Griffith Raw Water Facilities
Hazen and Sawyer provided planning, design, and construction administration services for the Frederick P. Griffith Jr. Raw Water Pump Station and associated intake and conveyance tunnels to deliver raw water to Fairfax Water’s new 120-mgd Frederick P. Griffith Jr. Water Treatment Plant. Our design provides flexibility for Fairfax Water to strategically expand its initial 120 mgd capacity to 160 mgd without taking the pump station out of service, minimizing impact to the community.
Hazen determined that the construction of a new 1000-foot 84-inch diameter steel suction main connecting to the existing suction piping in the Occoquan Reservoir via tunneling was the optimal approach to covey raw water across the Occoquan River to the new pump station. A 720-foot section of the main was excavated through a massive granitic gneiss formation some 40 feet beneath the river bed by drill and blast methods. Two shafts, 35 feet and 14 feet in diameter were constructed 80 feet deep on each side of the river to launch and retrieve the TBM, thus connecting the existing suction main to the new pump station.
Ball valves with self-contained electro-hydraulic actuators were provided for pump check/control service. New permanganate feed facilities were provided as well. Hazen worked closely with Fairfax Water in developing the pump station controls so that the pumps could be remotely controlled from the treatment plant, but also allow for local alternate manual and automatic operation at the pumping station.
From the pump station, raw water discharge piping had to be routed through a highly congested utility corridor to reach the treatment plant, representing a considerable design challenge. The project included detailed rock mapping of the adjacent Quarry Reservoir to determine the geotechnical stability of this major pipeline crossing, and also evaluated the potential integration of this facility with the raw water pumping operations. Ultimately, the 2,300-foot 84-inch diameter Pre-Stressed Concrete Cylinder Pipe (PCCP) was constructed along the quarry wall in solid rock. The discharge main was designed with connections for interim operation of the pump station to serve existing treatment facilities until the tie-in to the new Frederick P. Griffith Jr, Water Treatment Plant was made. The project also included installation of a 24-inch overflow decant line from the quarry to the river with 30-foot excavation cuts in solid rock.
Variable Capacity, Maximum Flexibility
The pump station is furnished with six horizontal split-case pumps ranging in size from 700 horsepower (HP) to 1,750 HP to provide variable raw water delivery capability in increments of 10 mgd over a range of 40 to 120 mgd using constant-speed drives and no throttling, thus providing maximum flexibility and ease in operation for meeting raw water demands.
Project Outcomes and Benefits
- Comprehensive engineering services from project planning through construction completion.
- The pump station design provides flexibility for Fairfax Water to strategically expand its initial 120 mgd capacity to 160 mgd to meet rising demand without taking the pump station out of service, thus minimizing impact to the community.
- The pump station exterior was designed to appear as a large colonial-era mansion keeping with the architecture of nearby buildings. Owing to careful attention to architectural and acoustical design, the building neither looks nor sounds like a pumping station to the public and inconspicuously blends in with the surrounding community.