Introduction
Secant pile walls are a type of continuous retaining wall system constructed by overlapping bored piles. They are widely used in deep basements, underground stations, cut-and-cover tunnels, and slope stabilization projects, particularly in urban environments where groundwater control and minimal ground movement are essential. By forming an interlocking barrier, secant pile walls provide both structural support and groundwater cutoff, making them an effective alternative to diaphragm and contiguous pile walls.
This article outlines the key design principles for secant pile walls in foundation support.
1. Structural Configuration of Secant Pile Walls
a) Primary and Secondary Piles
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- Primary piles (soft piles): Installed first, often with lower-strength concrete.
- Secondary piles (hard piles): Installed later, cutting into primary piles to form overlap.
b) Types of Secant Pile Walls
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- Hard–Soft Secant Walls: Secondary piles are reinforced with stronger concrete, while primary piles use weaker concrete.
- Hard–Firm Secant Walls: Both primary and secondary piles are reinforced but with different strengths.
- Hard–Hard Secant Walls: Both piles are reinforced with structural concrete, suitable for high load and severe water control.
2. Design Considerations
a) Pile Diameter and Spacing
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- Diameter typically ranges from 600 mm to 1200 mm depending on depth and loads.
- Overlap of 100–150 mm ensures continuity and groundwater cutoff.
b) Wall Thickness
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- Depends on excavation depth and lateral pressures.
- Greater thickness provides higher stiffness and reduced deflection.
c) Embedment Depth
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- Must extend into a stable stratum for effective toe restraint.
- Minimum depth: 0.5–1.0 times excavation height, depending on soil conditions.
d) Waterproofing
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- Hard–hard walls provide superior groundwater control.
- Additional sealing measures (grouting, membranes) may be required in aggressive soils.
e) Reinforcement Design
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- Reinforcement cages placed in secondary piles for bending and shear resistance.
- Design governed by Eurocode 7, ACI, or local geotechnical codes.
f) Structural Analysis
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- Wall analyzed as a flexible retaining system using earth pressure theories or finite element models.
- Consideration of construction stages, excavation sequence, and load redistribution.
3. Load Resistance
a) Lateral Earth Pressure
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- Designed to resist active and passive earth pressures.
- Earth pressure coefficients (Ka, Kp) depend on soil type and excavation geometry.
b) Groundwater Pressure
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- Hydrostatic and seepage pressures act on the wall.
- Requires conservative safety factors for seepage-prone soils.
c) Structural Loads
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- Overturning and sliding from building foundations, traffic loads, or adjacent structures.
- Anchors, struts, or slabs often supplement lateral resistance.
4. Construction Tolerances
- Verticality tolerance: typically 1 in 200 to ensure proper pile overlap.
- Concrete placement: must be continuous to avoid voids or cold joints.
- Overlap accuracy is critical—misalignment can create seepage paths.
5. Advantages of Secant Pile Walls
- Superior groundwater control compared to contiguous pile walls.
- Higher stiffness, reducing excavation-induced movements.
- Can serve as permanent structural basement walls, saving construction costs.
- Suitable for deep excavations in dense urban areas.
6. Limitations
- Requires precise drilling and alignment to achieve proper overlap.
- Construction costs are generally higher than contiguous piles.
- Installation in very hard rock can be challenging.
Conclusion
Secant pile walls are a robust and versatile solution for foundation support in deep excavations and groundwater-prone sites. Their design requires careful consideration of pile configuration, embedment depth, structural loads, and groundwater conditions. When executed with precision and supported by robust monitoring, secant pile walls provide long-term stability, reduced ground movements, and effective integration with permanent structural systems.
