A Little History The relationship between the frequency (i.e., pitch) and tension of a vibrating string (i.e., wire) goes back to antiquity and forms the operating principle of all musical string instruments. The recognition that this principle could be used in devices as a means of measuring changes in weight, position, stress and strain can be traced back to the late 19th century (1888) in Italy, by Pietro Cardani (1858-1924), a professor at the University of Palermo. It is reported that he plucked the wire manually using a small pair of tweezers and determined the frequency using a Stampfer Disc strobo- scope. Using this technique, he was able to measure deformations with an accuracy of approximately 0.003 mm (1/10,000 in). The next documented account is found in U.S. Patent No. 633,471, dated Septem- ber 10, 1899, where Edward McGarvey of Bellefonte, Pennsylvania, patented a Weight and Pressure Indicator in which the weight of coal carts could be determined using the resonant frequency of a support cable and a fulcrum, coupled to a secondary, shorter wire and fulcrum that vibrated in unison. However, it was not until the late 1920s and early 1930s that the vibrating wire sensor began to be produced commercially. Early pioneering in the development took place in 1928 in Russia by Davidenkoff and in 1931 in France when Andre Coyne patented a vibrating wire strain gage and then, in 1932, installed 36 vibrating wire strain gages in the thin-arch Bromme Dam. In 1947, Coyne along with J. Bellier went on to establish the company Telemac to promote the use of vibrating wire sensors in civil engineering applications. Around the same time, in Germany, the company Maihak was incorporated. These two companies enjoyed a virtual monopoly in Europe until 1957 when the Norwegian Geotechnical Institute (NGI) established Geonor and marketed a line of high-quality vibrating wire piezometers and load cells. NGI did a significant amount of research on vibrating wire sensors and proved what was claimed by Telemac and Maihak – that the long-term stability of the vibrating wire sensor was outstanding. Radio telemetry system for vibrating wire piezometers and in-place inclinometers In 1969, Gage Technique, in England, began commercially offering vibrating wire strain gages that had been designed by the British Transport & Road Research Labor- atory. Development of vibrating wire sensors in the U.S. followed in the mid-1970s by Irad Gage and Geokon. In 1978, Encardi- orite in India, began making vibrating wire sensors. In subsequent years, many other companies around the world, seeing the rapid spread of vibrating wire technology, decided that they too would begin manu- facture. Among them are Ace Instruments and Geovan in Korea, Sisgeo in Italy, Ofiteco in Spain, Soil Instruments and Geosense in England and numerous others in China. Applications Vibrating wire instruments are used primarily for monitoring the safety and stability of civil and mining structures, such as earth and concrete dams; subway systems and tunnels; mine openings; embankments and slopes; various shallow and deep foundations; cantilevered, braced and tied back earth retaining wall systems; underground powerhouses; bridges and culverts; pipelines; slurry wall excavations; nuclear waste repositories; and ground- water remediation schemes. In the author’s experience, the following examples serve to illustrate some of the more demanding applications. Monitoring for 30 to 50 Years Between 2009 and 2010, more than 28,000 sensors were installed across an area more than 100,000 sq m (1,076,391 sq ft) in footprint to monitor vertical displace- ments and geogrid deformation that could be brought about by subsidence when a new motorway was constructed in an area of near surface, poorly mapped, aban- doned mine workings near Krakow, Mesh of geogrid deformation transducers DEEP FOUNDATIONS • SEPT/OCT 2019 • 105
