# Secant Pile Wall Calculator Spacing, wall length and concrete for a secant-pile cut-off wall. - Page: https://jianzhujisuan.com/en/secant-pile - Category: Construction (https://jianzhujisuan.com/en/c/construction) - Chinese version: https://jianzhujisuan.com/zh/secant-pile.md - Data checked: 2026-10-25 ## Quick result With the default inputs (Pile diameter 800 mm, Pile length 20 m, Number of piles 50 pcs, Interlock overlap 150 mm, Overbreak allowance 15%): **Total concrete 578.1 m³** ## Inputs | Parameter | Type | Default | Unit | Note | |---|---|---|---|---| | Pile diameter | number | 800 | mm | | | Pile length | number | 20 | m | | | Number of piles | number | 50 | pcs | | | Interlock overlap | number | 150 | mm | Overlap between adjacent piles, usually 150-250 mm | | Overbreak allowance | number | 15 | % | Interlock + overbreak, higher than a plain pile | ## Formula - **Spacing**: s = D − overlap - **Wall length**: Lw = s·n - **Concrete per pile**: V₁ = πD²/4·L·(1+overbreak%) - **Total concrete**: V = V₁·n - **Per metre**: v = V / Lw ## Worked example 800 mm × 20 m, 50 piles, 150 mm interlock, 15% overbreak: spacing 0.65 m, wall 32.5 m; 11.6 m³ per pile, ~578 m³ total, ~17.8 m³ per metre. ### Quantities per metre of wall by diameter and overlap (from this page's formulas, 15% overbreak) | Diameter (mm) | Overlap (mm) | Effective spacing (m) | Piles per metre | Concrete (m³/m) | |---|---|---|---|---| | 600 | 150 | 0.450 | 2.22 | 0.723 | | 800 | 150 | 0.650 | 1.54 | 0.889 | | 800 | 200 | 0.600 | 1.67 | 0.963 | | 1000 | 200 | 0.800 | 1.25 | 1.129 | | 1000 | 250 | 0.750 | 1.33 | 1.204 | | 1200 | 300 | 0.900 | 1.11 | 1.445 | ### The soft pile and the hard pile are not the same pile | Item | Soft pile (primary) | Hard pile (secondary) | |---|---|---| | Reinforcement | None, or a inserted H-section only | Reinforced cage, sized for retained load | | Concrete | Retarded set, to the design's initial-set time | Normal concrete | | Role | Bored first; becomes the body the next pile cuts into | Cuts the soft pile to form a continuous cut-off | | Sequence | Alternate piles, then wait for strength | Bored through the soft pile once it has hardened | | Main risk | Hole collapse or necking - no body to cut | Deviation misses the interlock and the wall splits | | Check | Verticality, diameter, position | Verticality, overlap width, watertightness | ## Questions ### How does a secant wall differ from a plain soldier pile wall? A plain pile wall has gaps closed by a second process (jet-grouting or mixing columns). A secant wall seals by interlocking the hard piles into the soft ones, so one process does both retention and cut-off in a smaller footprint. The cost is that soft piles need slow-set concrete (60 h+ initial set) timed to the hard-pile interlock, demanding precise, well-sequenced work. It is chosen when a deep pit is tight against the red line. ### Why is the overbreak factor higher than a plain pile? Three reasons: the hard pile cuts away part of the adjacent soft pile (the overlap is lost), slow-set mixes lose more to slump and adhesion, and the interlock face over-breaks while drilling. Together they add 10-20% over a plain pile. Default is 15%; tight interlocks in soft ground can reach 20%. ### Secant piles or a diaphragm wall - which one? Three things decide it: depth, watertightness and the surroundings. Under about 30 m, on a tight site, where the wall both retains and cuts off water, secant piling is usually cheaper and needs smaller rigs. Beyond 30 m, where the wall forms part of the permanent structure, or where verticality tolerance is very tight (next to a running tunnel, say), the diaphragm wall is the safer call. Secant piling is brutally sensitive to verticality: 1% deviation at 20 m is 200 mm and the interlock is gone, so confirm the rig has correction and verticality monitoring before you mobilise. ### How much overlap should I use, and why not less? 150-250 mm is the usual band. A smaller overlap saves concrete but puts the whole cut-off at the mercy of construction tolerance: one deviated pile and the overlap goes to zero or the wall splits, which is a leak you cannot repair from inside - only by grouting from the outside. So go to 200-250 mm for deep basements, high water and sensitive neighbours; 150 mm is fine for shallow or temporary work. Match it to the diameter too: 250 mm on a 600 mm pile is a large fraction of the section and uneconomic. ### Why does a secant wall use more concrete than the same number of plain piles? Two effects stack. The overbreak factor is higher, typically 1.10-1.20 against 1.05-1.10 for a plain pile, because the hard pile cuts into the soft one and loses concrete into the cut. And the piles are packed closer: at 800 mm with 150 mm overlap you need 1.54 piles per metre of wall, far more than a row of independent piles. So the total divided by wall length lands around 0.9-1.4 m³ per metre, roughly double a free-standing pile of the same diameter. That is correct, not an error. ### Do the soft piles need reinforcement? It depends on the duty. For pure cut-off at modest depth, where earth pressure balances on both faces, an unreinforced soft pile (or plain concrete with an inserted section) is normal. But the soft pile is the body the hard pile cuts into: if it collapses or necks, the hard pile cuts nothing and the wall is broken. In loose sand, high water or at depth, many projects put nominal reinforcement in the soft piles as well. That item is easy to leave out of a quote, and if you leave it out you pay for it. ## Related tools - [Plaster and Render Calculator](https://jianzhujisuan.com/en/plaster-coat): Mortar, cement and sand by layer for internal plastering. - [Mortar Mix Design Calculator](https://jianzhujisuan.com/en/mortar-strength): Trial strength, cement, water and sand, and the mix by mass. - [Labour Days and Crew Calculator](https://jianzhujisuan.com/en/labor-productivity): Man-days, crew size and labour cost from output rates. - [Line-of-Balance Duration Calculator](https://jianzhujisuan.com/en/schedule-flow): Rhythmic flow duration with overlap and break adjustments. - [Material Storage Area Calculator](https://jianzhujisuan.com/en/material-storage): Net footprint, total area and a suggested bay size. - [Dewatering Calculator](https://jianzhujisuan.com/en/dewatering): Pit inflow by the equivalent-well method, then well count. --- Generated by BuildCalc. All calculations run in your browser; no data is uploaded. Formulas follow national standards and common trade practice; local codes govern.