Browsing by Subject "insinöörigeologia"

Weak zones in bedrock might have an impact on the environment, safety and costs of rock construction projects. It is possible to locate them already in the pre-investigation stage with geophysical measurements. This study utilizes ground penetrating radar (GPR) data, seismic refraction data and the data from geological mapping aggregated during the first stage of the West Metro project, more specifically from two of its station areas (Keilaniemi and Otaniemi). The GPR and seismic refraction surveys are cost-effective ways to investigate the ground and the bedrock. Both methods are based on detecting waves – seismic and electromagnetic, respectively – on the surface after they have been transmitted and have travelled through the ground. In this study the West Metro geophysical data was re-examined with new methods to improve the analysis, specifically, the detection of the weak zones. Tomographic velocity models were produced from the seismic data. The processing of the GPR data was done so that especially the bedrock structures would be interpretable. It was noticed in the early stage of this study that the available refraction data was not necessarily suitable for seismic tomography. Despite this, processing of the data with new methods did produce new, improved results. From all the weak zones that were mapped in this study, eight were located close to the reviewed geophysical survey lines. From those eight, 75 % were detected with geophysical methods. Also, other possible structures of the bedrock were discovered with geophysical methods but were not detected during the geological mapping. Anomalies were detected in both the seismic tomography and the GPR results. These anomalies could be interpreted as weak zones. However, without reference data, the interpretation of the source of the GPR reflections and the seismic velocity deviations can not be confirmed. The most important conclusion of this study is that by using geophysical measurements it is possible to detect weak zones, and that such measurements should be used more in rock construction projects for bedrock assessments. The exact purpose for the use of geophysical methods should be taken into account already when planning the geophysical surveys to ensure best possible data for the purpose.