
- Grout is injected at the lowest elevation and continues in stages upward.
- Common for ground densification and foundation stabilization.
2.Top-Down Grouting
- Used in shallow applications or where underlying strata are not suitable for bottom-up injection.
3.Targeted Grouting
- Focuses on specific areas such as beneath footings, slabs, or utilities, guided by settlement surveys or geotechnical data.
4.Real-Time Monitoring
- Use of pressure gauges, flow meters, and settlement markers to monitor ground response and avoid over-injection.
Equipment and Grout Mix
- Injection Pipes: Typically 38–76 mm diameter steel pipes inserted to the required depth.
- Grout Pump: High-pressure positive displacement pump capable of delivering controlled flow rates.
- Grout Mix: Low water-cement ratio (0.4–0.6), often with additives for workability and early strength gain.
Case Studies
1.Subway Station Underpinning – New York City, USA
Challenge: Active subway line running beneath a planned high-rise development required foundation stabilization without interrupting transit operations.
Solution: Compaction grouting was used to densify loose fill and granular soils beneath existing station structures. Grouting was conducted at night with vibration and settlement monitoring.
Outcome: Subsurface movement was kept below 2 mm. No service disruptions occurred, and excavation for the high-rise proceeded safely.
2.Sewer Tunnel Protection – Singapore
Challenge: A deep sewer tunnel was planned below a congested urban district with aging buildings.
Solution: Compaction grouting was used pre-tunneling to strengthen and compact surrounding soils, preventing surface settlement and damage to adjacent structures.
Outcome: Surface settlements were reduced by over 70% compared to untreated sections. No structural damage was reported.
3.Roadway Void Stabilization – London, UK
Challenge: Sinkholes were appearing along a major arterial road due to collapsing old brick sewers.
Solution: A rapid-response compaction grouting program was implemented to fill voids and densify surrounding fill.
Outcome: Ground stability was restored, and road reopening was completed within 48 hours.
Advantages in Urban Applications
- Non-Disruptive: No vibration, noise, or excavation required.
- Precise Control: Real-time monitoring allows safe injection near sensitive structures.
- Versatile: Can be used under roads, buildings, tunnels, and utilities.
- Immediate Effectiveness: Grouting produces measurable results immediately upon curing.
Considerations and Limitations
- Not suitable for highly cohesive or saturated clayey soils.
- Requires careful planning to avoid heave or over-compaction.
- Access for equipment must be arranged in tight or restricted urban sites.
Conclusion
Compaction grouting offers a powerful, minimally invasive technique for addressing complex geotechnical issues in urban environments. From mitigating settlement to stabilizing aging infrastructure, it provides a controlled and cost-effective solution adaptable to many conditions. As cities continue to grow and densify, compaction grouting will remain an indispensable method in the toolkit of urban geotechnical engineers.
