Introduction to 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 the surrounding environment. 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):
- Slope excavation
- Soil nail wall (composite soil nail wall)
- Cement-soil gravity retaining wall
- SMW method (steel-reinforced cement mixing wall)
- Soldier pile wall
- Diaphragm wall
1. Slope Excavation
Key Features:
- Slope ratio should be determined based on soil type and excavation depth.
- For depths exceeding 4m, multi-level slopes with platforms are required.
- In stable soil conditions, natural slopes are preferred.
- In soft soil areas, reinforced mesh should be applied to the slope surface.
Construction Requirements:
- If excavation is below the groundwater level, dewatering wells must be installed.
- Drainage ditches or water barriers should prevent water infiltration.
- The distance between the slope toe and any local deep pits should be at least twice the pit depth.
2. Soil Nail Wall (Composite Soil Nail Wall)
Applications: Used when space constraints limit slope excavation.
Construction Details:
- Soil nails can be steel pipes or reinforced bars, with shotcrete facing.
- Drainage holes are necessary behind the wall if water retention is present.
- Prestressed anchors (when used) should be placed in upper or weak soil zones.
3. Cement-Soil Gravity Retaining Wall
Structure:
- Typically constructed with double or triple-axis mixing piles (150–200mm overlap).
- A concrete cap is recommended at the top.
Technical Considerations:
- For depths <16m, double-axis piles are cost-effective; beyond 16m, triple-axis is preferred.
- Cement content should be increased in soft soil layers.
- Piles should be arranged in a grid pattern for stability.
4. SMW Method (Steel-Reinforced Mixing Wall)
Features:
- Mixing piles serve as both support and waterproofing.
- Steel sections (e.g., H400×400, H500×200) are inserted and later extracted.
- Rental periods should not exceed 6 months for cost efficiency.
Installation Options:
- Alternate insertion (“skip-one” or “skip-two”) or full insertion for higher stiffness.
- Anti-friction coatings facilitate steel removal post-construction.
5. Soldier Pile Wall
Waterproofing:
- A cutoff curtain (e.g., double/triple-axis mixing piles) is required if groundwater is present.
Design Guidelines:
- For depths <16m, double-axis piles are sufficient; deeper or sandy soils require triple-axis.
- Curtain depth varies:
- 6–7m below base in cohesive soils (e.g., Shanghai).
- Full cutoff in permeable layers (e.g., sandy soils in Wuhan).
“Pile-Wall Integration” Technique:
- Reusing temporary piles as permanent structural walls reduces material waste and costs.
6. Diaphragm Wall (“Two-Wall-in-One” System)
Advantages: Ideal for deep basements (>3 floors) in soft soil regions.
Construction Process:
- Serves as both temporary support and permanent basement wall.
- Common thicknesses: 600–1200mm.
Key Steps:
- Guide walls are mandatory on both sides.
- Pre-treatment with mixing piles is needed in sandy/loose strata.
- Bentonite slurry ensures trench stability during excavation.
- Post-construction, jet grouting can seal panel joints.
Conclusion
Selecting the optimal foundation pit support method requires balancing technical, environmental, and economic factors. Proper design ensures safety, cost savings, and minimal environmental impact. For specialized applications like “pile-wall integration” or “two-wall-in-one,” consult geotechnical experts to maximize efficiency.
