Pile Group Efficiency (Converse-Labarre)
Why a pile group carries less than the sum of its piles.
Example
You enter
- Pile rows n 3
- Pile columns m 3
- Pile diameter d (in) 12
- Center-to-center spacing s (in, >= d) 36
- Single-pile allowable Q (kip) 100
You get
- Angle theta = atan(d/s) 18.43 deg
- Converse-Labarre efficiency Eg 0.727
- Group (kip) 654
Checked against published worked examples from: Middle East Technical University.
Details, formula, and sources
ScopeThe stress bulbs of adjacent piles overlap, so the Converse-Labarre efficiency Eg = 1 - theta((n-1)m + (m-1)n)/(90 m n) with theta = atan(d/s) discounts the group. A 3x3 group of 12 in piles at 3d = 36 in spacing (100 kip each) runs Eg = 0.727 -> 654 kip, not the 900 kip the naive sum implies; squeeze to 2d = 24 in and Eg falls to 0.606 -> 546 kip, losing another 108 kip for zero added piles. Below about 3d, efficiency drops under 0.7, so close-spaced piles give diminishing returns. An empirical friction-pile hand check; block failure and settlement are separate. A design aid; the geotechnical engineer of record governs.
Formula or tabletheta = atan(d/s) [deg]; Eg = 1 - theta x ((n-1)m + (m-1)n) / (90 m n); group_allowable = Eg x (m x n) x Q_single.
Source and editionConverse-Labarre pile-group efficiency (standard geotechnical practice), an empirical friction-pile group hand check, by name; the geotechnical engineer of record and a load test govern.
Free accessThe Converse-Labarre efficiency is a published empirical relation in standard foundation-engineering texts; the single-pile allowable comes from a pile analysis or load test.
What governsEstimate. AHJ and licensed professional govern.
API field namesrows_n, cols_m, diameter_in, spacing_in, single_allow_kip, theta_deg, eg, group_kip
- Overlap discount group capacity < sum of piles because the stress bulbs overlapConverse-Labarre
- Spacing efficiency falls as s/d tightens; below ~3d Eg drops under 0.7Converse-Labarre
- Scope friction-pile hand check; block failure and settlement are separatescope of this tile