Design and Construction of the Cat to Miller Reservoir Interconnect Pipeline

In the late 1990's, Denver Water began a plan to acquire and convert gravel pits into water storage facilities. The main purpose of the facilities was to capture reusable wastewater effluent and store it for downstream water rights exchanges. Two facilities, the Cat and Miller gravel pits, were acquired on opposite sides of the South Platte River in Adams County, Colorado. A 42-inch steel pipeline was needed to connect both facilities. Both open cut and tunneling methods were examined to install the pipeline. Obstacles for the open cut design involved not only crossing the South Platte River, but also an excavation over 25 feet deep in clean sands through an Xcel Energy power line corridor. Beneath a 115 kVA line in the power corridor was also a 42-inch steel wastewater force main. Installation of the pipeline by tunneling also presented problems. The river alluvium and the high groundwater table presented risks for tunneling. The geotechnical trenchless recommendation was to microtunnel in the claystone deposit under the alluvium. This design would put the pipeline below both reservoir floors and cause major modifications to the Miller outlet tower, already in final design. These issues, coupled with a relatively short length of microtunnel, led Denver Water to bid the construction as an open cut contract. This paper will discuss the design problems, the final approach and the construction that resulted in a successful project.