October 18, 2014
Patuxent River Crossing HDD—230 kV Underground Transmission Line
Over the last 20 years, SMECO’s number of customers has doubled and energy demands has tripled. This project was developed to meet the demands of more capacity, improved reliability and minimized power outages.
The project would install two parallel bundles of five 8-in. fusible PVC pipes at 4,522 ft each using HDD underneath the Patuxent River to run new 230 kV underground transmission lines between St. Mary’s and Calvert counties in Solomons, Maryland.
This was an industry first with no previous installations close to this distance or complexity using PVC. The higher tensile strength of fusible PVC allowed this crossing to be completed without a steel casing. Although the maximum allowable bend radii and pull forces were not exceeded during the first pullback, higher than expected pull forces were generated, resulting in the team making several changes to the pipe layout to maximize the chances for success on the second bore.
Closely spaced vertical support posts at close intervals were driven at each horizontal curve to evenly support the pipe and the entire 4,522-ft length of each pipe string was placed on supports to reduce frictional drag forces. During pullback, five load cells were used for data collection — one on each pipe string.
They were re-programmed for the second bundle to not only broadcast real-time data to the receiver, but also to record the maximum load on each gauge throughout the pullback operation.
Oakley Station CSO 551 and CSO 553 Separation Sewer
The project objective was separation of storm and sanitary flows from an existing combined flow 32-in. brick sewer and eliminate CSOs.
This fast-track, municipal design-build project went from concept to completion in 13 months with a cost-savings of more than 20 percent on a budget of $12 million with $8.55 million in construction costs.Trenchless installation was necessary to facilitate redevelopment of an abandoned industrial area requiring additional storm and sanitary sewer capacity.
Scope of project included $5.3 million of deep tunneling, storm structures and utility relocation, including nearly a half mile of large diameter storm sewer (1,930 lf of 72-in. ID rib-and-board curved tunnel with a 60-in. HOBAS FRP liner in a roadway, 300 ft of 72-in. RCP jacked beneath an interstate highway, 11- and 12-ft diameter, 40- and 50-ft deep drilled tunnel access and manhole installation shafts and six manholes ranging from 72 to 120 in. in diameter and an outfall structure).
Among the challenges was the discovery of a buried building in the path of the proposed 72-in. RCP tunnel that was to be the system outfall, resulting in a re-design of the proposed crossing with a limited working area.
The project was essential to urban redevelopment and the municipal owner enjoyed substantial savings in both time and money by using Design-Build contracting, which had only recently been allowed in Ohio.