Application of Seismic Refraction and Reflection Techniques in the Preparation of Geotechnical Baseline Report (GBR) for Tunnelling Works-A review

The infrastructural developmental works, like constructing roads and railway networks in the Himalayas, is increasing. For that tunnelling is becoming inevitable. However, tunnelling work in the Himalayas involves uncertainties and risks. Due to this, the tunnelling projects face both time and cost overruns. The uncertainties are due to unidentified geological problems before the construction stage. In most of cases, geotechnical baseline reports (GBR) are prepared before the construction of tunnels to define the baseline conditions. Conventional geotechnical investigation like borehole drilling is still the backbone to obtain information on baseline conditions for the projects in India, from which the designers are unable to develop specific baselines and identify the geological problems. Here it has been proposed seismic refraction and seismic reflection surveys shall also be used as additional tools with traditional geotechnical investigation during the GBR stage to develop an appropriate geological model of the region in India. The seismic survey involves three steps- data acquisition, processing and interpretation. The seismic profiles generated during data acquisition are processed further to develop 3D images that help in the demarcation of faults, aquifers, water saturated zones, shear zones, etc. more accurately. The geotechnical properties of the groundmass can also be assessed through this. The seismic refraction and reflection surveys can produce information in a continuous array along the tunnel alignment up to the greater depth. It also helps in 3D visualisation of the geology of the concerned area.