Deep excavations in soft or saturated soils pose significant challenges for geotechnical engineers, particularly in urban environments or water-bearing strata. Ensuring stability, groundwater control, and minimal surface disruption is critical in such projects. One solution that has proven both versatile and effective is the Secant Pile Wall system — a robust, continuous retaining wall constructed from interlocking drilled piles. This article explores the methods, advantages, and applications of secant piling in soft and saturated ground conditions.
Introduction to Secant Piling
Secant piling involves the construction of intersecting (or overlapping) concrete piles to form a continuous wall. The technique uses primary and secondary piles, where:
- Primary piles are typically unreinforced and constructed first.
- Secondary piles are reinforced and drilled partially through the primary piles, forming a structural and often water-tight wall.
The interlocking nature of these piles provides both structural strength and hydraulic cutoff, making secant walls ideal for deep excavations in low-strength or high-water-content soils.
Construction Methodology
Secant pile walls are built using rotary drilling rigs equipped with tools capable of boring through soil and previously placed concrete. The general process includes:
a) Site Preparation and Surveying
Precise layout is essential to maintain alignment and interlock between piles. Positioning tolerances are typically within ±25 mm.
b) Primary Pile Installation
Unreinforced concrete is placed in alternate pile locations. These serve as the “soft” component of the wall.
c) Secondary Pile Drilling
After sufficient curing (generally 24–48 hours), secondary piles are drilled to cut into the sides of adjacent primary piles (typically 100–150 mm overlap). These piles are reinforced with steel cages or beams and cast with high-strength concrete.
d) Excavation and Support
As excavation proceeds, temporary props, anchors, or struts are installed to resist earth pressure and maintain wall integrity.
Performance in Soft and Saturated Soils
Soft clays, silts, and saturated sand layers offer limited shear strength and may be prone to heaving, slumping, or groundwater inflow. Secant piles address these issues through:
a) Hydraulic Cutoff
The interlocking nature of the piles and low permeability concrete reduce groundwater flow into the excavation. This eliminates or reduces the need for dewatering systems.
b) Structural Rigidity
Reinforced secondary piles provide resistance to lateral loads, even in weak soil layers. The system can support deep excavations exceeding 30 meters when properly braced.
c) Minimal Ground Movement
Rotary bored piling generates low vibration and minimal displacement, reducing the risk of settlement or disturbance to nearby structures.
d) Flexibility in Design
Pile diameter (600–1200 mm), depth, and overlap can be customized for varying soil profiles and hydrogeological conditions.
Applications in Challenging Ground Conditions
Secant piling is particularly effective in projects involving:
- Deep basements and podium structures in soft urban ground.
- Subway stations and cut-and-cover tunnels below the water table.
- Riverfront development and quay walls with high hydraulic pressure.
- Shafts for deep drainage or utility infrastructure in wet, unstable soils.
- Retaining systems for urban excavation adjacent to sensitive buildings.
Advantages of Secant Piling
| Feature | Benefit |
| Water-tightness | Excellent seepage control in saturated soils |
| Soil versatility | Can be used in soft clays, silts, sands, and fill |
| Structural strength | Capable of resisting significant lateral pressure |
| Low vibration | Suitable near existing structures or utilities |
| Flexibility | Customizable layout, reinforcement, and pile depth |
| Space-efficient | Allows vertical excavation close to site boundaries |
Limitations and Considerations
While highly effective, secant piling has certain limitations:
a) Tolerances and Overlap
Precise drilling is required to ensure adequate overlap; misalignment can compromise structural and hydraulic integrity.
b) Construction Time
Compared to sheet piling or contiguous piles, secant walls take longer to construct due to overlapping sequences and curing times.
c) Cost
The system is more expensive than other shoring methods, especially with reinforced and large-diameter piles.
d) Material Variability
Differential setting and strength between primary and secondary piles must be managed to avoid cracking or seepage paths.
Secant vs. Alternative Systems
| System | Water Tightness | Depth Capacity | Vibration | Installation Speed | Cost |
| Secant Pile Wall | Excellent | Very High (>30m) | Low | Moderate | High |
| Contiguous Pile Wall | Poor | Moderate | Low | High | Medium |
| Diaphragm Wall | Excellent | Very High | Very Low | Slow | Very High |
| Sheet Pile Wall | Moderate | Limited (to ~15m) | High | Fast | Low |
Case Study: Deep Shaft in Saturated Clay
In a metro construction project, engineers encountered soft marine clay with a high groundwater table. Traditional shoring was ruled out due to risks of basal heave and seepage. A secant pile shaft was designed with:
- 1000 mm diameter piles,
- 150 mm overlap,
- Reinforced secondary piles every 1200 mm.
The secant wall maintained groundwater control during excavation to a depth of 28 meters without significant inflow, enabling successful completion of the shaft with no reported ground loss or settlement damage to adjacent structures.
Conclusion
Secant piling techniques provide a dependable solution for excavation support in soft and saturated soils. Their ability to combine lateral support with hydraulic cutoff makes them indispensable in deep urban excavations, coastal infrastructure, and underground transport systems. While the method involves higher precision and cost, the gains in safety, control, and performance far outweigh the challenges in demanding geotechnical conditions.
As infrastructure projects continue to grow in complexity and depth, secant pile walls will remain a key technology in overcoming the toughest excavation environments.
