Logging rock quality and support needs

The degree of thoroughness with which TBM tunnel progress is logged in relation to rock conditions depends on many potential factors. The presence of continuous steel sets or pre-cast linings obviously forces geotechnical logging to be performed at the face (following partial withdrawal of the cutterhead) and along a narrow band of sidewall at the face and immediately behind the cutterhead ‘through’ the finger shield. Examples of such logging and a good illustration of the confined conditions were given by Warren et al. (1996) , from the UK Channel tunnel, which was driven during the early kilometers through jointed, overbreaking chalk marl, with poor to fair Q -values and difficult ring building conditions.