SUBSIDENCE OVER SOFT GROUND TUNNEL

Comments are made on the results presented by the authors, and it is observed that data covering a wide spectrum of jobs are needed so that all the factors that affect ground settlement can be more clearly understood and permit refinement of the tentative guidelines presented by the authors. Experience in using all-purposes remotely operated poling plates with greathead type shields has been indifferent with regard to the void caused by plowing, the shield, even without the extended poling plates, was very top heavy and excessive "look up" was required to keep the tunnel on grade. Even with a hood length of 2 ft to 4 ft, shields tend to dive and the main jacks around the invert are utilized to a considerably greater extent than those around the crown. The combination of relatively soft ground at the bottom of the face and the hard material at the crown has sometimes meant that the shield was driven at a pitch of 2 percent-2.5 percent greater than theoretical grade. However, under normal conditions the pitch of the shield does not differ from theoretical grade by more than 0.5-1 percent. The ratio of diameter to length is the most important factor in the controllability of the shield. Great reliance is placed on well-controlled workmanship by the contractor to minimize settlement. In many instances there is virtue in reverting to careful hand excavation and breasting with the somewhat lower (but steady) rates of progress which ensue.

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Filing Info

  • Accession Number: 00130622
  • Record Type: Publication
  • Report/Paper Numbers: ASCE #11743 Proceeding
  • Files: TRIS
  • Created Date: Apr 7 1976 12:00AM