Large Water Distribution Pipeline Calls for Challenging Trenchless Installation in Lake Oswego, Oregon

A large water supply pipeline has been designed to carry water from the Willamette River in Gladstone, Oregon, to the cities of Lake Oswego and Tigard, Oregon. The pipeline ranges in diameter from 24 to 48 in. and traverses more than 10 mi. The pipeline was part of an overall water supply project that included a raw water intake, a raw water pipeline, a water treatment plant, a finished water pipeline, and a reservoir. Finding a route for the pipeline was challenging as it traversed through many cities and required a number of interagency agreements. Along with the routing challenges, there were many technical challenges such as crossing beneath the Willamette River. This crossing is 36-in. in diameter through basalt that varies in unconfined compressive strength exceeding 40,000 psi. An extensive trenchless evaluation was performed to determine the method that offered the highest chance of success with the lowest overall risk profile. Horizontal directional drilling (HDD) was chosen as the preferred trenchless installation technique. The length of the river crossing is 3,800 ft, making the 36-in. diameter rock drill very challenging. In addition to the challenges of the drill, site and pipe layout was very restricted, further complicating and increasing the risk of the project. Many aspects of the drill had to be thoroughly laid out during design, leaving little flexibility to the contractor. This paper discusses the design of the project, the risk aspects of the horizontal directional drill and microtunneling alternatives, and the decision process leading to the selection of the trenchless technology.