EXPERIENCES IN THE MEASUREMENT OF ROCK DILATION WITH THREE-DEPTH, ROD-TYPE, BOREHOLE EXTENSOMETERS

Three-depth rod-type borehole extensometers were used to measure rock dilations in the foreground of advancing tunnels, commencing within 0.3 m of the face. The extensometer consists of 3 unstressed rods of different lengths protected by three copper sleeves and a three-holed headpiece. One end of the rods and protective sleeves is welded together to anchors to be set at the desired depths. The free ends of the rods pass into the headpiece and terminate about 25 mm short of the datum end. The unit is grouted into the drill hole with the datum end exposed. Initialy, the dilation caused by stress change and blasting is measured and extended to time effects up to 540 days. The results obtained and the measured primary stresses were used to derive a deformation modulus for stress-deformation studies of a large excavation. An alternative method was used in another project in which excess dilation (dilation due to crack opening and creep) was calculated for the design of a pressure tunnel lining. On long term pressurizing of the tunnel, the rock is considered to offer negligible support until the larger part of the excess dilation is taken up and thereafter to resist movement with the modulus of intact rock. This method was equivalent to the use of a deformation modulus which is not constant (as in the deformation modulus method) but becomes less at the lower loads and longer periods. In addition to these applications, the extensometers can give valuable information relating to the depth to which rock is disturbed and offers reduced support; the support offered by rock bolt patterns with different length and spacing of bolts; and the observations of time effects over long periods in underground works and open crust.

  • Supplemental Notes:
    • Presented at the 2nd Australia-New Zealand Conference on Geomechanics.
  • Corporate Authors:

    Institution of Engineers

    Science House, Gloucester and Essex Streets
    Sydney,   Australia 
  • Authors:
    • Alexander, L G
    • Fraser, C J
  • Publication Date: 1975

Media Info

  • Features: Figures; References; Tables;
  • Pagination: 7 p.

Subject/Index Terms

Filing Info

  • Accession Number: 00126968
  • Record Type: Publication
  • Report/Paper Numbers: #246
  • Files: TRIS
  • Created Date: Jan 14 1976 12:00AM