Planning Horizontal Directional Drilling for Pipeline Construction

The analysis and interpretation of the subsurface conditions encountered should
concentrate on factors pertinent to design and construction of the proposed HDD
installation, including:
• the occurrence of cohesionless and coarse granular surficial materials;
• the lithology and structural characteristics of the bedrock formations;
• data on groundwater levels and the presence of artesian conditions, if
anticipated to be present;
• identification of overburden or bedrock intervals where loss of circulation
may be a concern; and
• bedrock strengths.
Bedrock strength is often described in terms of the International Society for Rock
Mechanics (I.S.R.M.) classification. Data on expected unconfined compressive
strengths can be provided, either based on laboratory testing or estimated from the
I.S.R.M. classification.
Bore hole logs and the results of laboratory testing should be appended to the
report. The report should also be accompanied by a surveyed cross-sectional
profile showing the soil and bedrock conditions encountered and the
recommended No Drill Zone.
Issues that present a challenge to HDD design and construction, or that may affect
the drilling contractor’s assessment of the risks associated with completing a
successful crossing, should be identified and discussed. Preliminary design
recommendations, including the proposed No Drill Zone, expected entry/exit
conditions and anticipated drilling considerations, should be provided.
6.3.1 Preliminary Design
A recommended No Drill Zone accommodating geotechnical and, if appropriate,
hydrological and hydrogeological considerations should be developed for the
crossing. As far as possible, the intention should be to maintain the bore within
surficial overburden units or bedrock formations that are favourable for
directional drilling. The No Drill Zone will typically be defined in terms of the
minimum recommended depth of cover below the valley bottom, based on
crossing area geometry, expected drill orientation, anticipated ground conditions,
etc. and maximum entry and exit angles.
6.3.2 Entry and Exit Areas
The entry and exit areas for the proposed directional drill should be discussed,
given the proposed drill orientation, “low to high” or “high to low” (wherever
feasible, the former configuration is preferred), and expected near surface
soil/bedrock conditions. Any requirement for casing to isolate the bore from
gravels and other problematic materials on entry should be identified. Similarly,
any need to excavate a bell hole at the exit to control drilling fluids and/or retrieve
the drill string should be noted. Finally, an assessment of terrain conditions in the
September 2004 Planning Horizontal Directional Drilling for Pipeline Construction Page 6-7