Secant/Tangent Piles: Construction, Use, Advantages and Disadvantages
Secant pile walls are formed by constructing intersecting reinforced concrete piles. Secant pile walls are reinforced with either steel rebar or with steel beams and are constructed by drilling under mud. Primary secant piles are installed first with secondary reinforced secant piles constructed in between primary piles once the latter gain sufficient strength. Pile overlap is typically in the order of 3 inches (8 cm). In a tangent pile wall, there is no pile overlap as the piles are constructed flush to each other. The main advantages of secant pile walls are:
1. Increased construction alignment flexibility.
2. Increased secant pile wall stiffness compared to sheet piles.
3. Can be installed in difficult ground (cobbles/boulders).
4. Less noisy construction.
5. No need to install timber lagging.
The main disadvantages of secant pile walls are:
1. Verticality tolerances may be hard to achieve for deep piles.
2. Total waterproofing is very difficult to obtain in joints.
3. Increased cost compared to sheet pile walls.
Above: Secant pile wall types, all included in our DeepEX deep excavation software
Secant pile wall section drawing in DeepEX
DeepEX can Design any Secant or Tangent Pile Wall Model in Minutes! Review Software Capabilities
Selecting the appropriate secant pile wall depends mostly on local practice, material, and installation costs. In the US, secant piles are commonly reinforced with steel I or W beams. In Europe and South America, where labor costs are smaller, reinforced concrete secant pile walls are commonly used. Occasionally, some designs opt to have two or three unreinforced secant piles in between the reinforced piles.
Above: Secant pile wall excavation with struts for the Dodulu Subway Station in Instabul, Turkey, designed with our DeepEX software.
Hard-Soft Secant Pile Considerations
Very often unreinforced secant piles are constructed with weaker concrete to allow for easier drilling of the reinforced secant piles. Other times, two or three unreinforced secant piles are utilized to achieve greater economy. In all cases though, a secant pile wall designer needs to consider compression arching stresses in the concrete as well as shear at the intersections. For deeper secant piles, installation tolerances might have to be also taken into consideration.
In circular secant pile shafts, it might be possible to only have unreinforced piles if a compression ring action is ensured throughout the secant pile wall perimeter.
Tangent pile walls, in contrast to secant pile walls do not interesect. As a result, all tangent piles need to be reinforced. With tangent pile walls, it is common to create a pile cap so that a more homogenous behavior can be achieved. Advantages:
1. Increased construction alignment flexibility.
2. Increased wall stiffness compared to sheet piles.
3. Can be installed in difficult ground (cobbles/boulders).
4. Less noisy construction.
5. No need to install timber lagging.
6. No need to create an overlap as required with secant piles.
The main disadvantages of tangent pile walls are:
1. Verticality tolerances may be hard to achieve for deep piles.
2. Walls are not waterproof
3. Tangent piles cannot share load with adjacent pile
4. Soil must be able to span in between any small openings between the piles. As a result tangent piles are not recommended for cohesionless sand soils.
Training Videos: An Anchored Secant Pile Wall Design and the available Secant and Tangent Pile Sections in DeepEX:
