Secant Pile Wall Calculator

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.

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%.

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.

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.

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.

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.