Pile Group Spacing for a Target Efficiency
The inverse of the pile-group-efficiency tile: the center-to-center spacing that reaches a target Converse-Labarre efficiency, theta = (1 - Eg) x 90 m n / ((n-1)m + (m-1)n), then s = d / tan(theta). A 3x3 group of 12 in piles targeting Eg 0.75 needs about a 39.6 in (3.3d) spacing. Answers 'how far apart for this efficiency' instead of the efficiency from a set spacing. Spacing must be at least one diameter. An empirical hand check; block failure and settlement are separate; the geotechnical engineer of record governs.
Formula and source
theta = (1 - Eg) x 90 m n / ((n-1)m + (m-1)n) [deg]; s = d / tan(theta). The inverse of Eg = 1 - atan(d/s) x ((n-1)m + (m-1)n) / (90 m n); spacing must be at least one diameter (theta <= 45 deg).
Converse-Labarre pile-group efficiency (standard geotechnical practice), solved for the spacing, by name; the geotechnical engineer of record and a load test govern.
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