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Technical note: Automatic magnet extensometer for monitoring vertical ground movement

By Ian Longworth, BRE

This paper was first publsihed in GE’s November 1992 issue.


Since development at the Building Research Establishment (BRE) by Burland et al (1972), the magnet extensometer has been used extensively to monitor vertical ground movement. Various modifications have been introduced by BRE over the years to deal with different ground situations. These have ranged from an extensometer with a permanently installed reference tape, developed for use in soft rocks (Smith & Burland 1976), to a highly flexible extensometer for measuring ground strain in very compressible ground (Marsland & Quarterman 1974, Charles et al 1977, Burford 1991).

Prior to the instrumentation described here, all BREmagnet extensometers have been read at time intervals by simple manual devices. On remote sites the intervals between readings have necessarily often been long, sometimes leading to loss of valuable data on ground movement events. Participation in study of collapsing limestone mines in the UK, where it was essential to determine the timing of ground subsidence events, has led to the development of an automatic operating magnet extensometer. Two of the new extensometers were installed at the 30m deep Castlefield limestone mine, Dudley.

This mine was worked in a 7.3m thick bed of Upper Wenlock Limestone in the period 1750-1910,by the pillar and room method, with an extraction ratio of 80%. One extensometer was installed in a borehole down through the centre of a mine pIillar and the other in a borehole which terminated just above the roof of an adjacent room (Figure 1). These extensometers were used over a five year period to observe ground movement before, during and subsequent to an experimental infilling of the mine (Longworth 1988).

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