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The diameter of each drilled pile.
The DESIGN diameter of the bore. Construction tolerance is what this calculation is really about: a secant wall that overlaps on paper by less than the rig's positional tolerance is a contiguous wall in the ground, with a gap between the piles for water to find. If the piles are of two diameters — primary and secondary — this arrangement assumes they are equal, and an unequal-diameter wall needs its own geometry.
The distance between adjacent pile centers.
For a secant wall, this must be less than the pile diameter to create overlap.
Pile overlap (or gap)
0.5 ft overlap
True secant configuration — adjacent piles physically overlap, forming a continuous water-resistant wall (assuming the secondary piles are cut into the primary piles as designed).
- Spacing-to-diameter ratio
- 0.75
They open the calculator with your figures already in it
Secant / Contiguous Pile Wall Overlap Calculator: 0.5 ft overlap — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Secant pile geometry: adjacent piles overlap when center-to-center spacing is less than the pile diameter; overlap distance = diameter minus spacing (a negative result indicates a gap, i.e. a contiguous rather than secant wall)
Inputs used
- Pile Diameter
- 2 ft
- Center-to-Center Spacing
- 18 in
Intermediate steps
- Spacing-to-diameter ratio
- 0.75
Confidence note: True secant configuration — adjacent piles physically overlap, forming a continuous water-resistant wall (assuming the secondary piles are cut into the primary piles as designed).
What this calculation does not cover
- This is a geometry check on the two numbers you type, not a wall design. It says nothing about pile capacity, embedment below formation level, wall stiffness, or the earth and water pressures the wall has to hold, and it does not stand in for a temporary works design.
- Verticality is not in the arithmetic. A bored pile drifts off plumb with depth, so a nominal overlap that holds at the top can be gone at the toe; the deeper the wall, the more design overlap it needs, and this does not tell you how much to add.
- It assumes every pile is full diameter and set exactly on its layout point. Setting-out error, casing or auger wander, and over- or under-break in the bore all eat into overlap, and none of them are modelled — the number is nominal, not as-built.
- Nothing here covers the concrete or the sequence: primary pile mix strength (the soft, firm or hard secant distinction), the window in which the secondary pile must be cut into the primary, reinforcement, or guide-wall setting-out. A geometrically valid overlap can still be undrillable.
- A positive overlap is not proof the wall will hold water. Sealing depends on joint quality, construction tolerance and any grouting or membrane treatment, and a contiguous wall — spacing at or beyond the pile diameter — needs a separate water-control measure whatever this returns.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-02 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Secant pile geometry: adjacent piles overlap when center-to-center spacing is less than the pile diameter; overlap distance = diameter minus spacing (a negative result indicates a gap, i.e. a contiguous rather than secant wall)
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