TECHNICAL NOTE. AUTOMATIC EXTENSOMETER FOR MONITORING VERTICAL GROUND MOVEMENTS

Two automatic magnet extensometers designed by the Building Research Establishment (BRE) for use in remote sites have operated successfully for five years, monitoring ground movement over a collapsing limestone mine. Details are given of extensometer design which is based on axially magnetised ferrous ring magnets acting as markers and reed switches acting as sensors. The two extensometers were in 120mm core drilled boreholes, one centrally through a mine pillar and the other 0.5m above the roof of a mine room, enabling a continuous log of the strata to be made to a depth of 5m below the mine level. A data logger is used to read the extensometers at 12 hour intervals, and stored data was transferred weekly from the logger to BRE over a telephone modem link and processed on a desktop computer. Examples are given of the spreadsheet layout for calculating ground displacements. Most of the magnets showed small settlements relative to the bottom reference magnet. It is also possible to arrange for an alarm to be raised via the modem if ground movements exceed pre-set levels. The short term repeatability of the system is shown to be generally within +/- 0.1 mm; long term repeatability is difficult to determine but it is estimated to be within +/- 0.4 mm.

• Availability:
  • Find a library where document is available. Order URL: http://worldcat.org/issn/00174653
• Corporate Authors:

  Thomas Telford Limited

  London,   United Kingdom 
• Authors:
  • LONGWORTH, I
• Publication Date: 1992-11

Language

• English

Media Info

• Features: References;
• Pagination: p. 36-9
• Serial:
  • GROUND ENGINEERING
  • Volume: 25
  • Issue Number: 9
  • Publisher: EMAP CONSTRUCT LIMITED
  • ISSN: 0017-4653

Subject/Index Terms

• TRT Terms: Design; Extensometers; Ground settlement; Mines; Test procedures
• Uncontrolled Terms: Reproducibility
• ITRD Terms: 9011: Design (overall design); 5117: Mine; 5927: Reproducibility; 5792: Settlement; 5562: Strain gauge
• Subject Areas: Design;

Filing Info

• Accession Number: 00636040
• Record Type: Publication
• Source Agency: Transport Research Laboratory
• Files: ITRD
• Created Date: Sep 24 1993 12:00AM