Pile Group Efficiency (Converse-Labarre)
Why a pile group carries less than the sum of its piles: the 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 and source
theta = 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.
Converse-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.
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