The Gainesville to Haymarket 230kV Underground Transmission project for Dominion Energy consisted of four horizontal directional drills (HDD) under Interstate 66 in Prince William County, Virginia.
The HDD installations were separated into two segments of two crossings each. The installations were each approximately 2,500 ft long and were reamed to 60 in. in diameter in moderately hard rock. Although large-diameter installations have become common in the HDD industry, certain challenges are presented when creating a 5-ft diameter borehole underground. The HDD installations of this project were merely a single component of the trenchless work required to facilitate the project. The aspects of the project that make it unique and outstanding are related to the methods of installing electrical and other conduits inside the 48-in. diameter HDPE casing.
The project was initially conceived and designed by POWER Engineers, but the HDD portion of the project was awarded to Laney Directional Drilling Co. as a design-build project. Laney’s trenchless engineering group designed the HDD crossings while working with POWER to maintain the required ampacity for the 230kV electrical cables. This allowed the Laney construction team to develop the HDD execution plan while the engineering team was finalizing initial designs.
During construction, several design modifications were required to facilitate the HDD installations; these design adjustments were made rapidly because of the integrated relationship between the engineering and construction teams.
Seventeen HDPE conduits were installed inside each 48-in. diameter casing through an innovative conduit bundler that allowed each individual casing to be installed in the desired configuration. To reduce ampacity losses in the electrical cables, the annular space between the conduits and casing was filled with a thermal grout designed specifically to reduce heat and ampacity loss in the electrical conduits. After the thermal grouting was completed, a concrete bulkhead was cast around the entire bundle to provide a stable platform to connect to the electrical system.
Why Project Is Outstanding:
The installation of four 48-in. diameter HDPE conduits under an active interstate highway was just the start of this project. When the HDDs were completed with the installation of the 48-in. diameter HDPE casing, 17 HDPE conduits had to be installed inside the casing within a specialized thermal grout designed to protect the electrical conduits. Making design and construction modifications in real-time illustrates the benefits of an integrated approach to the design and construction of trenchless projects.
The trenchless project involved more than 120,000 working hours without a single lost-time incident while installing 10,000 ft of 48-in. diameter HDPE casing, and more than 170,000 ft of HDPE conduit.
The engineering phase of the project began while the construction phase was ramping and had to balance the need for the profile to be in a competent stratigraphic layer to reduce the risk of hydraulic fracture while also limiting the depth to avoid a loss of ampacity that results from increased depth. The Laney and POWER teams worked together to find a depth that met both criteria but presented some construction difficulties. As a result, the construction team employed a series of construction practices to reduce the risk of hydraulic fracture. These included a revised drilling fluid program, using the pilot hole intersect method and planned trips to condition the hole and remove cuttings.
Because electrical cables were to be installed inside the conduits, the beads resulting from the fusion process were removed prior to installation. An innovative bundler was designed and fabricated to place each conduit in the proper location and orientation before being pulled into the casing. In addition, the conduits needed to remain filled with water to prevent collapse. Rather than fill each conduit with water, a specialized pullhead was fabricated to allow water to fill the conduits as they were being pulled into the water-filled casing. The installation of the four bundles of 17 conduits was executed according to the plan without difficulties.
Following the installation of the bundles, a specially formulated thermal grout was pumped into the annular spaces between the conduits and casing using HDPE grout tubes pulled into the casing with the bundle. As the thermal grout was pumped into the casing, the grout displaced the water filling leaving the annular space filled with the thermal grout.
Project Owner: Dominion Energy
Engineers: Laney Directional Drilling Co. and POWER Engineers
Contractors: Laney Directional Drilling Co., Wilson Construction Co., inTerra Innovation Inc.
Equipment and Suppliers: Herrenknecht, Sharewell, Brownline, Derrick Corp., Shale Pumps and ISCO