over 212 F) and will rise to more than 150C (302 F) as the mine expands and deepens. It is essential that the piezometers used for monitoring at this site can withstand not only the elevated temperatures but also the highly saline conditions. The vibrating wire sensors provided employ an all welded construction, high temperature coils and cables encased inside stainless steel tubing, and with calibrations per- formed at 25C, 60C, 100C, 140C, 180C and 220C (77 F, 140 F, 212 F, 284 F, 356 F and 428 F) for maximum accuracy. In 1993, the LNG storage facility at Close up of geogrid deformation sensor (protective cover removed) Poland. Vibrating wire instruments were selected, as the monitoring is required to be undertaken for more than 30 years. The instrumentation consists of more than 2,500 horizontal extensometers, each equipped with vibrating wire displacement transducers, 12,500 vibrating wire geogrid deformation transducers, 25 vibrating wire settlement sensors and 30 vibrating wire instrumented rebar. At the Underground Hard Rock Laboratory at Äspö, Sweden, tests are underway to assess the behavior of bentonite seals that surround nuclear waste contained within their repository. Given the aggressive nature of the environment (i.e., saline groundwater) and the required operating life (more than 50 years) of the monitoring instrumentation, vibrating wire sensors were selected with the additional proviso that they be manufactured from titanium. As a result, a number of all- titanium vibrating wire piezometers and pressure cells, with cables inside titanium tubing, were constructed and supplied to monitor the swelling pressures as the bentonite seals hydrate. Monitoring at Temperature Extremes Because there are no electronics other than the electromagnetic coil used to excite the wire, vibrating wire sensors more easily lend themselves to modification for use in environments subject to temperature extremes than do other commercially 106 • DEEP FOUNDATIONS • SEPT/OCT 2019 Roda Sten Rock Laboratory at Chalmers University in Gothenburg, Sweden, was instrumented with a variety of vibrating wire sensors designed to measure temperature, displacement and stress changes in the rock mass due to cooling of the cavern. The instruments installed were required to withstand ice growth and operate at temperatures between -196 C and +7 C (-321 F and +45 F). The sensors used special O-ring seals and cables and were checked in liquid nitrogen for correct function before shipping. The results indicated a radial deformation of 1 mm up to 4 m (0.04 in to 13.1 ft) into the rock mass and joint openings as a result of the cooling. Monitoring Deep Shafts & Sockets Wireless data acquisition system for vibrating wire piezometers available sensor types. Similarly, by carefully selecting materials and using techniques that are both generally available and proprietary, vibrating wire sensors can also be constructed that are highly corrosion resistant and capable of long- term use in highly aggressive environments. To monitor the effectiveness of dewa- tering operations at the Minifie open pit gold mine on Lihir Island, Papau New Guinea, it was critical to measure ground- water pressures. As Minifie lies within an active geothermal system, the temperatures range from 50 C to over 100 C (122 F to Although it attracted frequent protests from environmentalists during drilling of Shaft No. 1, the Nuclear Waste repository at Gorleben, in Saxony, Germany, started in 1986. In 1996, the punch line between Shafts No. 1 and No. 2 was realized at a depth of 840 m (2,756 ft). Shaft No. 1 has a diameter of 7.5 m (24.6 ft) and was sunk using freezing, drilling and explosives. At a depth of about 230 m (754.6 ft), inhomogeneous stresses in the Tertiary Clay formation required that the precast concrete block lining be supplemented with welded steel ring segments. At seven levels, the rings were instrumented with vibrating wire strain gages installed in clusters of four at six locations around the periphery to determine the tangential stresses in the lining. To ensure the instrumentation would function during construction and operation, the gages were required to be waterproof to a pressure of
