Rock Mass Classification of Granite by TBM Cutting Force

Since tunnel boring machine (TBM) is a full-face machine, the face cannot actually be observed. This is disadvantageous when excavating rock, which has rapidly changing characteristics, as is commonly the case for rock in Japan. Therefore, measuring geological characteristics forward of the face is essential. In the previous paper, a method for estimating the rock strength of face using TBM cutting force, such as thrust, torque and cutting depth, is proposed. In the present study, rock strength along the Hiraya tunnel that mainly consists of granite is estimated from actual excavation data. The length of the tunnel excavated by TBM is 3 km. A full-shielded type TBM having a diameter of 2.6 m was used to excavate the tunnel. Firstly, the estimated rock strength was compared with the characteristics of rock obtained using the bedrock surveys (inflow water, alteration etc.) and Schmidt-hammer rebound hardness. Estimated rock strength was found to agree well with those found using these surveys. Therefore, the reliability of the estimated rock strength was very high. Secondly, the relationship between rock mass classification and the estimated rock strength is examined. The results indicated that the higher the rock mass classification, the higher the estimated rock strength. In case of granite, spacing of rock discontinuities and weathering were determined the rock strength. Therefore, the estimated rock strength provides a useful method for properly selecting a supporting pattern for rapidly changing rock mass characteristics. Thirdly, the relationship between the cutter-head rotational rate and estimated rock strength was discussed because the cutter-head rotational rate of the TBM was variable (4~12 rpm). The lower rotational rate was found to be useful to excavate soft rock.