Contractors Share 6 Considerations For Designing and Constructing Large-Diameter Urban Pipelines

Alan Hutson

Co-authored by Jared Barber, P.E., Treatment, Transmission and Utilities

Large-diameter pipelines are a critical component of a utility’s infrastructure. Weaving a large piece of infrastructure through city streets, easements and existing infrastructure has unique challenges that must be studied in the planning, routing and preliminary design of the project. Working closely with contractors enables easier navigation of challenges and contributes to a project’s timely and cost efficient completion.

Freese and Nichols enlisted a large group of experienced contractors to discuss some of the difficulties and important considerations for designing and constructing large-diameter pipelines in urban areas. Here are six challenges and ways to address them that they have observed during their many years in the field:

Trucking. The most common challenge voiced by the contractors was the difficulties involved with bringing in and removing construction material. Large pipelines can potentially require huge amounts of embedment and backfill material to be imported to the job site. The contractor’s ability to do this greatly impacts the cost and schedule of the project. It is critical to ensure adequate access to the construction area during allowable working hours. To help address this issue, consider using native material or some combination of native and imported material to reduce the amount of imported material. On-site recycling of asphalt and concrete was also suggested as an alternative. Removing material also creates a large demand for trucking. A good rule of thumb is that the maximum amount of material that can be removed in a given day is 1,000 CY.

Traffic Control Planning. Early communication and coordination with the public to mitigate traffic concerns can help reduce scheduling issues and public impact. If possible, implement detours and/or complete closures at street and intersection crossings in lieu of constructing the intersection half at a time. This will speed up construction and reduce the overall time that traffic is impacted.

Working Room. A lack of working room can severely limit the contractor’s ability to move and efficiently construct the pipeline. In an urban area, conditions may require narrow working room but considerations must be given to: 1) where the contractor will string out the pipe before it’s installed, 2) how an excavator can move within the working area, and 3) where dump trucks can come in and out to bring in or remove material.

Depth. Depth of cut should be minimized to decrease cost and difficulty of construction. Any increase in depth may require larger working rooms and creates more spoil material and import material needed.

Existing Utilities. The value of locating existing utilities ahead of construction cannot be overstated. Any utilities that are unknown to the contractor ahead of construction are likely to slow down construction, increase costs and create safety risks. Make sure the contactor is aware of all existing utilities.

Overhead Obstructions. Often overlooked, overhead obstructions can impact the speed of construction and create new safety concerns for the contractor. Overhead power and signalized intersections are the most common source of overhead obstruction and need to be observed carefully during construction and all OSHA requirements for working near these facilities must be observed.

This article is an excerpt from the Freese and Nichols paper “Big Pipe – Tight Quarters: Lessons Learned from Large-Diameter Urban Pipelines” to be presented at the 2015 ASCE Pipelines Conference in August.