Introduction
In modern geotechnical and foundation engineering, piled retaining walls are essential for supporting deep excavations, basements, and underground infrastructure in urban and challenging environments. Two of the most widely used systems are Secant Pile Walls and Contiguous Pile Walls. While both serve as vertical excavation support, they differ in construction method, cost, performance, and suitability for groundwater control.
This article provides a comparative study of secant piles and contiguous piles, highlighting their advantages, limitations, and applications in retaining structures.
1. Construction Method
- Secant Pile Walls
- Built by installing primary (soft) piles followed by secondary (hard/reinforced) piles that cut into the primary piles, creating overlap.
- Overlap ensures a nearly continuous wall, with minimal gaps.
- Contiguous Pile Walls
- Constructed by drilling piles at a certain spacing, leaving small gaps (typically 100–200 mm).
- Not fully watertight—gaps may allow seepage unless grouted or sealed.
2. Groundwater Control
- Secant Piles: Provide better groundwater cutoff, especially in hard–hard configurations. Suitable for high water table conditions.
- Contiguous Piles: Allow groundwater seepage unless treated. Suitable where water inflow is minimal or controlled by grouting.
3. Structural Strength and Stiffness
- Secant Piles: Higher stiffness due to continuous overlap and reinforcement, making them suitable for deep excavations and heavy surcharge loads.
- Contiguous Piles: Lower stiffness compared to secant piles; best suited for moderate-depth excavations with lower lateral loads.
4. Material and Cost Efficiency
- Secant Piles: Require more concrete, reinforcement, and precise drilling—leading to higher construction costs.
- Contiguous Piles: More cost-effective, using less material and simpler construction methods.
5. Construction Tolerances and Accuracy
- Secant Piles: Require high drilling accuracy (verticality tolerance ~1 in 200). Misalignment can cause gaps, defeating the purpose of overlap.
- Contiguous Piles: Less demanding on alignment, easier to construct in restricted sites.
6. Environmental Considerations
- Secant Piles:
- Less seepage, but higher carbon footprint due to material use.
- Generates more spoil from overlapping pile drilling.
- Contiguous Piles:
- Uses less material, lower carbon footprint.
- Allows natural groundwater flow, reducing impact on hydrogeological balance.
7. Applications
- Secant Piles
- Deep basements in high groundwater conditions.
- Metro stations, tunnels, and water-sensitive sites.
- Projects requiring low ground movement and high stiffness walls.
- Contiguous Piles
- Basements in dry or low-permeability soils.
- Temporary shoring where seepage is manageable.
- Cost-sensitive projects with moderate depth requirements.
Comparative Summary Table
| Parameter | Secant Pile Walls | Contiguous Pile Walls |
| Construction | Overlapping piles (primary + secondary) | Spaced piles with small gaps |
| Water Control | Excellent (hard–hard system) | Limited; seepage possible |
| Stiffness & Strength | High; suitable for deep excavations | Moderate; best for shallow to medium depths |
| Cost | Higher (more material, precision drilling) | Lower (economical option) |
| Accuracy Required | Very high | Moderate |
| Environmental Impact | Higher CO₂ footprint due to more concrete/steel | Lower material use; eco-friendlier |
| Best Applications | Deep basements, metro stations, tunnels | Urban basements, temporary works, low groundwater areas |
Conclusion
Both secant pile walls and contiguous pile walls play vital roles in geotechnical engineering. Secant piles are best suited for deep, water-bearing, and high-load conditions, where strength and groundwater cutoff are critical. Contiguous piles, on the other hand, offer a cost-effective and efficient solution for moderate-depth excavations in drier soils. The choice between the two depends on soil conditions, groundwater regime, project depth, and budget constraints.
