Planning Horizontal Directional Drilling for Pipeline Construction

Anticipated ground and site access conditions will determine the type of drill rig
to be employed. Typically, bore holes are completed using a truck-mounted or
track-mounted rig capable of drilling the soil and bedrock materials expected at
the site. As noted, where ground access into the proposed bore hole locations is
not available, helicopter-transportable drilling equipment may have to be used.
For some bedrock investigations, diamond coring equipment can be utilized. The
scope of the drilling program, in terms of number of bore holes and depths, will
depend on the length of the HDD, projected length of the design drill path and the
anticipated complexity of subsurface conditions. The bore hole locations should
be chosen to minimize the risk of interception by the HDD and subsequent
inadvertent returns following the bore hole to the surface.
The soil and bedrock strata being penetrated are logged from cuttings returned to
the surface during drilling, samples from standard penetration tests (SPT), spoon
and Shelby tubes, as available, and observations of drill performance. If soil
conditions are suitable, cone penetration tests may also be carried out.
Groundwater levels may be documented either by installing and monitoring
piezometers or based on observations of groundwater seepages during and after
drilling.
In addition to logging and characterizing the subsurface materials and
groundwater conditions, the field investigation should focus on identifying
conditions that could impact the feasibility of designing and constructing an
HDD, including:
• the distribution/characteristics of surficial overburden deposits, in particular
the occurrence of cohesionless clean silt-sand deposits and distribution and
characteristics of any coarse-textured granular layers (i.e., gravel, cobbles or
boulders);
• depths to and nature of the bedrock formations, including the presence of hard
and/or abrasive units (that could delay the project or impacts rates of wear on
drilling equipment); and
• the presence of discontinuities, joints, fractures and fissures, that could
contribute to instability of the bore during drilling and/or provide a path to the
surface for drilling fluids.
6.2.4 Geophysical Surveys
When the results of bore hole investigations are inclusive or incomplete, use of
shallow geophysical techniques may be considered. Seismic surveys, ground
penetrating radar (GPR), electromagnetic (EM) surveys and electrical resistivity
tomography (ERT) can be employed, often in combination. The application of
geophysical methods is affected by the soil/bedrock conditions at the site and
some techniques may not be appropriate in all situations.
Depending onsite conditions and expected subsurface materials, geophysical
techniques can be used to supplement the bore hole investigation results.
However, use of geophysical techniques as a substitute for, rather than as a
September 2004 Planning Horizontal Directional Drilling for Pipeline Construction Page 6-5