What are Secant Pile Walls?
Secant piles are constructed so that there is an intersection of one pile with another. The usual practice is to construct alternative piles along the line of the wall leaving a clear space of a little under the diameter of the required intermediate piles. The exact spacing is determined by the construction tolerances which can be achieved. These initially placed piles do not have to be constructed to the same depth as the intermediate piles which follow, depending on the way in which the wall has been designed and reinforced.
Concrete is added and before it has fully set the intermediate holes are drilled along a parallel, but slightly offset, line so that the holes cut into the first piles. The intermediate piles are placed through a heavy casing whose cutting edge is toothed and enables the casing to cut into the concrete of the initial piles on either side. Subsequent concreting results in a continuous wall. The concrete chosen will often have a slow rate of setting in order to ease the problem of cutting one pile into another.
What Are Applications For Secant Wall Constructions?
Secant walls are utilized to transfer lateral, vertical or a combination of both loads through weak soil layers to a suitable bearing layer. They can be drilled in all types of configurations that can form a interlocking watertight wall, or provide for a structural overlapping wall. Secant walls are mainly used for foundation load transfer and earth retention systems. They are used in land-based structures, top-down tunnel, cofferdams and subway structures where the surface soil layers cannot provide the required load-bearing capacity. They are also used in marine-based structures and dams.
Secant piles are one of the most economical methods of creating an effective water control barriers for building structural walls, top-down tunnel and dams, when water tables and dewatering are major problems. Various drilling techniques are used to construct overlapping concrete cylinders through saturated soils, rock, extending well below the water seepage elevation. Sequenced drilling and concreting of the individual cylinders that make up the finished barrier allows the concrete to cure, ensuring a tight seal between the cylinders for complete water cutoff. Secant piles can be constructed to depths that exceed 150 plus feet, depending on the units Kelly bar/ auger length.
Typical applications for secant walls include:
- Groundwater barriers
- Retaining walls
What Methods are used in Secant Wall Construction?
Secant piles can be constructed either with conventional drilling methods or through the use of CFA techniques. Secant pile walls typically include both reinforced secondary and unreinforced primary piles. The secondary piles overlap the primary piles, with the primary piles essentially acting as concrete lagging. The reinforcement in the secondary piles generally consists of rebar cages or steel beams. The normal diameter of the secant piles are 36”in/1.0 m and center to center distance between the piles are 30”in/0.8 m. Therefore, each pile overlaps with the adjacent piles.
Secant pile wall formed by the primary (female) and secondary (male) piles by drilling the piles to the specified diameter and required depth. The secant pile wall is constructed in two stages. The primary piles will be constructed at Stage 1 with the specified strength of mass concrete and the secondary piles will be constructed at stages 2 with the required concrete strength after the reinforcement cage is lowered into the bored holes. The secondary piles are positioned between the primary piles and secant with the primary piles in order to form the interlocking joint.
Schematic Plan View of a Secant Pile Wall
A. Primary Pile (Female Pile)
- The female pile will be constructed as the primary piles for the secant pile wall. The primary pile will be filled with the specified strength of mass concrete without any steel reinforcement.
B. Secondary Pile (Male Pile)
- The secondary pile will be constructed in between the primary piles and overlap with the primary piles with the specified strength of concrete after the reinforcement cages are lowered into the bored hole. The secondary pile formed by cutting into the primary piles to form the interlocking joint by using the specified coring tools. The boring process for the secondary pile shall only commence upon initial set of the primary piles.
A secant wall is made up of several steps:
- The initial step is to construct a guide wall to set up the positioning of the secant pile wall.
- Drill and Installation of the 1 meter temporary casing.
- Drilling of the Primary Columns, 1,3,5,7, etc etc.
- Concreting of Primary Columns.
- Drilling of the Overlapping Secondary (Reinforced) Column, 2.4.6. etc, etc.
- Install the Steel Cage or I-Beam.
- Concreting of Secondary Column.
- Repeating the above steps.
Advantages and Disadvantages of Secant Piles?
Secant pile walls are formed by construction intersecting reinforced concrete piles. The piles are reinforced with either steel rebar cages or with steel beams. They are constructed by either casing the columns, drilling under mud or auger drilling. Primary piles are installed first with secondary piles constructed in between primary piles once the latter gain sufficient strength. Pile overlap is typically in the order of 3 inches (8 cm).
The main advantages of secant pile walls are:
- Increased construction alignment flexibility.
- Increased wall stiffness compared o sheet piles.
- Can be installed in difficult ground (cobbles/boulders).
- Used in high water table conditions without excessive dewatering.
- Properly placed columns can be as close as 3” to 5” from existing buildings.
- Less noisy construction
The main disadvantages of secant pile walls are:
- Verticality tolerances may be hard to achieve for deep piles.
- Total waterproofing is very difficult to obtain in joints.
- Increased cost compared to sheet pile walls if depth is less than 40’ feet deep.
- Noise and vibration are generated, if casing is driven in lieu of hydraulically pushed and retrieved casings.
Secant Wall Conclusion
Secant walls are an economical, commercially available solution for many civil structural shoring walls providing unequaled support of existing foundations and environmental groundwater control problems. The method has many civil applications including using the walls as a structural wall, shoring and dewatering solution.