Monitoring a Tailings Dam
Huckleberry Mine, BC, Canada: Engineers employed several interesting techniques when they instrumented a tailings dam at this gold and copper mine in north-central British Columbia. They grouted-in piezometers, installed inclinometers and piezometers in the same borehole, and implemented a lightning protection scheme that has already proven its value.
The TMF-2 starter dam is the first in a series of dams that will be constructed for tailings and waste management at Huckleberry mine. The dam will be raised each year for the next five years to reach its final height of 110 meters.
Geotechnical instrumentation installed in the dam included inclinometers, vw piezometers, standpipe piezometers, and surface survey monuments. The instrumentation will be used throughout the life of the mine, helping engineers assess the performance of the structure, optimize future construction, and detect any geotechnical problems early enough to allow effective remedial actions.
Installation of Piezometers: The VW piezometers are installed in boreholes that are backfilled with a low-permeability, bentonite-cement grout. The grout effectively limits the intake zone for the piezometer, preventing migration of water upwards or downwards through the borehole. This allows several piezometers to be installed in the same borehole, each monitoring a discrete zone. [More information on grouting-in piezometers] Standpipe piezometers, which will be used to monitor water quality as well as pore-water pressure, are installed with the traditional sand intake zone and bentonite seal.
Dual-Use Boreholes: Engineers installed VW piezometers in the same boreholes used for inclinometers. The bentonite-cement grout provides a dimensionally stable backfill for the inclinometer casing and discrete zones for the piezometers. Eventually, there will be three VW piezometers installed with each inclinometer.
Lightning Protection: Signal cables from the piezometers are extended vertically along the axis of an established instrumentation borehole or laterally in a trench leading to a readout location at an abutment. Since vulnerability to lightning damage increases with cable length, some form of lightning protection was required.
Slope Indicator proposed a two-tier lighting protection system, based on recommendations proposed in its publication: Sensors in the Real World. The first tier of protection is a lightning diversion system, comprising one or more lightning rods connected to bare, heavy-gauge, low- impedance, copper wire that directs the transient voltage away from the instrument zone.
The second tier is an equipotential grounding system. It comprises bare, heavy-gauge, low-impedance, copper wires that run parallel to each signal cable and extend about 2 meters deeper than each sensor. Spacers were used to keep the wires about 10 cm from the signal cables, as shown in the photo at right. The low-impedance wires keep electric potentials the same at either end of the signal cables, thus minimizing destructive current flow and eliminating damage to sensors and signal cable.
The protection system was put to the test when an electrical storm hit the mine a few months after the instruments were installed. The storm severely damaged the mine's telephone system, which took nearly a month to restore, but all of the piezometers survived.
Thanks to Slope Indicator Canada for providing this story.