Case study: Crossing of existing PCCP aqueducts A and B Southern Nevada Water Authority Las Vegas, Nevada

The Southern Nevada Water Authority (SNWA) is the principal wholesaler of water to purveyors in the Las Vegas Valley. Approximately 85 percent of the potable water supplied by SNWA is currently conveyed from the Alfred Merritt Smith Water Treatment Facility at Lake Mead to the Las Vegas Valley via Aqueducts A and B. These aqueducts are two parallel large diameter pipelines (2,438 mm [96-inch]) constructed of prestressed concrete cylinder pipe (PCCP). A third large diameter (2,286 and 2.438 mm [90 and 96-inch]) water supply aqueduct is being constructed to increase the capacity of SNWA's system. The alignment selected for this new aqueduct requires crossing the existing PCCP aqueducts at two locations. The existing aqueducts were designed for about 1.5 to 3 meters (5 to 10 feet) of earth cover at both crossing locations. Also, neither aqueduct can be taken out of service for more than a few days and that outage may only take place during the winter low demand period. The new pipe (full) would weigh more than 4,470 Kg/lineal meter (3,000 lb/lineal foot) and could cause an undesirable loading condition if placed directly over the existing aqueducts. A crossing configuration that does not pose a significant structural or construction related risk to the performance of the existing aqueducts is required. As part of design activities, several alternatives were considered for crossing over or under the existing aqueducts. They included: Open Cut Undercrossing Tunneled Undercrossing in In-Situ Soils Tunneled Undercrossing in Chemically Hardened Soils Structurally Supported Overcrossing The structurally supported overcrossing was selected because of its lower cost and lower risk characteristics relative to other alternatives. The structurally supported overcrossing consists of a structural support, earth cover fill material, and a welded steel pipe overcrossing The structural support includes a corrugated arch with a reinforced concrete load carrying cap straddling each existing aqueduct. The earth cover includes mounded and compacted native soils filled to a configuration that provides visual and thermal cover over the entire crossing. The welded steel pipe overcrossing includes appropriate air release and blowoff appurtenances. The design concept included provisions for excavating the trench for the new pipe into the compacted soil cover and using a controlled low strength material (CLSM) for pipe zone and bedding. Design development information and sketches depicting the structural support and the Aqueduct C overcrossing are presented.