What is Foundation Pit Support Engineering?
Foundation pit support engineering refers to the structural measures taken during excavation to ensure slope stability, protect underground structures, and prevent damage to surrounding environments. The selection of support methods depends on factors such as excavation depth, site conditions, adjacent structures, geological and hydrological conditions, and project timelines.
Cost Comparison of Common Support Methods (From Low to High)
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Slope excavation
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Soil nailing wall (composite soil nailing)
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Cement-soil gravity retaining wall
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Steel-reinforced cement mixing wall (SMW method)
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Soldier pile wall
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Diaphragm wall
1. Slope Excavation
Key Considerations:
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The slope ratio should be determined based on soil properties and excavation depth. For depths exceeding 4m, multi-level slopes with platforms are required.
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In stable soil conditions, natural slopes are preferred.
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In soft soil areas, slope surfaces should be reinforced with steel mesh and shotcrete.
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Groundwater control: If excavation occurs below the water table, dewatering wells must be installed.
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Slope drainage: Install drainage ditches or water barriers to prevent erosion.
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Local deep pits: Ensure the distance between the slope toe and any deep pit is at least twice the pit depth; otherwise, slope stability must be re-evaluated.
2. Soil Nailing Wall (Composite Soil Nailing)
Ideal for sites with space constraints where steep slopes are impractical.
Key Features:
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Uses steel pipe or rebar nails with shotcrete facing.
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Drainage: Install weep holes behind walls in water-bearing layers to reduce hydrostatic pressure.
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Prestressed anchors: When used, place them in upper sections or areas with high soil pressure and weak strata.
3. Cement-Soil Gravity Retaining Wall
Construction Methods:
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Typically built using double or triple-axis deep mixing piles with 150–200mm overlaps. A concrete cap is recommended.
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Depth considerations: For depths <16m, double-axis piles are cost-effective; beyond 16m, triple-axis is preferred. Increase cement ratio in soft soils.
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Layout: Piles should form a grid pattern for enhanced stability.
4. Steel-Reinforced Cement Mixing Wall (SMW Method)
Best Practices:
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Uses overlapping double/triple-axis mixing piles for water cutoff.
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Steel beam selection: Common H-beams (e.g., H400×400, H500×200) are leased for ≤6 months. Arrangement options:
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Alternate insertion (every other pile)
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Double insertion (every two piles)
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Full insertion (max stiffness)
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Extraction: Apply anti-friction coatings to facilitate post-construction removal.
5. Soldier Pile Retaining Wall
Critical Requirements:
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Waterproofing: For excavations below groundwater, install cutoff curtains outside piles.
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Curtain selection:
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Double-axis mixing piles for depths <16m.
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Triple-axis for deeper excavations or dense sandy layers.
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Depth rules:
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In cohesive soils, extend curtains 6–7m below excavation.
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In permeable layers (sandy/silty soils), curtains should fully penetrate the aquifer.
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“Pile-Wall Integration”: Reuse temporary piles as permanent structural walls to reduce costs and improve space efficiency.
6. Diaphragm Wall (“Two-Wall Integration”)
Cost-effective for deep excavations (e.g., 3+ basement levels). The wall serves as both temporary support and permanent basement structure.
Construction Tips:
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Guide walls: Install reinforced concrete guide walls on both sides.
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Pre-reinforcement: Use cement mixing piles to stabilize sandy layers before trenching.
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Slurry walls: Prefer natural slurry; use artificial slurry only if necessary.
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Waterproofing: Enhance joints with jet grouting and install internal drainage systems.
Common Thicknesses: 600mm, 800mm, 1000mm, 1200mm.
Conclusion
Selecting the optimal foundation pit support method requires balancing technical feasibility, environmental impact, and cost. Early engagement with geotechnical experts ensures safety and efficiency throughout the project lifecycle.
