a site investigation was carried out prior to the tunnelling trials in order to select the best tunnel alignment, predict chalk conditions along this alignment and evaluate the site investigation procedures used. the site investigation consisted of several phases: preliminary investigations, a two stage borehole investigation and measurements made in a 3 m diameter shaft. in addition, chalk quality encountered in the tunnel was assessed during the tunnelling experiments. following a description of the techniques and results of each phase of the site investigation, the data are discussed and compared to evaluate the best techniques for predicting tunnel conditions. detailed descriptions of each phase of the programme are given in the appendices.(a)

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