Page 246 - Trenchless Technology Piping Installation and Inspection

210 Cha pte r F i v e

Setback Distance
The shortest setback distance corresponds to a bore path segment
comprising a straight line extending from the drill rig directly toward
the point of entry (e.g., point 1; see Fig. 5.16):
S = d /Tan β
1 1
≈ d /β (5.5)
1
where S = shortest setback distance from entry point to point of
1
interest, ft
d = depth of point of interest, ft
1
β = bore entry angle, radians or (approximately) percent
grade.
This formula corresponds to a drop at a constant grade angle. It
should be noted that the depth of the bore path would continue to
increase beyond this point, over the distance required for the drill
rods to develop a concave-up curvature and achieve a horizontal
trajectory.
If it is specified that the bore path be essentially level at the depth
and point of entry, a longer setback distance will be required to allow
the path to reach the desired depth, and a horizontal trajectory, at a
decreasing rate of descent. This corresponds to the geometry leading
to point 2 in Fig. 5.16. In this case, it is assumed that the bore is initi-
ated along a straight path, without any curvature or steering, for a
distance equal to one full drill rod length (e.g., 10 ft, for typical Mini-
HDD machines) in the ground. This is a recommended industry prac-
tice to avoid lateral bearing loads at the front of the drill rig. The
upward desired curvature may be introduced during the placement
of subsequent drill rods. The minimum setback distance corresponds
to a path with the first few rods inserted such as to continue the
straight (sloping) trajectory, and the subsequent rods placed with the
leading rod then creating a path at the minimum allowable radius of
curvature, as follows:
(S ) = l C os β + (R ) Sin β
2 min rod min
+ {d – l Sin β – (R ) (1 – Cos β)}/Tan β
2 rod min
2
≈ l + (R ) β + {d – l β – (R ) β /2}/β (5.6)
rod min 2 rod min
where (S ) = minimum setback distance from entry point to point
2 min
of interest, with level trajectory, ft
d = depth at point of interest, ft
2
l = drill rod length, ft
The setback distances of Eqs. (5.5) and (5.6) are shown in Fig. 5.17
for a typical 10 ft long drill rod and an allowable 100 ft radius of