What is the Converse–Labarre formula?

η = 1 − θ·[(n−1)·m + (m−1)·n] / (90·m·n), where m and n are the rows and columns of the group and θ = arctan(d/s) in degrees, with d the pile diameter and s the centre-to-centre spacing. It estimates the efficiency of a friction pile group.

What is a typical pile group efficiency?

At the common 3-diameter spacing, Converse–Labarre gives roughly 0.65–0.80 depending on group size; efficiency approaches 1.0 at spacings beyond about 6 diameters. Driven piles in sand can exceed 1.0 due to densification, so many codes don't reduce sand groups.

Does group efficiency apply to lateral loading?

Not this formula — lateral group action is handled with p-multipliers that reduce soil resistance row by row (leading rows carry more than trailing rows). PileCalc's group analysis applies p-multipliers for lateral and efficiency/block checks for vertical loading.

Pile Group Efficiency Calculator (Converse–Labarre)

How to use it

Enter the layout

Number of rows and columns, centre-to-centre spacing, and pile diameter (any consistent length unit).

Read the efficiency

The Converse–Labarre efficiency η is computed and applied as a multiplier on the sum of single-pile capacities.

Check block failure separately

For closely spaced piles in clay, also check the equivalent-block capacity — the full app runs both and takes the governing value.

The Converse–Labarre formula

η = 1 − θ·[(n−1)·m + (m−1)·n] / (90·m·n),   θ = arctan(d/s) in degrees

Piles spaced closely enough that their stress zones overlap can't each mobilize the full soil resistance a single pile would — the group is less than the sum of its parts. Converse–Labarre captures that geometrically: m and n are the rows and columns, d the pile diameter and s the centre-to-centre spacing. At the conventional 3-diameter spacing a 3×3 group comes out around η ≈ 0.73; by 6 diameters the piles behave nearly independently. The full derivation, typical values, and its limits are in pile group efficiency, explained.

What this formula does not cover

Two things. In clay, a tight group can fail as a single block — the perimeter shears soil-on-soil and the whole plan area bears at the tip — which must be checked separately and can govern below about 3-diameter spacing. And under lateral load, group action is modeled with row-by-row p-multipliers (shadowing), not an efficiency factor. PileCalc's group analysis runs the individual-pile and block checks for vertical loading and p-multiplier analysis for lateral — see the pile group documentation.