Design of a Drop Shaft Pressurized Steel Liner to Connect Pipeline and Tunnel Segments of the Clean Water Coalition's System Conveyance and Operations Program Effluent Interceptor

Highly treated effluent from the regional wastewater treatment plants in the Las Vegas basin currently discharges into the Las Vegas Wash and is conveyed to the Las Vegas Bay of Lake Mead. The Systems Conveyance and Operations Program's (SCOP) goal is to collect the majority of the region's highly treated effluent and convey the reclaimed water directly to a deep basin of Lake Mead, bypassing the Las Vegas Wash and Bay, thereby preserving the water quality of the bay. A conveyance system of gravity and pump pressurized pipelines and tunnels are being designed to deliver the reclaimed water directly to Lake Mead. This paper addresses the design methods for the pressurized steel liner of the Reach 3 Tunnel drop shaft which connects the in-trench pipeline and tunnel segments of the project. The vertical drop between the 10-foot diameter steel pipeline and 10-foot diameter steel lined pressure tunnel is approximately 99 feet. The conveyance system is designed to transport a maximum flow of 617 million gallons per day (mgd), the peak hour flow in the year 2050. The tunnel portion of the project is required to convey the water under the Las Vegas Wash and through the River Mountains with minimal impact to the surface above. The drop shaft changes the flow from the horizontal direction of the pipeline to the vertical direction of the shaft and then back to the horizontal orientation of the tunnel thereby creating large unbalanced hydraulic thrusts which must be resisted by the design. The directional and elevation changes of the drop shaft must be achieved with minimal head loss to preserve pressure head at the terminus of the tunnel which feeds two 8 megawatt hydroelectric turbines. Access to the shaft and tunnel must also be provided. The drop shaft serves as a high point to vent air when filling or draining and as an air trap for pressurized air release during normal operation.