Mechanism of Geogrid in Reducing Settlement
Stress Diffusion Effect
The tensile stiffness of the geogrid distributes the load over a wider area, reducing soil stress (by 10%–30%) and thus minimizing compressive deformation.
Measured data: In soft soil subgrades, laying geogrids can reduce the stress concentration factor at the base from 1.5 to 1.2–1.3.
Lateral Constraint Effect
The frictional resistance between the geogrid and the fill material restricts lateral soil displacement, forcing vertical loads to be transferred more effectively to deeper layers.
Experiments show that reinforced subgrades experience a 40%–60% reduction in lateral displacement, indirectly reducing settlement.
Uniformly Distributed Differential Settlement
The flexible tension characteristics of the geogrid can coordinate uneven settlement, preventing cracks caused by abrupt settlement differences.
Settlement Control Effect under Different Working Conditions
Soft Soil Foundation (Silt, Silt-like Soil)
| Treatment Solution | Post-construction Settlement (Without Geogrid) | Settlement After Adding Geogrid | Reduction Ratio |
|---|---|---|---|
| Direct Filling (No Treatment) | 30~50cm | 20~35cm | 25%~40% |
| Replace with 2m Crushed Stone Soil | 15~25cm | 8~15cm | 40%~50% |
| Combined with Drainage Board Preloading | 10~20cm | 5~12cm | 30%~50% |
General Embankment Subgrade (Good Soil Quality)
Settlement can be reduced from 8-12cm to 5-8cm (a reduction of approximately 30%), mainly by controlling differential settlement.
Key Influencing Factors
Grating Performance
- High-strength grating (such as PET/glass fiber, tensile strength ≥80kN/m) improves settlement reduction by more than 20% compared to ordinary PP grating (20-30kN/m).
- Multi-layer laying (2-3 layers) improves settlement reduction efficiency by 15%-25% compared to single-layer laying.
Laying Location
- Optimal Location: Bottom of the subgrade (0.5-1.0m from the top surface of the subgrade) and at the cut-fill junction.
- Effect Comparison: A single layer of grating at the bottom can reduce settlement by 30%; if an additional layer is added on top of the subgrade, the total settlement reduction can reach 45%.
Foundation Conditions
The effect is more significant on highly compressible soils (such as silt), while the impact on gravel foundations is smaller (settlement reduction of approximately 10%).
Engineering Application Recommendations
Selection and Design
- For soft soil areas, use bidirectional high-strength geogrid (TGSG30-30 or equivalent), with longitudinal and transverse tensile strength ≥30kN/m.
- Number of Layers: Deep soft foundations (such as 8~10m silt): 2~3 layers (1 layer each at the bottom, middle, and top of the subgrade); General fill: 1 layer (bottom of the subgrade).
Construction Control
- Tension: Pre-tension 2%~3% strain during laying to avoid loosening.
- Overlap Length: Longitudinal ≥50cm, transverse full-width continuous laying.
- Fill Particle Size: ≤15cm to avoid puncturing the geogrid.
Combined Process
- Geogrid + Drainage Board: Accelerates consolidation, settlement reduction can reach 50%~60%.
- Geogrid + Micropiles: Used in sensitive areas (such as bridge approach transition sections), effectively eliminating differential settlement.
Case Studies
Case 1: Soft soil section of a highway (6m thick silt)
- Measures: Replace with 2m of crushed stone soil + 2 layers of TGSG40-40 geogrid.
- Results: Post-construction settlement decreased from the predicted 35cm to 18cm, with a cumulative settlement of 21cm over 3 years of operation (40cm in the unreinforced section).
Case 2: Municipal road expansion (miscellaneous fill subgrade)
- Measures: Single-layer geogrid (20kN/m) laid at the bottom of the subgrade.
- Results: Differential settlement decreased from 10cm/m to 3cm/m, preventing reflective cracking of the asphalt layer.
Limitations
1. Cannot completely eliminate settlement: Especially for deep silt, geogrids mainly slow down the settlement rate and need to be combined with preloading or pile foundations.
2. Long-term creep: Plastic geogrids may experience 3% to 5% creep deformation under long-term loads, requiring the selection of creep-resistant materials (such as fiberglass).
Conclusion
Geogrids can effectively reduce subgrade settlement (30% to 50%), especially in soft soil foundations where they offer excellent cost-effectiveness. However, it is important to note that: deep soft soil foundations require combination with other construction methods (drainage consolidation, pile foundations); strict control of construction quality is essential to prevent geogrid damage or displacement.
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