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Anti-floating Measures for Metro Underground Stations: A Case Study on Large-Diameter Manual Excavation Piles

Introduction

With the rapid development of metro construction, underground stations are evolving to serve multiple functions. These stations often require large underground spaces, which may result in insufficient soil cover above the roof slab, leading to buoyancy issues. This article examines the anti-floating measures for a metro transfer station designed as a “cross-shaped” interchange. Due to functional requirements, the soil cover above the bus interchange section is only 0.7m–1.0m, while the transfer section consists of three underground levels. Anti-floating calculations confirmed the need for additional measures, leading to the adoption of large-diameter manual excavation piles as anti-floating piles.

This paper explores various anti-floating solutions and details the design considerations for anti-floating piles, focusing on a real-world case study.

1. Project Overview

The metro station is located beneath Shennan Avenue, serving as a temporary terminus for Line 1 and intersecting with the planned Line 2 in a cross-shaped layout. Key features include:

  • Location: Adjacent to a famous tourist attraction in the south, surrounded by high-rise buildings, with residential and commercial areas to the north.

  • Structure:

    • Total length: 334m

    • Width: 48.0m (bus interchange section), 45.0m (metro section)

    • Two underground levels with an inverted “T” cross-section; three levels at the transfer node

    • Construction method: Cast-in-situ reinforced concrete frame structure

  • Retaining Systems:

    • South side: Manual excavation piles (due to geological and traffic conditions)

    • North side: Soil nailing wall

    • Lower levels: Manual excavation piles

2. Geological Conditions

The site features a complex geological profile with multiple layers:

  1. Artificial Fill Layer

  2. Slope Alluvial Layer

  3. Marine Alluvial Layer

  4. Quaternary Residual Layer (critical for pile design)

    • Sandy Clay: Hard to soft, thickness 0m–21.8m

    • Gravelly Clay: Predominant layer, stable distribution, thickness 0m–21.0m

  5. Yanshanian Granite

The frictional resistance of the pile-soil interface (Quaternary residual layer) is summarized in Table 1, with adjustments based on pile type and soil properties.

3. Comparison of Anti-Floating Measures

Two primary anti-floating strategies were evaluated:

  1. Bottom Reinforcement: Anti-floating piles, anchor cables, or grouted bolts.

  2. Drainage Systems: Filter layers and blind drains beneath the structural slab.

A comparative analysis (Table 2) concluded that anti-floating piles were optimal for this station.

4. Anti-Floating Pile Design Process

4.1 Buoyancy Verification

Calculations for the bus interchange section (8.1m column spacing) yielded:

  • Total Anti-Floating Force: 47,388.1 kN (including soil cover, structural weight, and friction)

  • Buoyant Force: 44,724.0 kN

  • Safety Factor: 1.06 (<1.15 required) → Anti-floating piles needed

4.2 Pile Design

  • Single Pile Specifications:

    • Diameter: 1.4m

    • Length: 15.5m (including 3.2m enlarged base)

    • Base diameter: 3.0m

    • Self-weight: 688.5 kN

  • Frictional Resistance:

    • Calculated per *JGJ 94-94* (l/d = 11.1 > 5)

    • Ultimate uplift capacity: 1,715.2 kN

  • Quantity: 2 piles per column

4.3 Construction Measures

  • Ventilation and safety protocols for deep excavation.

  • Dewatering for subsequent piles (extended length by 2.5m, no enlarged base).

5. Key Design and Construction Considerations

5.1 Design Notes

  1. Water Table: Use the historical highest level for buoyancy calculations.

  2. Load Calculation: Include only permanent loads; deduct openings in soil cover.

  3. Friction Adjustments: Account for pile type (e.g., drilled shafts require lower friction values).

  4. Pile Geometry: L/d > 5 alters failure mechanics.

  5. Group Effects: Avoid clustering to maximize individual pile efficiency.

  6. Crack Control: Ensure reinforcement meets *GB 50010-2002* crack width limits.

5.2 Construction Notes

  1. Safety: Prioritize ventilation and dewatering in deep excavations.

  2. Soil Challenges: Mitigate risks like sand inflow or unstable strata.

  3. Blasting: For rock sockets, control intensity to prevent collapse.

  4. Timely Concreting: Prevent water infiltration post-excavation.

  5. Structural Integrity: Secure connections between piles and base slabs.

Conclusion

This case study demonstrates the efficacy of large-diameter manual excavation piles in addressing buoyancy challenges in metro stations. By integrating geological data, structural requirements, and construction safety, the design ensures long-term stability while adhering to industry standards. Future research should explore (rock-socketed anti-floating piles) to expand technical options.

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