What are the biggest risks involved with trenchless crossing installations?

Q:

What are the biggest risks involved with trenchless crossing installations?

A:

Pipelines installed using trenchless crossing methods face many engineering and geological challenges. Trenchless crossings are frequently used to install pipelines under rivers, highways, railway embankments, buildings or sensitive environmental habitats.

Unique risk factors come with the nature of the installation — almost all of the work performed below the earth’s surface, unseen from above. Though bore paths for projects are planned using reports derived from extensive geotechnical site investigation, survey, environmental studies and trenchless engineering design, there is always the inherent risk of encountering unforeseen and unavoidable circumstances underground.

Waterbody crossings — rivers, wetlands, creeks, shorelines, harbors etc. — are uniquely challenging. When crossing a watercourse or environmentally sensitive habitat it is necessary to understand its geotechnical, aquatic and environmental setting. Though horizontal directional drilling (HDD) is a commonly utilized crossing method, improper application of HDD, lack of information, or disregarding crossing-specific risks can lead to problems such as, delays, environmental damage, project cost overruns and ultimately, failure of the crossing attempt.

The risk of inadvertent drilling fluid release to the surface could have a negative environmental impact. Cleaning operations often involve heavy equipment in order to contain and clean drilling fluid in order to remove it as best as possible from the surface. Fluid release into a waterbody increases the turbidity of the water, and depending on which drilling fluid additives are used, may cause additional impact to flora, fauna and other aquatic life.

Significant unknown risks may arise from insufficient geotechnical investigation, improper planning, shortage of understanding site specific constraints, and lack of skilled engineering design or inexperienced construction execution. Subsurface risks are associated with the type and behavior of encountered soil and rock geology along the crossing path.

When crossing infrastructure, such as pipelines, utilities, roadways or railways, a lack of understanding the geotechnical setting or improper design of the crossing path below a surface obstacle could lead to excessive heave or settlement of the existing infrastructure causing potentially significant damage.

This can cause potentially dangerous circumstances such as sink holes, improper alignment of railway tracks, utility leaks, electrical short circuits, and building foundation impact.

In areas where subsurface infrastructure such as utilities, cables and pipelines already exist, it is important to know exactly where that infrastructure is in order to design a trenchless trajectory that avoids these subsurface obstacles.

A significant risk of an improperly planned trenchless crossing is cross-boring into an existing subsurface utility or pipeline. Cross-boring into an existing utility, may result in groundwater contamination, or other significant environmental and safety concerns, depending on the product conveyed by that pipeline, such as water, sewer, electrical or hydrocarbon.

Identifying the geotechnical conditions, environmental setting and site-specific constraints enables engineers and contractors to select the most appropriate trenchless method, equipment and tooling for a project.

Trenchless risk management requires a thorough understanding of potential problems that are site specific to each crossing on a project.

Appropriate risk mitigation measures can then be incorporated into the design and crossing details of the selected crossing method.

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Written by Bertus Vos
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Bertus Vos is one of the co-founders and principal engineers at BlueFox Engineering. He holds a bachelor of engineering degree from the University of Alberta, and a master of business degree from the University of Fredericton.

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