What Is The Most Effective Method For Mitigating The Risk Of Soil Liquefaction During Seismic Activity In A Deep Excavation For A Large-diameter Tunnel In A Region With High Plasticity Index Soils And A High Groundwater Table, Considering The Need To Balance The Use Of Deep Soil Mixing And Jet Grouting Techniques With The Environmental Concerns Of Minimizing Groundwater Contamination And Preserving Adjacent Soil Structures?

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The optimal approach to mitigating soil liquefaction in a deep excavation for a large-diameter tunnel, considering high plasticity index soils and a high groundwater table, involves a balanced and hybrid strategy. Here's a structured plan:

  1. Hybrid Technique Combination:

    • Deep Soil Mixing (DSM): Use DSM for general soil improvement, especially in larger areas. This method enhances soil stability by mixing it with non-toxic binding agents like cement, reducing liquefaction risk without significant environmental impact.
    • Jet Grouting (JG): Apply JG in critical areas requiring precise reinforcement, such as around tunnel openings or where groundwater is active. This creates impermeable barriers, preventing water infiltration and enhancing structural support.

  2. Environmental Considerations:

    • Non-Toxic Materials: Ensure all binding agents and grouts used are environmentally friendly to prevent groundwater contamination.
    • Containment Measures: Implement casings and monitoring during JG to contain grout flow, avoiding leakage beyond target areas.

  3. Protection of Adjacent Structures:

    • Strategically apply JG near existing structures to create protective barriers, minimizing disturbance to surrounding soil. Use DSM in areas with more space to avoid affecting nearby structures.

  4. Design and Monitoring:

    • Hybrid Design: Integrate DSM for base and wall stability with JG for targeted reinforcement, ensuring a balanced approach.
    • Monitoring Systems: Install instruments to track groundwater levels, soil movement, and liquefaction signs. This allows early intervention if issues arise.

  5. Cost and Feasibility:

    • While JG may be more expensive, its precision justifies use in critical zones. DSM offers cost-effectiveness for larger areas, balancing overall project costs.

  6. Long-Term Stability:

    • Both methods provide durable results, but ongoing monitoring and maintenance are essential for sustained stability.

By combining these strategies, the approach effectively mitigates liquefaction risk while addressing environmental and structural concerns.