Pile Group Efficiency Calculator: Converse-Labarre Method

Nine piles, each good for 800 kN, do not make a 7,200 kN foundation. Each friction pile transfers its load into a bulb of surrounding soil, and when piles stand close together those bulbs overlap: the same soil is being asked to carry shear for two or three piles at once. The group settles more, mobilizes less total resistance, and carries less than the sum of its parts. Group efficiency is the honest bookkeeping for that effect, and the Converse-Labarre formula is its longest-serving estimate.

The formula

eta = 1 – theta x [(n-1)m + (m-1)n] / (90 m n), with theta = arctan(d/s) in degrees

for an m by n rectangular group of piles with diameter d at center-to-center spacing s. The structure is intuitive once unpacked: theta grows as piles crowd together (bigger d relative to s), and the bracketed term counts the interior adjacencies where bulbs overlap, normalized by group size. Wide spacing drives theta toward zero and efficiency toward one.

Worked example

• A 3 x 3 group of 0.4 m piles at 1.2 m spacing (the classic 3-diameter layout), each pile good for 800 kN
• theta = arctan(0.4/1.2) = 18.4 degrees
• eta = 1 – 18.4 x 12/810 = 0.727
• Group capacity: 0.727 x 9 x 800 = about 5,230 kN, a 27 percent haircut on the naive sum

The judgment that goes around the formula

Converse-Labarre is a geometric rule, and its blind spots are soil-behavioral. In soft clays, closely spaced friction piles can fail as a single block, the whole group plus the soil trapped inside it shearing along the group perimeter, and that block check (perimeter friction plus base bearing of the equivalent big pier) can govern below the formula’s answer, so both get computed and the lower wins. In sands, driven piles actually densify the soil between them, and full-scale tests have measured group efficiencies above 1.0, making the formula conservative there, which is a comfortable direction to be wrong in. End-bearing piles socketed to rock share almost nothing through the soil and take little or no group reduction. And one settlement reminder that efficiency arithmetic hides entirely: even at full capacity efficiency, a group settles substantially more than a single pile at the same load per pile, because the group’s stress bulb reaches several group-widths deep into soil a lone pile never bothers. Capacity and settlement are separate checks, and the formula only speaks to the first.

Related Calculators and Reference

The single pile capacity this multiplies comes from the pile bearing capacity estimator, the soil strength inputs trace to the SPT corrections calculator, and the shallow-foundation alternative is checked in the Terzaghi bearing capacity calculator. A good open overview of group behavior is the deep foundation article.