Retrogressive rotational translational landslide complex along South Saskatchewan River Valley in Southern Alberta, Canada.

Assessing the Difficulties Associated with Slope Drills and Navigating the Risk These Projects Present

Horizontal directional drilling (HDD) plays a crucial role in the installation of pipelines in challenging terrain, including slopes and areas with complex geology. HDD projects in such environments must emphasize thorough planning, risk mitigation strategies, and the utilization of advanced or novel techniques to ensure successful outcomes.

Slope drills have become a more common feat for HDD projects to overcome, whether for laying new pipelines, replacing existing ones that are impacted by various geohazards or those that have outlasted their design age.

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Comparison showing the satellite imagery and the LiDAR at the same location
Comparison showing the satellite imagery and the LiDAR at the same location. It is important to note the differences and the amount of detail that can be discerned form LiDAR.

Valley Geology and Geohazard Risk

There are many geohazards that can influence previously open cut pipelines, however one of the most common is slope instability. Understanding the causes of slope failure is essential for engineers to design and execute projects effectively. Factors such as decreased shear strength, increased pore water pressure, cracking, swelling, and cyclic loading can all contribute to slope instability, highlighting the importance of comprehensive geotechnical assessments and design considerations.

Valley geology presents its own set of challenges when it comes to HDD operations. Terraces forming within the valley extents and the potential type of slope failure must be carefully studied to determine the most suitable approach for pipeline installation.

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By leveraging technologies like LiDAR, engineers can obtain detailed information on slope angles, differential elevations, and slope aspects, aiding in the development of more precise project plans and risk management strategies.

Generally, some of the most problematic geotechnical formations are present near areas of high topographic relief, such as valleys or slopes.

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Some of these geotechnical challenges include alluvial sediments, artesian groundwater conditions (or groundwater at high elevation) and fractured bedrock, each of which requires specific attention and mitigation measures to ensure the stability and integrity of the borehole during HDD installation.

This graphic shows a typical valley geology cross section.
This graphic shows a typical valley geology cross section. Slope drills at locations such as these must balance construction feasibility and geohazard risk.

Geohazard Risk Mitigation

Geohazard avoidance is a critical aspect of HDD projects in challenging terrain such as slope drill alignments. While open-cut methods may be suitable in some cases, HDD offers a more practical and less disruptive solution, particularly in high hazard zones, where avoiding geohazards near the surface is paramount.

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By conducting thorough routing assessments, adhering to regulatory requirements, and engaging with stakeholders, engineers can optimize the design and execution of HDD projects while minimizing environmental impacts.

Conventional open cut installation of a pipeline
This image shows a conventional open cut installation of a pipeline. Deeper burial could be achieved through use of HDD methods and decrease the geohazard risk, if required.

HDD Is Versatile; However Construction Risks Still Persist

During design of these challenging projects there are limited options for execution during construction. Trenchless methods such as microtunnelling, auger boring, pipe ramming or Direct Pipe Installation (DPI) are often unsuitable for managing the geometry change that may be required for certain sites.

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Specialized mitigation techniques and contractor execution plans may be necessary to navigate through these challenges, underscoring the importance of expertise and innovation for HDD slope drill projects.

HDD is a versatile way of successfully installing large elevation change crossings, however care must be taken to manage the risk profile. Since there is an elevation difference between the entry and exit locations the drilling fluid support pressure will not be present throughout the entire length of the crossing.

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The length of drill path that is at an elevation above the entry elevation is commonly referred to as a “dry hole” because no fluidic support will be available to stabilize the borehole sidewall. In these areas granular soil would be expected to be prone to collapse and must be avoided or stabilized by some physical means (usually temporary surface casing).

Another risk for these “low to high” geometry crossings are groundwater elevations above entry elevation, or artesian groundwater conditions. Because the borehole is not cased or supported by high fluid pressure head throughout its length groundwater can readily flow through the borehole sidewall to the entry point at the lower elevation.

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Artesian groundwater conditions encountered during a geotechnical investigation for an HDD crossing
Artesian groundwater conditions encountered during a geotechnical investigation for an HDD crossing. Water is shown flowing out of the drill pipe filling bucket to measure flow rate.

This may cause flooding of the entry lease, uncontrolled inflow of cuttings or soil materials and other unwanted challenges. Potential mitigations for these types of groundwater conditions need to be determined at the design stage in order to allow construction to be successful.

In conclusion, careful consideration needs to be given to the complexities involved in slope drills, as nearly all of these great topographical changes would be expected to encounter challenging geotechnical conditions.

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By leveraging advanced technologies and experience for analysis, and implementing proactive risk mitigation strategies, engineers can navigate these challenges effectively and ensure the successful completion of HDD projects in these demanding environments.

The combination of technical expertise, innovative approaches, and a proactive mindset is essential for overcoming the challenges posed by slopes and challenging geotechnical conditions in HDD projects, ultimately contributing to the safe and efficient installation of pipelines in landscapes with large topographic relief.

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A flooded lease at the HDD entry pad location
A flooded lease at the HDD entry pad location caused by challenging groundwater conditions.

Stefan Goerz, M.Sc., P.Eng., P.E., is the geotechnical manager for CCI Inc. and CCI & Associates Inc.

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