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208 Cha pte r F i v e

5.4.2 Bore Path Layout and Design—Vertical Trajectory

Depth of Cover
The nominal depths will be specified by the owner, including mini-
mum and maximum, and consistent with other existing utility loca-
tions, as determined by the preliminary investigations. A minimum
depth of cover of 36 in. is desired to reduce the potential for drilling
fluid penetration to the surface and avoid the tendency for the drill
head to rise to the free surface, thereby complicating the steering
operation during the initial pilot bore. In addition, typical industry
guidelines recommend a minimum ratio of 10-to-1 for depth of cover
to final borehole diameter to avoid surface heaving effects, for a pos-
sible compaction process during reaming and pullback in appropri-
ate soil conditions. In such cases, spoils disposal is minimized since
the displaced soil is compacted into the surrounding walls of the
borehole. For shallower burial or noncompactable soil conditions, the
drilling operation must use appropriate drilling fluids designed to
remove at least a portion of the spoils during the initial boring or
reaming operations. A greater volume of drilling fluid would gener-
ally be required to remove the soil from the borehole in a soil removal
process, in comparison to a compaction process. It is the responsibil-
ity of the contractor to understand and utilize drilling fluid technol-
ogy to form the borehole without surface heaving.
Figure 5.15 shows recommended minimum depth of cover for
conditions compatible with the nominal 10-to-1 ratio for compactable
soils, as well as for the case including a spoils removal process––
assuming a 5-to-1 ratio. The increasing depth as a function of bore-
hole (and pipe) diameter allows some degree of inefficiency of soil
removal, reduced potential for any surface heave or settlement, and

160
140
120
Depth of cover, in. 100 Compaction

80
60
40
20 Spoils removal (Mini-HDD)
0
0 2 4 6 8 10 12 14 16
Final borehole diameter, in.
FIGURE 5.15 Recommended minimum depth of cover of borehole. (Source:
Outside Plant Consulting Services.)