SBT – Experience horizontal exploratory drillings

The geological model of the design stage covers the needs of the support concept, but more detailed information is required during the construction process to ensure safe and efficient tunneling. Especially in tunnel sections within fault zones the prediction of the geological conditions is essential for getting the indications for a modification of the excavation concept on time. Therefore, an extensive program of exploratory drillings was part of the tender design. In difficult conditions, overlapping exploratory drillings in front of tunnel drive are essential to assess geotechnical risks. However, in the encountered geological‐hydrogeological conditions, the well‐established exploratory drilling methods reached their limits. Horizontal core drillings did not reach the desired length due to plastic deformation and water sensitivity of parts of the fault material. Consequently, the exploration method changed to overburden drillings without core recovery. A combination of exploratory methods such as mineralogical analyses, geophysical borehole logs, drilling data and physical measurements provide the principal data for the geotechnical engineer from a wide range of oblique information. The article describes the efforts in establishing exploratory drillings in the construction process in major fault zones of lot SBT1.1, an evaluation of the drilling methods used and the development of an alternative drilling concept.