Drains are most efficiently placed in a triangular pattern, but they can be arranged in a square
pattern (although a triangular pattern is nearly 2.5 times more efficient). The diameter of the sphere
of influence (D in formula 1) is based on the existence of a soil cylinder, and to account for overlap
and gaps between such cylinders. The design drain spacing for triangular pattern should be 0.95,
and that of a square pattern should be 0.88 times that calculated value of D.