Planning Horizontal Directional Drilling for Pipeline Construction

6 GEOTECHNICAL CONSIDERATIONS
The design drill path must be developed taking into account the geological setting
for the project and geotechnical and hydrogeological issues at the crossing site.
6.1 Geotechnical and Hydrogeological Issues
From a geotechnical perspective, a number of issues should be taken into account
during the HDD feasibility investigation as well as during design and construction
of the directional drill, including:
• the distribution and characteristics of the surficial overburden deposits;
• the presence of high plastic clay and bentonitic shale bedrock materials; and
• the occurrence of structurally complex, hard and/or abrasive bedrock
formations.
These issues are briefly addressed below.
6.1.1 Surficial Overburden Deposits
In general, cohesive soils, such as clays, silty clays and silty-clayey tills, are selfsupporting
and an open bore should be achievable. An open bore can often also be
maintained through “dirty” sands and clayey silts, and even cohesionless clean silt
and sand materials (provided the bore is full of drilling mud). However, medium
to coarse-textured granular materials (e.g., gravels, cobbles and boulders) can give
rise to a number of problems during HDD construction, including:
• bore instability or collapse during drilling of the pilot hole and subsequent
reaming passes, that may result in the drill string becoming stuck;
• loss of drilling fluids to the formation; and
• release of drilling fluids to the environment.
Mitigative measures may include:
• maintaining drilling mud in the bore hole at all times by locating the entry and
exit points above cohesionless silt-sand zones;
• evaluating alternative drill paths that avoid or minimize exposure to the
problematic soil materials;
• casing or excavating through near surface silt, sand or coarse-textured
deposits; and
• using drilling additives to consolidate and reduce the permeability of these
materials.
Strict monitoring of fluid volumes, annular pressure and cutting returns will assist
in ensuring that bore hole plugging and fluid losses are detected and addressed.
6.1.2 Clays and Shales
Clays and soft shale formations of low to medium plasticity (based on Atterberg
Limits), have a low potential to swell and typically can be readily penetrated
September 2004 Planning Horizontal Directional Drilling for Pipeline Construction Page 6-1