Page 247 - Trenchless Technology Piping Installation and Inspection

Pr oject Considerations for Horizontal Dir ectional Drilling 211

Setback distance
(10 ft rod, 100 ft radius of curvature)
100
10° (18% grade)
90
15° (27% grade)
80
20° (36% grade)
70 S 1
Setback, ft 60 (S ) (d )
2 min
50
2 min
40
30
20
10
0
0 20 40 60 80 100 120 140 160 180
Depth, in.

FIGURE 5.17 Setback distances. (Source: Outside Plant Consulting
Services.)

curvature, for various entry angles. Figure 5.17 also indicates the
minimum depths at which the trajectory may become level. Thus, for
a bore entry angle of 15°, a depth of 72 in. will be achieved at a set-
back distance of slightly in excess of 20 ft using a straight trajectory
segment. In comparison, a setback distance of at least 35 ft is
required to reach the same depth with a level bore angle at that
point. Figure 5.17 indicates that this particular entry angle, for the
assumed radius of curvature, is not consistent with achieving a level
bore at depths shallower than 72 in. In such cases, the trajectory
would exceed the desired depth (i.e., beyond the point of interest,
such as the entry pit). If it were necessary to remain within a specified
maximum depth along the entire path, including near the entry point,
a shallower entry angle or sharper (smaller) radius of curvature, or
both, would be required, as quantified in the following section.

Minimum Depth at Level
Due to the recommendation that the first rod be placed in the ground
such as to define a straight entry, and its subsequent path curvature
consistent with the minimum allowable radius of curvature, there is
a minimum depth at which the trajectory will become level, depend-
ing upon the entry angle. This depth may be calculated by:

(d ) = l Sin β + (R ) (1 – Cos β)
2 min rod min
2
≈ l β + (R ) β /2 (5.7)
rod min
Figure 5.17 shows the minimum depth value for the case of a 15°
entry angle.