Introduction
Jet grouting is a versatile ground improvement technique that involves eroding and mixing in-situ soil with high-pressure grout to form cemented columns or panels. Its adaptability makes it suitable for a wide range of applications, including foundation support, excavation stability, and groundwater control. However, the performance of jet grouting is strongly influenced by soil type and groundwater conditions. Understanding these factors is crucial for evaluating effectiveness, durability, and long-term performance.
Jet Grouting Mechanism
- Process: High-pressure grout jets (typically 300–600 bar) break down soil structure, mix it with grout, and form soilcrete with improved strength and reduced permeability.
- Performance Factors:
- Soil erodibility and permeability
- Grout mix and injection parameters (pressure, flow rate, lift rate)
- Groundwater flow and pressure
Performance in Different Soil Conditions
1. Cohesionless Soils (Sands and Gravels)
- Performance: Excellent, as sands and gravels are easily eroded and mixed.
- Advantages: Large column diameters achievable, uniform soilcrete formation.
- Challenges: In coarse gravels, grout consumption increases; requires higher energy.
- Applications: Liquefaction mitigation, seepage control in permeable strata.
2. Silty Soils
- Performance: Moderate to good, depending on fines content.
- Advantages: Silts can be treated effectively if jet energy is sufficient.
- Challenges: Fine-grained silts reduce erodibility, risking irregular columns.
- Applications: Groundwater cutoff walls, underpinning of foundations.
3. Cohesive Soils (Clays)
- Performance: Variable; stiff clays resist erosion, leading to smaller or irregular columns.
- Advantages: Soilcrete strength can be high if mixing is achieved.
- Challenges: Jetting efficiency is reduced; may require triple-jet systems.
- Applications: Excavation support and stabilization where permeability control is critical.
4. Mixed or Heterogeneous Soils
- Performance: Highly variable due to alternating layers of sand, silt, and clay.
- Advantages: Jet grouting is one of the few methods applicable in such conditions.
- Challenges: Non-uniform column diameters; requires careful design and monitoring.
- Applications: Urban tunneling, deep foundation support in complex stratigraphy.
Influence of Groundwater Conditions
1. High Groundwater Table
- Impact: Increased grout washout and dilution risks.
- Performance: Soilcrete strength may decrease; permeability reduction is less effective if grout is diluted.
- Mitigation: Use of anti-washout additives, higher cement content, or staged grouting.
2. Flowing Groundwater
- Impact: Can erode fresh grout, leading to incomplete or weak columns.
- Performance: Difficult to achieve hydraulic cutoff in high-velocity flows.
- Mitigation: Pre-grouting, temporary dewatering, or using rapid-setting grout.
3. Low or Absent Groundwater
- Impact: Favorable for grout retention and soilcrete formation.
- Performance: Higher strength and more uniform columns.
- Applications: Structural support and underpinning where strength is critical.
Performance Indicators
1. Strength
-
- Soilcrete unconfined compressive strength (UCS) typically ranges 1–5 MPa in sands, up to 10 MPa in clays.
2. Permeability Reduction
-
- Permeability can be reduced by 2–4 orders of magnitude, depending on soil type and grout composition.
3. Column Geometry
-
- Achievable diameters:
- 0.6–1.2 m in clays
- 1.0–3.0 m in sands and gravels
- More irregular in mixed soils.
- Achievable diameters:
4. Durability
-
- Long-term durability depends on grout composition and exposure to aggressive groundwater chemistry (e.g., sulfates, chlorides).
Case Examples
- Metro Excavation in Sandy Soil (with groundwater table near surface): Jet grouting successfully formed bottom plugs and cutoff walls, enabling dry excavation.
- Historic Building Underpinning in Clayey Soil: Soilcrete columns provided sufficient strength, though smaller diameters were achieved compared to design.
- Seepage Control in Gravelly Riverbed: Triple-jet grouting reduced permeability, but grout consumption was higher due to coarse material and flowing groundwater.
Conclusion
The performance of jet grouting is highly dependent on soil erodibility and groundwater conditions.
- Best performance: sands, gravels, and low groundwater conditions → large, strong, uniform soilcrete columns.
- Moderate performance: silts and clays → smaller, irregular columns requiring advanced jetting systems.
- Challenging conditions: flowing groundwater and heterogeneous soils → require additives, staged grouting, or hybrid improvement techniques.
Overall, with proper design, material selection, and monitoring, jet grouting remains one of the most adaptable and reliable soil improvement techniques across varied geotechnical environments.
