Progress Energy Duct Bank - St. Petersburg, FL
Clearwater, Florida was a sub-contract project. Progress Energy awarded the $42 million duct bank
project to Kiewit Southern Co., a subsidiary of Kiewit Corporation. This project included procuring
and installing four miles of a 16-duct utility in the corridor from the Bartow power plant to its
northeast substation. The duct bank includes sixteen 8-inch-diameter PVC ducts, installed in an
existing 100-foot-wide right-of-way, paralleling two existing high-voltage lines and a hot-oil line.
The duct bank, which will carry a high-voltage power line, is encased in concrete.
Trench boxes were used during the duct bank construction to prevent the earth from caving into the
9-foot deep trench and to act as a forming system for the 6.5-foot-tall duct bank construction. Sixty
pre-cast manholes 7 feet deep and 14 feet long were installed in sheeted excavations every 2,000 feet
for cable pulling and splicing.
The part of this project that TJ Morris was involved with, entailed placing two 42-inch steel casings,
each of 1,570 feet, and placing bundled eight 8-inch high-density polyethylene conduits under a major
intersection. The pipe casing was pulled through a predrilled hole that utilized a directional drill
process. Six additional bores were performed utilizing micro-tunneling and conventional jack and bore
technology. These procedures allowed the duct bank to be constructed without demolishing the intersection
and roadways, or interrupting traffic and to minimize the environmental impact. Grouting was done in a
non-stop process, pumping grout inside the 2, 42-inch casings to keep the 345 KV transmission cables cool.
Morris Power Construction has experience in duct bank construction, thermal backfill and grouting for
underground power transmission lines. Knowledge and experience go hand in hand. You can't have one without
the other. In building a wind farm, the buried power cables create heat as voltage is passed through the
cables. As wind speed increases, the amount of current passing through the buried cable increases as well.
As the flow of current increases the buried cables can become very hot and, in some instances, the heat will
melt the protective insulation causing a short circuit and literally burn up the cable.
Dirt surrounding the buried cables does not have thermal properties to dissipate this heat. This problem
can be eliminated by placing thermal backfill in the trench, on top of the buried cable. The thermal backfill
sets up quickly with little or no delay in backfilling the trench. The thermal backfill remains cool and will
absorb the heat. As heat rises with the increase of wind speed, the thermal backfill will quickly dissipate
the heat, avoiding a cable burnout. If you are building a wind farm, you should do it once and do it right.
Every effort should be made to protect the investment by using thermal backfill over the buried cables.
Thermal grout is used inside of casings, when cables are routed through a casing such as under a road or a
highway. The use of thermal grout, as with thermal backfill is for keeping the cables cool by dissapating heat.
Thermal grout and thermal backfill should be mandatory for all buried power cables on all wind farms. If you
are building a wind farm, you should do it once and do it right, include thermal backfill and thermal grout as
insurance to prevent cable burnout. Morris Power Construction is very knowledgeable and well experienced in
working with both thermal backfill and thermal grout.