For over a decade, an existing 42-inch wastewater interceptor has plagued the Austin Water Utility (Utility) with higher than usual maintenance demands. Located in the Walnut Creek flow line, this interceptor is partially exposed with manholes rising several feet above the creek bed due to years of erosion. The interceptor suffers from external erosion and is victim to debris floating down the creek during rain events.
On two separate occasions, the Utility designed a new parallel interceptor to replace the existing one only to have the project halted by the public and local home owners’ association. The environmental concerns, coupled with stream bank erosion and stabilization issues, made this project a heated debate for City residents. To further aggravate the situation, in 1999 the Utility was put under an Administrative Order by the U.S. Environmental Protection Agency (EPA) to eliminate sanitary sewer overflows in their wastewater conveyance system by December 31, 2007.
This project utilizes trenchless technology coupled with traditional construction methods to develop a variety of solutions - and ultimately the best solution for the project. It was imperative to keep the public impacts to a minimum and have the project substantially complete by the mandated date of the EPA’s Administrative Order. The fast tract schedule mandated work in double shifts for at least a portion of the project and did not leave much time for changed or unforeseen conditions. Because of these specific project conditions, it was important to assure that the element of risk on the project was kept to a minimum and shared by the Utility and the Contractor. In order achieve this several items were incorporated into the project, and are listed below:
- Differing Site Conditions Clause
- Geotechnical Baseline Report
- Disputes Review Board
- Escrow Bid Documents
- Pre-Construction Survey
- Noise Abatement
- Monitoring Ground Movement
A win-win for the Utility and the residents, this project won acceptance from the residents, minimized impacts to the community and the environment, and eliminated land acquisition requirements. The project began construction in June of 2004 and the tunneling operation was complete in November of 2005. In January of 2006 the City Manager received a letter from a resident of the Little Walnut Creek Community stating the following:
“Where’s the new sewer line we’ve been promised?”
“The City promised us the line down Northeast”
With the tunnel mining operation complete two months prior to the resident’s letter, it was clear that the resident was unaware that the construction for the sewer line down Northeast Drive had already been completed. Clearly a sign that impacts to the community were kept to a minimum.
The project reached substantial completion in July of 2006 well within the projects fast track schedule. The project is ultimately a success with neighborhood acceptance and minimal impact to the community and the environment.
Due to the simple laws of physics, wastewater collection systems flow by gravity to sewer basin creeks and rivers. Over time the soils and soft rock formations along the creek beds erode and shift and wastewater collection systems become exposed or misaligned and damaged. The Austin Water Utility (Utility) is no stranger to this phenomenon with what they commonly referred to as the Little Walnut Creek Interceptor.