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Thermal Grout, HDD Make 6,200-Foot Electrical Transmission Installation Possible
As grout consultant to the Wolf Bay project, the scope of the Constellation Group’s responsibilities included mix design, technical consulting and operational supervision. The thermal grouting was done after installation and testing of the HDPE conduit bundle. Before the grout was installed, a completed bore path profile was generated and used in developing a strategy for the thermal grout operation in order to fill the extraordinary length and volume of fill specified by design requirements.
“Engineering required thermal grouting to minimize heat build-up as well as physically cementing the bundle of HDPE conduits inside the casing,” said Dickes. “There were four 10-inch conduits for power cables and one 8-inch conduit for fiber optic cable.”
In order to achieve pumping distances in excess of one-half mile, the thermal grout needed to be extremely fluid, yet have the necessary thermal characteristics required by Waldemar S. Nelson & Co. engineers. Grout also needed to be homogenous and not separate during the pumping process. The grout had the consistency of latex paint, yet weighed 118 pounds per cubic foot.
Dickes said the grout was manufactured by a local ready-mix supplier, Baldwin Concrete, who dedicated its plant entirely to the project on two non-consecutive days. Specialty admixtures were supplied by Euclid Chemical. Four hundred cubic yards of grout were pumped on one day, 270 cubic yards more two days later.
Drilling contractor Southeast Directional Drilling (SEDD) performed the required thermal grouting in-house and under the supervision of Constellation Group. A Schwing truck-mounted pump was used, and because of the low pressure required, it never was operated above idle speed. The highest pumping pressure recorded was 110 psi, at 20 to 25 cubic yards per hour using several four-inch diameter HDPE grout pipes spaced in the bore. The pumping rate was established to keep pace with the grout delivery schedule and not over stress the HDPE grout pipes. Low pumping pressures indicate easily flowing grout without back pressure.
Mel Olson, SEDD vice president, said the innovative grout mix design resulted in a highly fluid material that was easily pumped and distributed throughout the entire 6,200 feet of casing pipe, effectively interacting with the transmission line while satisfying specified compressive strength of the HDPE bundle contained inside the casing and thermal resistivity parameters for the cable.