Prediction of TBM performance in mixed-face ground conditions

A significant number of TBM performance prediction models in full-face soil or rock conditions are available in literature. However, very few prediction models exist for tunnels excavated in mixed-face ground conditions, mainly due to the complexity of the ground. Tunnelling in areas where two or more materials with significantly different geotechnical properties are simultaneously encountered on the tunnel face has often been described as one of the most challenging tunnelling scenarios. In such cases tunnel face instabilities are common and a thorough geotechnical assessment is necessary in order to select the most appropriate tunnelling technique. TBM performance for tunnels in mixed-face conditions was studied, based on the analysis of data from a tunnel project in Singapore excavated into Bukit Timah Granite. The areas considered are located within granite characterised by different weathering grades, which frequently results in mixed-face ground conditions represented by varying percentages of rock and soil simultaneously present at the tunnel face. A new TBM performance prediction index was proposed for mixed-face ground conditions, named as Mixed face Penetration Index (MFPI). MFPI accounts for the total thrust force exerted by the TBM on the tunnel face (MN) and for the penetration rate (mm/rev). The index was correlated with: (a) the percentage of rock on the tunnel face, (b) the uniaxial compressive strength (UCS) of intact rock and (c) the rock quality designation (RQD) of rock mass, by using a weighted rock mass rating of the tunnel face (RMRm). Although no correlation was determined between MFPI and the geotechnical properties of the soil, its presence was considered in the TBM performance prediction model by assigning an RMR value to the soil portion equal to zero. MFPI and RMRm were correlated using regression analysis tools. The proposed relationship between these two indexes has been validated based on the recorded data of TBM performance in mixed-face ground along the excavated tunnel in Bukit Timah granite.