Introduction
Loose and compressible soils often exhibit low bearing capacity and excessive settlement, posing significant challenges to foundation performance. Compaction grouting is a ground improvement technique widely adopted to densify such soils and enhance their engineering properties. Evaluating the performance of compaction grouting is essential to ensure that the desired improvements in strength, stiffness, and settlement control are achieved.
Mechanism of Compaction Grouting in Loose and Compressible Soils
Compaction grouting involves the injection of a stiff, low-slump grout into the ground under controlled pressure. Instead of permeating soil pores, the grout forms bulbs or lenses that:
- Displace surrounding soil
- Rearrange soil particles
- Reduce void ratio
- Increase soil density
In loose and compressible soils, this mechanism leads to significant improvement in soil behavior.
Performance Indicators for Evaluation
1. Reduction in Settlement
The primary performance criterion is settlement control. Post-grouting measurements often show:
- Immediate reduction in ongoing settlement
- Improved resistance to future load-induced deformation
Settlement markers and precise leveling are commonly used for evaluation.
2. Improvement in Bearing Capacity
Densification increases effective stress and shear strength, resulting in:
- Higher allowable bearing pressure
- Reduced foundation deformation
Plate load tests and load tests on foundations help quantify these improvements.
3. Increase in Soil Stiffness
Compaction grouting enhances stiffness, reflected by:
- Increased modulus of elasticity
- Reduced compressibility
Pressuremeter and dilatometer tests are effective tools for assessing stiffness improvement.
4. Changes in In-Situ Test Results
Performance is often evaluated using:
- Standard Penetration Test (SPT): Increased N-values
- Cone Penetration Test (CPT): Higher cone resistance and sleeve friction
These tests provide direct evidence of soil densification.
Performance in Different Soil Types
Loose Sands and Gravels
- High effectiveness due to particle rearrangement
- Significant increase in density and strength
- Excellent settlement control
Loose Fills
- Effective in heterogeneous materials
- Requires careful pressure control to avoid surface heave
Compressible Soils
- Moderate improvement depending on soil composition
- Performance reduces with increasing clay content
Monitoring and Quality Control Measures
Effective performance evaluation relies on:
- Continuous monitoring of grout pressure and volume
- Observation of surface movements and heave
- Instrumentation such as settlement gauges and inclinometers
Real-time feedback allows adjustments during grouting operations.
Limitations in Performance Evaluation
- Improvement zones are difficult to predict precisely
- Risk of excessive heave if injection pressure is high
- Limited effectiveness in soft cohesive soils
These factors must be considered while interpreting performance results.
Case Performance Outcomes
Field studies commonly report:
- Settlement reduction of 50–80%
- Increase in SPT N-values by 2–3 times
- Improved structural performance without excavation
Conclusion
Performance evaluation of compaction grouting in loose and compressible soils demonstrates its effectiveness in settlement control, strength enhancement, and foundation stabilization. Proper selection of soil conditions, controlled execution, and comprehensive monitoring are essential to achieve reliable and measurable improvements in ground performance.
