Introduction
Retaining structures are critical in controlling earth pressures and supporting deep excavations, especially in urban or constrained sites. Contiguous piles and secant piles are two widely used pile wall systems. Both provide lateral support and prevent ground collapse, but their design, construction, cost, and performance differ significantly. Understanding these differences is essential for selecting the most suitable solution for a given project.
1. Overview of Pile Systems
(a) Contiguous Piles
- Definition: Closely spaced piles with small gaps (usually 75–150 mm) between them.
- Purpose: Forms a near-continuous wall that resists lateral earth pressures.
- Applications: Shallow basements, retaining walls, cut-and-cover tunnels.
(b) Secant Piles
- Definition: Overlapping piles formed by drilling primary (usually unreinforced) piles first and then secondary (reinforced) piles to interlock with the first.
- Purpose: Forms a water-tight, continuous wall suitable for deeper excavations or groundwater control.
- Applications: Deep basements, underground parking, high-rise building excavations, and cofferdams.
2. Construction Techniques
| Feature | Contiguous Piles | Secant Piles |
| Pile Overlap | Minimal gaps, usually unreinforced | Overlapping, secondary piles reinforced |
| Sequence | Piles installed side by side | Primary piles first, secondary piles drilled later to interlock |
| Water Tightness | Not watertight; may require additional lining | Water-tight due to interlocking design |
| Construction Complexity | Simple drilling and grouting | More complex; requires precise alignment and sequencing |
| Construction Time | Faster | Longer due to sequential drilling |
3. Structural Performance
| Aspect | Contiguous Piles | Secant Piles |
| Lateral Load Resistance | Moderate; suitable for shallow depths | High; suitable for deep excavations |
| Groundwater Control | Limited; may require dewatering | Excellent; can act as a cut-off wall |
| Settlement Control | Adequate for shallow sites | Better for sensitive structures in deep excavations |
| Bending and Shear Resistance | Lower; relies on close spacing | Higher; reinforced secondary piles resist higher stresses |
4. Economic Considerations
- Contiguous Piles:
- Lower material and construction costs
- Faster installation
- Suitable for projects with limited budget and shallow depth
- Secant Piles:
- Higher costs due to reinforced secondary piles and more complex installation
- Longer construction duration
- Cost justified in deep excavations or sites with high groundwater levels
5. Environmental and Site Constraints
- Contiguous Piles:
- Minimal vibration; suitable for urban areas
- May require additional waterproofing in saturated soils
- Secant Piles:
- Provides better groundwater control, reducing dewatering and associated environmental impact
- Suitable for restricted sites requiring watertight excavation support
6. Selection Guidelines
- Choose Contiguous Piles When:
- Excavation depth is moderate
- Groundwater is not critical
- Budget or construction time is limited
- Choose Secant Piles When:
- Excavation is deep
- Groundwater control is necessary
- High lateral loads or sensitive adjacent structures are present
Conclusion
Both contiguous and secant piles offer effective solutions for retaining structures, but their suitability depends on depth, groundwater conditions, load requirements, and budget. Contiguous piles provide a faster and cost-effective solution for shallow, dry sites, while secant piles offer superior structural performance and groundwater control for deep or complex excavations. Selecting the appropriate system ensures safety, durability, and efficiency in foundation and retaining wall projects.
