2015 Issue

45 Repairs The existing pipeline has been uncovered multiple times to repair leaks. In general the concrete pipe wall and cylinder are in good condition, but failure of the pipe is occurring at the rubber gasket joints because of age, suspected pressure surges in the system, and tree root growth into the joints. The leaks have compromise pipeline function and added risk to the system and to the District. Historically temporary repairs have been made to the joints by tightly packing the exterior of the joints with grout or concrete encasing the joints. However this method has not always been successful and often serves as a temporary fix. Figure 4 shows a joint of the 27-inch RCP being packed with grout during a leak that required repair in the spring of 2013. After multiple failed attempts at repairing leaky joints in the spring of 2013 the District was required to make an emergency repair to the pipeline which included slip lining approximately 1000 feet of the existing 27-inch pipeline with 24-inch outside diameter HDPE pipe. The District was able to install some temporary emergency cross connections with Layton City’s distribution system which al- lowed the pipeline to be taken out of service for about a 5 day time period. This down time put a major stress on the rest of the water system and luckily the repair was made in the early spring when the pipeline was not operating at peak demands. The 1000 foot section eliminated 8 of the 16 leaks found during the leakage test conducted in May of 2012. Figure 5 shows slip lining the existing 27-inch pipe during an emergency repair. Hydraulic Capacity and Increasing Demands The existing pipelinewas designed to provide a design flowof 8,400 gpm at the upstream end and 7,700 gpm at the downstream end. The pipeline operates under gravity flow conditions, with certain reaches operating at pressures as low as 10 psi. These low operat- ing pressures make it difficult for air valves to seat properly and are below the recommended AWWAminimumoperating pressures of 20 psi for a drinkingwater pipeline. At these lowoperating pressures any down surges in the pipeline can cause the air valves to become unseated and then quickly slamclosedwhen pressures are regained. The 2012 peak hour summer demands in the pipeline were 8,050 gpm. Increasing flow rates in the pipeline will continue to reduce seating pressures at air valves and increase operation and main- tenance (O&M) requirements. The District’s forecasted 50-year demands require doubling the pipe capacity (8,050 to 15,600 gpm). Rehabilitation and replacement alternatives will need to consider the current and long-term demand requirements. REHABILITATION AND REPLACEMENT ALTERNATIVES Following the condition assessment of the existing pipeline, it was apparent that the pipeline could not be taken out of service for any extended period of time and rehabilitation/replacement alternatives investigated would require the installation of a sepa- rate pipeline on a parallel corridor that could at least meet existing and near future demands. The District established the following primary objectives. • Meet Long Term Demands: The rehabilitation and replace- ment options consideredmust address the 50-year future (2060) peak hour demand of 15,600 gallons per minute (gpm). The 2012 peak hour demandwas 8,050 gpmwhich is approximately the design capacity of the existing pipeline. • Increase SystemReliability: The East Layton Pipeline provides the primary drinkingwater source for Layton, Kaysville, and Fruit Heights and cannot be taken out of service for repairs during the peak summer months (June through September). During other times of the year the pipeline can be taken out of service for no more than a 24-hour period. A project objective is to increase the reliability of the District’s water delivery system by formulating cost-effective ways to replace the existing pipeline and identify rehabilitation options for the existing pipeline that can improve system reliability and facilitatemaintenance needs. • Reduce Operation and SystemMaintenance Costs: The Dis- trict desires to implement rehabilitation/replacement options and system improvements that reduceO&Mcosts. The existing pipeline is near or at the end of its useful design life. TheDistrict has experienced frequent pipe break and leaks causing high O&Mcosts. Additionally, many of the air valves on the pipeline are located in the back yards of residential homes and are hard to access. Pump station surges on the line are contributing to the leaks and are causing increased maintenance on air valves that seat and unseat when the pump stations shut off. Figure 5. Slip-lining existing 27-inch RCP with 24-inch HDPE CULINARY PIPELINE | continued on page 46