Everything You Need to Know About Secant Pile Walls

What are Secant Pile Walls?

Secant pile walls are bored pile retaining walls developed by connecting primary and secondary piles, creating a continuous wall. Unlike diaphragm retaining walls, they offer flexibility in design and shape, providing a stronger advantage over sheet-piled walls.

These walls consist of overlapping piles to form structural or cut-off walls, reinforced using steel bars or beams. They are constructed to resist groundwater utilising the CFA or cased CFA technology.

Technical Specifications

Temporary or permanent earth-retaining structure.
Ideal for short-term retaining of water or groundwater ingress.
Applicable to widely ranged soil conditions because of rotary drilling practices.
Inappropriate for hard clay as well as hard rocks.
Build with a slight overlap, ensuring proper depth or reaching the impermeable clay layer as per site needs.
Designed to withstand lateral bending forces, typically connected at the top with a capping beam to guarantee they function as a single unit.
The introduction of pile cage reinforcement restricts the excavation depth.
Strutting systems and retaining walls are needed to withstand lateral earth pressure.
Capable of supporting axial loading

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Distinctive Types of Secant Pile Walls

Secant walls have diverse compositions for the overlapping piles, providing a wide range of blends, including:

Hard/soft: The primary piles in hard/soft compositions are made of a “soft” cement-bentonite mix with a strength of 1N per mm2 and may not offer overall water-tightness.

Hard/firm: The primary piles for hard/firm composition use weak-mix or full-strength concrete with a strength of 10 N per mm2. A reinforced concrete lining is needed for permanent works on the secondary pile wall.

Hard/hard: The hard/hard composition used primary piles with higher-strength concrete, offering an alternative to diaphragm wall construction.

Advantages of Secant Pile Walls

Flexibility

Because of their flexibility, Secant walls can adapt to various soil types and ground settings, making them a practical choice for multiple construction projects. They can be customised to meet your requirements, no matter the soil type.

Space Efficiency

They do not require a separate construction space, making them the perfect choice in urban surroundings. Because they do not require plenty of space for installation, these walls can be constructed in restricted areas without changing the structural integrity.

Enhanced Water-Tightness

The overlapping design of piles effectively blocks water intrusion, which can be reliable for projects where inhibiting groundwater leakage is critical, such as excavations, waterfront developments or buildings near bodies of water.

Longevity & Durability

Secant pile walls are designed to be enduring and sturdy, providing structural support for ages. They are ideal for residential and commercial projects, such as high-rise structures and infrastructure developments, where lasting stability is paramount.

Compatibility with Deep Foundations

These walls are compatible with deep foundation systems, which adds to their adaptability and suitability for numerous construction set-ups and provides complete support for tall structures.

Disadvantages of Secant Pile Walls

There are different disadvantages to using secant pile walls, including:

Achieving verticality tolerances for deep piles is challenging
Obtaining total waterproofing in joints is very difficult
More expensive when compared with sheet pile walls

Application of Secant Pile Walls

Secant pile walls are adaptable and operational and can be used in a comprehensive range of applications. They are preferred because they are strong, watertight, and hassle-free to set up.

Slope stabilisation

These walls can stabilise slopes susceptible to erosion or landslides. The interconnected piles of a strong, continuous wall can withstand gravitational and water forces.

Excavation support

They are excellent for providing support to excavations, including building tunnels, basements, and bridges. They also offer adjacent support to the soil to prevent collapse.

Bulkhead support

Secant pile walls support bulkheads, which create a watertight barrier and prevent water from entering areas like tunnels and basements.

Water cut off

These walls can create a continuous, watertight barrier to prevent a water flow.

Different Steps for Secant Pile Walls Construction

To create overlapping (primary and secondary) piles and get a preferred water tightness. There are some steps for the construction process of the secant pile walls.

Excavation & Site Preparation
Primary Pile Installation
Secondary Pile Installation
Anchoring or Strutting (Optional)
Pile Spacing and Overlap

Conclusion

Secant pile walls offer wide-ranging benefits for urban environments with tight spaces. This structural system maximises the site area, reduces noise for the community, and can be used for various ground conditions. Effective implementation of secant piles depends on close coordination and collaboration among the engineering, architectural, and construction teams. Their interconnecting nature creates a stable barrier against lateral pressures, making them perfect for construction projects that require deep excavations or retaining structures.