Importance of Geophysical Investigations in Underground Urban Metro Tunnel Projects

Subsurface geophysical investigations are useful for minimization and optimizing the association of conventional direct investigation methods, resulting in faster and effective advancement of underground development systems. Geophysical techniques provide expensive and cost-effective ways to enhance data obtained by direct exploratory techniques such as borehole drillings, trial test pits and in situ testing, confirming local anomalies that may not be distinguishable by other methods of investigations.In the present study, the operation of Seismic Refraction Method (SRT) in urban tunnelling is investigated during the planning phase (before construction). The lithology interpreted using the SRT technique is compared to a nearby executed borehole log and found to be comparable. A cost and duration comparison between the direct system (boring) and the seismic refraction method for a 1.0 km tunnel stretch is also carried out. It is revealed that the use of SRT required only 2.5 days, whereas borehole drilling takes a much longer duration, 17 days, to perform and complete the same task. The use of SRT reduces exploration duration by over 7.0 times compared with the boring method. The cost required for conducting SRT is 425,000 INR, whereas the borehole drilling technique is 2,800,000 INR, which is almost 6.5 times less than the boring method. This demonstrates that SRT is significantly more efficient in time and cost than borehole drilling.