Rock mass characterization for Copenhagen Metro using face logs

An extension of the existing Metro in central Copenhagen is currently under construction. We present a comparison of the different field logging techniques available from a large number of borehole logs and face logs carried out during the construction in cooperation with the constructor and client’s representatives, which illustrate and approve the applied methods. The new ‘Cityringen’ Metro will consist of two 16 km single track tunnels, with 17 stations and 3 construction and ventilation shafts. The geological ground conditions are dominated by glacial and postglacial deposits overlying Paleocene Greensand deposits and bedrock of Paleogene ‘Copenhagen limestone’ characterized by alternating indurated and less indurated beds with local variations. The limestone contains layers and nodules of flint. Prior to construction works, a geological model of the Copenhagen area was set up, combining geological, geotechnical, geophysical and hydrogeological informations based on around 500 project boreholes located in close proximity to the alignment, combined with around 5000 existing boreholes in the area. Furthermore, 14 km of seismic data were included in the evaluations. Characterization and determination of relevant rock mass properties for tunnelling in Danian limestone has previously been difficult, as core logging shows a high degree of induced fracturing and core loss due to drilling disturbance, with an underestimation of the RQD values, and other rock mass properties, compared to face logging. However, describing rock mass characteristics using detailed face logging with geological description and recording of induration and fracturing, giving a field RQD value during excavation, combined with televiewer logs, when available, has shown to be a valuable tool for rock mass characterization compared to traditional classification methods only using core logs.