Trenchless crossings inherently present a greater number of risks and installation challenges than traditional open-cut construction methods. This is especially true for waterbody crossings where a unique set of engineering challenges must be overcome for a successful trenchless installation.
Waterbody installations can consist of rivers, creeks, wetlands, harbors, shoreline crossings or combinations thereof. Historically, waterbodies were crossed using open-cut practices with minimal mitigation measures for sediment control and environmental impacts. In today’s age, these types of construction practices are often not preferred by regulators, legislators and/or public entities pushing for the use of trenchless methods to complete waterbody crossings. Each trenchless technology carries an inherent risk associated with its specific method of installation.
Often, horizontal directional drilling (HDD) construction methods are viewed as the preferred method for crossing any and all waterbodies. However, blind application of HDD methods to a waterbody crossing with disregard to crossing-specific risks has led to major failures, significant project delays, higher construction costs and greater environmental impacts.
When a lack of appropriate information exists, crossing unknowns can manifest themselves into undocumented project risks. These risks can lead to a greater probability of encountering a differing site condition during construction, construction delays, higher costs, damage to corporate image, shareholder implications, and can potentially lead to ultimate project failure.
Managing risks requires thorough characterization of challenges associated with a particular crossing and proper designs to mitigate identified risks and reduce the overall risk profile to an acceptable level. Relying solely on a contractor to overcome design deficiencies presents, in itself, a significant risk to a successful waterbody crossing. Similarly, applying a specific trenchless method to a crossing without due consideration of the impacts associated with the construction method may increase impacts to local areas and negate the reasons or decisions for undertaking the trenchless crossing in the first place.
Specific engineering challenges associated with waterbody crossings typically include geotechnical based-risks, environmental considerations, site-specific constraints, access and workspace considerations, historical land use, and trenchless construction method-specific challenges. These challenges must be addressed during the design phase of the project to enhance the crossing success while avoiding significant impacts.
Geotechnical-based risks are associated with the type, properties and behavior of the encountered geotechnical materials/soils. For shorter crossings, it is recommended that boreholes be completed on or as close as possible to each bank or edge of a crossing feature. Boreholes completed far away from a waterbody crossing due to an inability to access or to mobilize equipment to the crossing location introduce risks associated with unknown geotechnical conditions until the time of construction, when the construction contractor may not be able to effectively manage the encountered geotechnical materials. Larger waterbody crossings can require boreholes completed within the waterway from a barge (see above right) to collect pertinent information associated with the ground conditions and anticipate behaviors.
Planning of geotechnical programs must consider the number of boreholes, termination depth, and spacing between boreholes. The termination depth must account for at least the anticipated depth of a trenchless crossing and should collect information at deeper depths in the event unfavorable ground conditions are encountered and the anticipated installation depth is deepened. If additional depth is not completed, the probability of needing to return to the field to collect additional geotechnical information increases.
The results of the geotechnical investigations should be used to verify the selected trenchless method for a crossing. If the risk factors are too great to overcome for a specific trenchless methodology, an alternative construction method must be considered. It is not sufficient to complete a geotechnical program just as a “check-the-box” process or push the risks to an HDD contractor. If geotechnical risks are identified, the trenchless design must mitigate the risks to an acceptable level.
Environmental considerations include evaluating the impacts of performing the geotechnical program, creation of access roads to the crossing feature, construction of large workspaces for setting up trenchless equipment, and potential false right-of-way requirements to fabricate, test and stage the product pipe. Often, the impacts associated with a trenchless construction method are far greater than the impacts associated with a short open-cut crossing with mitigation measures employed to minimize work space requirements across the critical feature while maintaining water flow and minimizing water quality and environmental concerns.
Trenchless method specific challenges need to be considered and fully evaluated to determine potential impacts to a crossing feature. For example, HDD methods require drilling fluids throughout the drilling and product pipe installation process. Managing and controlling drilling fluid flow within an HDD bore is critical to a successful crossing. If adequate depth of cover cannot be afforded due to site constraints or other factors, a potential exists for these fluids to impact the water resource through an inadvertent drilling fluid return. If this potential is deemed to be high for a critical waterbody feature and cannot be mitigated properly, then this method of construction may not be suitable for completion of the crossing as the risks could outweigh the benefit of the method. Site-specific constraints can include site topography, directional changes of the pipeline alignment close to the waterbody, inability to attain a proper depth of cover, inability to attain an appropriate setback distance between a trenchless entry/exit location and the critical feature, presence of piles and structures, proximity to existing wells, presence of deep buried river channels, and other related factors.
Shoreline crossings present additional challenges for pipeline or conduit installations that are not typical of land-to-land based installations. These challenges arise from the inability to readily access the exit side of the installation, elevation differences between entry and exit locations, trenchless method specific risks (e.g., management and control of drilling fluids and annular pressures), working in a marine environment, exiting the trenchless installation through soil materials, developing an installation strategy that maximizes onshore construction activities, and other site-specific items.
It is important for trenchless designers to manage the specific risks and constraints while balancing the challenges associated with each plausible trenchless method for a given waterbody crossing. If risks are not adequately identified and mitigated, increased construction costs and damage to reputation may occur.
Once the geotechnical risks and crossing specific constraints have been thoroughly identified, the most appropriate trenchless or non-trenchless construction method can be selected for the crossing. The design can then be tailored to the selected construction method with appropriate risk mitigation measures incorporated into site specific crossing details, designs, drawings and specifications.
Overly optimistic trenchless designs and use of trenchless methods where they may not be appropriate will continue to attract scrutiny from regulators, legislators, and the public when failures arise. It is recommended that project owners, designers, consultants, contractors, agencies and regulators work collectively to evaluate risks and address design/constructability challenges to provide optimum opportunities for project success.
Reference: Duyvestyn, G., Perkins, J., Petta, C., and Foltz, T., 2016. “HDD Used to Conquer Ohio River Crossing in Downtown Urban Environment”, Proceedings of North American No-Dig 2016 Conference, Paper MA-T2-03, North American Society for Trenchless Technology, Dallas, TX, USA, March 2016.
Tags: Engineering, HDD, horizontal directional drilling, June 2018 Print Issue, Mott MacDonald, water crossing
Glenn Duyvestyn, Ph.D., P.E., P.Eng., is vice president of the pipelines unit at the engineering firm Mott MacDonald.