
Effective Trenchless Design Requires Understanding and Communicating Risks
A recent pipeline crossing in southeast Ohio offers a prime example of conditions that are challenging for horizontal directional drilling (HDD).
Irish spiritual leader Emmet Fox said, “Make friends with nature by working in harmony with her and she will make friends with you.”
While I doubt that Mr. Fox was thinking about trenchless construction, Mother Earth often dictates how to approach a project and whether it will be successful.
Trenchless projects often involve site conditions that are not ideal. Challenging topography, restrictive workspace, and adverse subsurface conditions are just a few examples. In many cases, these are the very conditions that result in the need for a trenchless solution.
When faced with these types of challenges, an understanding of the risks they present and how they can best be mitigated is a key component of the design process.
Project Scope
In late 2022, J.D. Hair & Associates Inc. (JDH&A) was engaged by Civil & Environmental Consultants Inc. (CEC) to design a 20-in. diameter HDD crossing of Interstate 77. The crossing, which would be placed primarily through soft, sedimentary rock over a length of about 2,100 ft, presented three principal challenges:
An elevation differential of more than 220 ft between the crossing’s endpoints that could possibly cause hole stability and groundwater flow issues.
Highly fractured rock over roughly a third of the crossing length that could negatively impact pilot hole drilling or potentially fall into the hole, creating obstructions during subsequent passes.
Limited workspace for pull section staging on both ends of the crossing, requiring multiple tie-in welds and possibly a rig move-around.
After discussing these challenges with the project owner and explaining the associated risks, JDH&A and CEC worked together to optimize the crossing design.
Optimizing the HDD Design
With competent bedrock present beneath the lower (eastern) end of the crossing, the design was placed as deep as possible to minimize the risk of impact to both the interstate and an adjacent creek. This also maximized the length of the crossing in competent rock and allowed for installation of surface casing to bedrock on the lower end if necessary.
The less competent rock encountered on the higher (western) end of the crossing was described as weathered and very soft. It was believed that this material may behave more like hard or dense soil due to its relatively soft, weathered condition. However, the fact that highly fractured rock often poses challenges that cannot be mitigated by design could not be ignored.
When potentially adverse conditions cannot be avoided, design should be accompanied by efforts to inform the owner and potential contractors of the identified risks.
To avoid the complications associated with transporting and handling the product pipe on the higher end of the crossing, workspace for pull section staging was obtained on the lower end. Unfortunately, the limited space would require breaking the pull section into three segments.
While not ideal, the risk associated with tie-in welds during pullback could not be avoided, and in this case was believed to be relatively low. Placing the pull section workspace on the lower end would also complicate operations to some degree since it is generally preferable to position the rig on the low end of the crossing for drilling fluid management.
For this reason, the entry point was also placed on the lower end of the crossing, bearing in mind that the rig would need to be moved to the higher end prior to pullback.
Finally, with the exit point being more than 220 ft above the entry point, it would be difficult to maintain drilling fluid in the portion of the hole that is above the entry elevation, creating a risk that the empty portion of the hole could deteriorate and cave in.
An additional risk is that groundwater from the higher end of the crossing could flow to the lower end, causing a number of potential issues. However, neither the geotechnical investigation nor the site walk indicated that groundwater flow would be a concern.
These risks often cannot be remedied in the design phase and may require mitigation during construction. Again, when potentially adverse conditions cannot be avoided, all parties should be made aware of the identified risks.
Effective design has the potential to reduce risks associated with trenchless construction. However, some risks cannot be mitigated through design alone.
Engineers who design trenchless crossings should understand the risks that may be present at a specific location and make sure they are communicated to the owner and potential contractors. This can be accomplished through discussions during the design phase, notes on the drawings, inclusion in the design report, or in person at a pre-bid meeting.
Engineers should also understand what level of risk may be too great for a given project and be prepared to advise the owner.
Owners who are aware of specific challenges can use this information to determine minimum contractor qualifications, such as having completed comparable crossings with similar challenges, while contractors can use it to evaluate risk, determine means and methods, and develop contingency plans.
Success
The crossing addressed in this article was successfully completed in October of 2023 with minimal issues. Ultimately, the challenges identified during the design phase did not have a significant impact on construction operations.
While a carefully considered design certainly contributed to the project’s success, it could be argued that an awareness of the crossing’s risks led to selection of a qualified contractor who was able to plan and execute the crossing with those risks in mind.
Every trenchless crossing is unique, often involving challenges that can be detrimental if not recognized early and managed effectively. Successfully negotiating such challenges requires a combination of effective design and skilled execution.
Jeff Puckett is president of J. D. Hair & Associates Inc., part of the Michels Family of Companies.