Introduction
In modern urban environments, deep excavations and underground structures such as basements, tunnels, metro stations, and parking facilities require retaining systems that are both structurally reliable and environmentally adaptive. Secant pile walls, constructed by overlapping primary and secondary bored piles, offer a versatile solution by combining structural strength, excavation support, and groundwater control. Their integration with underground works ensures excavation stability, minimizes ground movements, and protects adjacent structures.
1. Role of Secant Piles in Deep Excavations
- Earth Retention: Secant pile walls act as rigid barriers resisting soil pressures and external surcharge loads during excavation.
- Groundwater Cutoff: Hard–hard pile configurations provide an effective seepage barrier, essential in high groundwater table environments.
- Minimized Settlement: Their stiffness reduces ground deformation, protecting nearby buildings, utilities, and transport infrastructure.
2. Structural Integration with Underground Facilities
2.1 Basements and Parking Structures
- Secant piles can serve as permanent basement walls, reducing the need for additional retaining walls.
- They integrate seamlessly with basement slabs, transferring axial and lateral forces into the foundation system.
- Waterproofing membranes or shotcrete linings are often applied to improve durability and watertightness.
2.2 Metro Stations and Tunnels
- In congested cities, secant piles form diaphragm-like cutoffs for metro boxes or cut-and-cover tunnels.
- They act as both temporary excavation support and permanent external walls.
- Tieback anchors or struts are integrated into the pile wall to resist high lateral pressures during staged excavation.
2.3 Retaining and Underground Utilities
- Secant walls provide stability for deep shafts, underground utility corridors, and pumping stations.
- Their adaptability to irregular alignments makes them suitable for confined urban geometries.
3. Construction and Design Considerations
- Pile Alignment & Overlap: High drilling accuracy is crucial to maintain overlap and ensure watertightness.
- Embedment Depth: Piles must extend into competent strata to provide lateral fixity and prevent toe heave.
- Reinforcement Detailing: Secondary (hard) piles are reinforced to withstand bending and shear under excavation loads.
- Load-sharing: Integration with slabs, struts, or anchors distributes stresses, enhancing wall performance.
- Seepage Control: For water-bearing soils, additional measures such as jet grouting or waterproof membranes may be integrated.
4. Monitoring and Performance Evaluation
During excavation, continuous monitoring ensures safety and validates design assumptions:
- Inclinometers to measure wall deflection.
- Strain gauges to assess bending moments in piles.
- Piezometers to track groundwater pressure changes.
- Settlement markers to monitor movements of nearby structures.
Integration of monitoring with staged excavation helps maintain excavation stability and minimize risks.
5. Advantages of Integration
- Dual function: temporary excavation support and permanent retaining wall.
- Reduced construction footprint in dense urban areas.
- Improved cost-efficiency by eliminating separate retaining structures.
- Enhanced sustainability, with minimized excavation spoil compared to diaphragm walls.
6. Limitations and Challenges
- Requires precise equipment for drilling and alignment.
- Higher material consumption compared to contiguous pile walls.
- Not always fully watertight without supplementary measures.
- Quality control of concrete and overlap joints is critical.
Conclusion
The integration of secant piles with deep excavations and underground structures offers a robust, versatile, and space-efficient solution in geotechnical engineering. By combining excavation support, groundwater control, and structural functionality, secant pile walls address the complex demands of urban infrastructure projects. Their successful implementation, however, depends on careful design, precise construction, and comprehensive monitoring to ensure both short-term excavation stability and long-term durability of underground structures.
