Horizontal directional drilling (HDD) has made many aspects of pipeline construction easier. For example, it has eliminated the need for trenching, one of the most disrupting aspects of traditional pipeline construction methods. It has also reduced the carbon footprint generated per project, is more environmentally friendly. (Read Understanding Construction Emissions: What’s Causing Greenhouse Gases & How We Can Improve.)
Plus it has made it possible to install pipelines through areas inaccessible by traditional methods, and takes significantly lesser time.
However; there’s a complex side of HDD that is often overlooked while admiring its benefits. Since HDD works by drilling deep under the earth’s surface, these projects encounter sub-surface soil and rock strata that may sometimes change drastically.
HDD used under city roads also face the challenge of navigating through a maze of underground utilities that are sometimes old and fragile, and sometimes carry hazardous fluids.
HDD Can Face Complex Situations When:
- Projects pass through densely populated areas with a maze of underground pipelines and conduits.
- Projects have to pass through several crossings some of which are long.
- Projects pass through archeological sites and environmentally sensitive locations.
- Projects pass through mine sites or contaminated sites.
- Projects that have to pass through rapidly changing subsurface geology.
- Large diameter pipes have to be installed.
Importance of Geotechnical Information
The first step to a successful bore is having the right geotechnical information beforehand. (Read Getting Technical: Information Required in a Geotechnical Investigation Report.)
Defining the geology of the borepath or crossing should neither be neglected nor underestimated. The planning of a project also involves costing, and cost overruns are not desirable to any contractor.
The downhole tooling, type, and mix of drilling fluids and project schedules are all developed based on the geotechnical information obtained from the project site. Site-specific investigations should only be undertaken once site reconnaissance and geological review have been done in order to narrow down the scope of the investigation.
The purpose of a thorough geotechnical investigation is to obtain sufficient information that can be relied on to design and install permanent works. It is equally the responsibility of the HDD contractor as much as the clients to ensure that the investigation prepared by the client contains relevant information that will help him plan the bore. In some cases the contractor may need further information regarding specific locations; it is the duty of the client to provide the same.
Any previously available information should not be taken at face value but should be investigated and verified.
Geotechnical Investigation Planning Guidelines
- If complexity is anticipated the scope of investigation will increase. For longer bore lengths such as those greater than 300 meters minimum, two test holes should be bored.
- The boreholes should be offset from the centerline of the HDD borehole in the perpendicular direction by 10 meters.
- The depth of the borehole should be at least 3-5 meters below the depth of the proposed bore path.
- The location of the boreholes should be such that it can pick up geological sequencing and areas where the geology is changing.
- The possibility of contaminating groundwater or soil should be taken into consideration.
- The investigation report should describe the soil and rocks encountered and samples should be recovered for laboratory testing (Read Understanding Key Laboratory Testing Methods for Site Investigation.)
- For soils, standard penetrometer testing (SPT) within the borehole should be taken at selected depth intervals.
- For frac-out modeling, geotechnical parameters such as unit weight, cohesion, friction angle, shear strength, and Young’s modulus should be determined during the investigation.
- The final geotechnical report should address the sampling program, laboratory test analysis, engineering properties, bore logs and the profile of sub-surface conditions.
Limited but Beneficial
Geotechnical engineering dealing with soil mechanics as we know it today can be traced back to Karl Terzaghi. (Read Geotechnical Engineering Factors to Consider in Underground Pipeline Design and Installation.)
He developed the principle of effective stress in soil and its effect on shear stress. The progress in the field has helped design structures that are reliable and strong.
It is especially beneficial when it comes to constructing structures such as pipelines below the earth’s surface. Studies have shown that trenchless technology projects carry a higher risk of failure, especially in large diameter construction due to the associated uncertainty. The mitigation of this risk is mainly done by obtaining adequate information about the site geology which helps in taking alternatives such as revising the route.
It needs to be noted that even the most thorough geotechnical report may not be able to provide enough details of the sub-surface conditions along the entire path of the bore.
However; that cannot undermine the fact that geotechnical reports, especially for complex HDD projects, are the only way to ensure that a project is successful. Studies have been done that assert the prudence of geotechnical investigation prior to undertaking construction including its benefits to cost and time, and potential mitigation and management options.