It Looks Good on Paper, but What’s Going on Underneath?

Due to the ongoing desire to separate combined sewers and increase stormwater capacity, the City of Lancaster engaged Jacobs to design a new stormwater trunkline. The main purpose of the project is to reduce combined sewer overflows and increase the capacity of the stormwater collection system through the addition of a new north-to-south stormwater trunkline with connections to smaller collection branches through a busy area of the city. Following the planning phase of the project, the engineering team kicked off the design refinement for their draft design submission. While the initial horizontal alignment looked good visually, once the vertical alignment got underway, the vast amount of existing utilities and grade changes challenged the original alignment. The team had to look at multiple alternatives through several reaches of the project to be able to navigate these obstacles while keeping the needed capacities. Additionally, the existing combined sewer system’s pipes which are 120-in. and 96-in. in diameter run parallel to the proposed stormwater trunkline through a tight corridor with steep slopes and critical utilities around as both try to reach the same outfall point. Geotechnical conditions and the applicability of trenchless installation were evaluated and implemented into the design. This paper will go over some of those challenges mentioned, how the project will be phased, as well as how the team tried to maintain overall hydraulic capacity while detailing the design of over 4,000 LF of 12-in.–66-in. sewers.