ETL - Impacts of Overhead vs. Underground Construction
Download "Impacts
of Overhead vs. Underground Transmission Line Construction" (PDF). If you have any additional questions, please
contact Paul DeFrank.
As stated in the Consultant's report, " An overhead line can generally span and not disturb sensitive features such as cultural resources sites, streams, most wetlands, isolated steep slopes, a sensitive species location, etc. Undergrounding however, requires construction of a trench and results in a disturbed area of approximately 40' in width for the entire length of the line" (see pictures of underground projects). Even with direct boring construction, there is an area of disturbance of approximately 40'-50'W X 100'-150'L due to the installation of a vault and room for cable pulling and conduit installation equipment every 2000'.
Reliability
Transmission lines have very high reliability because of their physical design
and the fact that most transmission lines are in a "loop" configuration
that provides a back-up source if one of the transmission lines experiences
an outage. The City's transmission system has experienced only 2 significant
outages in the last 17 years. One of these incidents was related to Hurricane
Kate and the other was caused by a tree in the late 1990's. The outage that
occurred in the late 1990's resulted in power restoration to the transmission
grid within thirty minutes. If an outage occurs on an underground transmission
line to this substation, the line could be out for weeks. Since this will
be the only feed to the substation for some time, this will mean that a portion
of the City's customers may be without power for this same period of time.
Trees
Another point that is commonly mentioned with respect to underground electric
facilities is that trees no longer have to be trimmed or cut in order to
maintain an appropriate, safe clearance for an overhead line. Therefore,
not only is the annual expense of tree trimming saved, but also the aesthetic
value of the tree is saved. There are avoided tree trimming maintenance costs
associated with underground installations. However, while trimming trees
for overhead lines may create an impact on the crown of a tree, underground
lines may affect the root structure of a tree. Since the majority of roots
are within the top four feet of the soil, significant damage can occur to
any tree with open-trench construction. At least a portion of the tree can
be killed, depending upon circumstances. In order to minimize the impact
to a tree, "trenchless" construction (boring) can be done on this
project. Boring a hole through the root of a tree does reduce, but not eliminate,
the impact on the tree; however, this is at an additional expense for this
project of 1.63 times the cost of the open trench construction.
Maintenance Cost Benefits
In many cases there is an expectation that the economic benefits associated
with underground electric installations are significant enough to offset
the additional cost of those facilities. Unfortunately, the economic benefits
associated with underground electric facilities are, in most cases, minimal
compared to the difference in the cost of installation.
The most quantifiable savings associated with the installation of underground transmission facilities compared to overhead transmission facilities is the reduction of exposure to the trees that must be trimmed on a regular cycle. For every mile of line the City installs underground, the reduced overhead electric transmission facility exposure reduces the City's tree trimming cost by approximately $7,300 annually. For every million-dollar increase per mile in construction costs to support underground construction, the City would incur approximately an additional debt service cost of $65,000 per year for 30 years. In addition, when underground facilities require maintenance, their costs are significantly higher than that of overhead facilities.
Minimum Corridor Widths
Underground Construction (Open Trench)
40' (25' permanent, 15' temporary) Underground Construction (Direct Bore) 10'
Wide (no above ground disturbance). However, every 2000' along the route, there
is an area of disturbance of approximately 40'-50'W x 100'-150'L due to the
installation of a vault and room for cable pulling and conduit installation
equipment.
Regardless of the underground construction method used, the Vaults that would be needed for this project are 10' W x 24' L.
Overhead Construction (Mahan Drive) 22' Wide Overhead Construction (on Welaunee Property) 100' Wide.