The installation of underground power cables can require sophisticated preconstruction soil testing, as demonstrated by the Connecticut Light and Power Company's (CL&P) effort to install a 69-mile, 345,000-volt transmission line between Middletown and Norwalk.
As part of the project, Burns & McDonnell Engineering Co., the utility company's design agent, hired Norwood, Mass.-based GZA GeoEnvironmental Inc. to provide geotechnical services that included determining the thermal resistivity, or measurement of heat flow, of the soil along the route of the cable.
While water and sewer engineers and contractors are fully acquainted with the possibility of pipe failures should they use unsuitable soil in backfilling trenches, they are usually concerned with compaction qualities, moisture content and sieve analyses, but for the high-voltage cable installer, soil thermal resistivity is a key consideration.
Thermal resistivity is a significant characteristic of soils that can greatly affect the ampacity, or capacity of different types of cable to carry electrical current. And thermal resistivity of soils can vary greatly along the route of the cable, which means the designers have to take into account worst-case scenarios to select the proper type of cable for the desired loads. In addition, even the way the cable is installed can affect the ampacity. Utilities have to consider all of these variables to optimize the design for obtaining the most amperes from a cable installation.
If the thermal resistivity is not accounted for in design, elevated temperatures arising from the use of unsuitable backfill can cause failure, with disastrous consequences. Not only will the improper backfill have to be removed, a costly task in itself, but the transmission cable will have to be taken off-line with ensuing high loss of revenues.
GZA and its team of specialists collected soils thermal resistivity information and other subsurface data for Burns & McDonnell, who made final design decisions. Among the members of the team were AI Engineers Inc., responsible for survey, maintenance and protection of traffic inspection, and permitting; E-L Engineering Support LLC, which provides traffic control services; Geotherm Inc., which handles in-situ and laboratory thermal resistivity testing of soil and rock; New England Boring Contractors of Conn. Inc., which performs test boring and barge drilling services; and GZA Geotechnical Laboratory, which conducts soil and rock testing services.
For the testing, the team drilled and logged more than 150 test borings between 14 feet and 100 feet deep; in-situ thermal resistivity testing at more than 30 boring locations; and geotechnical laboratory testing of some 200 soil samples.
Geotechnical engineering reports were prepared for each segment of the project, with such information as foundation recommendations and design parameters for the underground duct bank and manhole structures, and a discussion of subsurface conditions and the impact on selecting methods of trenchless pipe installation. In cases where the route was changed because of the data collected, the team conducted additional geotechnical investigations along the new path.
Such soils studies have become increasingly important to utilities as they plan the routes of power cables in the current multi-billion-dollar effort to upgrade the nation's power transmission capability.