Secant Piles Walls | Construction & Benefits

Secant piles are bored pile retaining walls that consist of a system of reinforced concrete piles that are placed in an overlapping manner in terms of their positioning. This overlapping avoids the possibility of gaps between the adjacent units.

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Secant Pile Casted
Image Courtesy: cordek.com

Secant piles are one of the techniques developed to allow the construction of the wall within the ground prior to the full excavation of the basement void. These structures create a perimeter retaining wall, around the basement areas, to safely excavate the soil. These are permanent embedded retaining walls.

Construction of Secant Piles - Secant Piling Procedure

Secant pile walls consist of overlapping piles that are constructed to create cutoff or structural walls, to obtain the required water tightness.

Arrangement of Primary and Secondary Piles in Secant Wall Construction
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Secant piling process involves retaining wall construction, formed by primary or female piles and secondary or male piles. The primary piles are unreinforced and the secondary piles are reinforced. It is constructed such that, the secondary piles are cut into either side of the primary piles in order to form a continuous impervious structure, in the form of a secant.  The primary pile secanted by the secondary piles provide a close structure

The primary unreinforced piles are constructed using cement or bentonite or weak concrete. After which the secondary reinforced piles are installed with rebar cages or I-beams, W or H beams

Different Arrangement of Secant Piles
Image Courtesy: DEEP EXCAVATION

When the deflection criteria are tight, the secant pile walls can be retained by anchors or a strutting system that can provide temporary or permanent retention.

The piles in the secant wall are spaced at 0.8 to 0.9 pile diameters. The pile overlap is in the order of 3 inches.

Secant Pile Design Considerations

The design and construction of secant pile the depends on the 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, as the labor costs are high, reinforced concrete secant piles are used. 

The success of a secant pile system requires that the individual piles be structurally sound over their full length, and that adjacent piles be constructed to tolerances that maintain a reasonable overlap between the primary and secondary piles.

Verticality tolerances of 0.5% (1 in 200) or stricter are typically necessary for secant piling projects, compared with standard requirements of 1% to 1.5% (ACI 336.1) for drilled piers. Pile spacing is maximized for economy, and therefore successful secant piling projects require exceptional attention to drilling procedures, equipment, and quality control to ensure overlap is maintained. Drilling methods and equipment selection are integrally linked in the construction process

Primary or female piles are usually casted in situ, leaving space between them to place secondary piles later. Based on the composition of secondary piles, the primary piles constructed can be of the following types:

1. Hard/Soft
2. Hard/Firm 
3. Hard/Hard

Hard/Soft Primary Secant Pile	Hard/Firm Primary Secant Pile	Hard/Hard Primary Secant Pile
The primary pile uses a soft cement-bentonite mix of strength  (typically 1 N/mm2), i.e. unreinforced.	Primary pile uses a weak mix concrete or full-strength concrete of strength 10 N/mm2	High-strength concrete is used. They can be reinforced.
Do not provide full water-tightness	Used where permanent works, reinforced concrete lining is demanded	Alternative to diaphragm wall construction

Applications of Secant Pile Construction

The main applications of secant pile construction are in:

1. Slope stabilization works
2. Providing supports for excavations
3. Providing supports for bulk-head
4. Provide water cut off to conduct excavation
5. As cut-off barriers
6. Retaining walls against liquefaction, in hard soil, in sloped areas.

Advantages of Secant Pile Wall Construction

1. Cost-effective option for short-term water retention
2. Provide greater pile wall stiffness compared to sheet piles
3. Can be constructed in areas with low headroom and difficult ground conditions
4. Have increased construction alignment flexibility
5. No requirement for installing timber lagging
6. The construction is less noisy
7. Less vibration during construction, hence less deformation and settlement on the outside
8. Can carry high loads from the surrounding structure

Disadvantages of Secant Pile Walls

1. The total waterproofing between the joints is difficult to achieve
2. The cost of construction is higher compared to sheet pile walls
3. The verticality tolerance of deep steel helical piles is less compared to sheet pile walls.

Also Read: Classification of Piles Based on Function and Design