Advanced seismic investigations during construction of hydro tunnels

In order to maintain the economic and safety demand in underground projects for hydropower there is a vital necessity of advance geological prediction by using the seismic reflection prediction technique. Tunnelling in many projects suffered due to uncertain or even unknown geological problems such as fault and shear zones, cavities and ingress of water and high level of seismicity etc. By recording the full wave-field of reflected signals, P-and S-waves, prediction of the rock mass quality in term of physical and mechanical parameters up to 150 m ahead of the face is possible. Moreover, 3D velocity distribution images are generated by means of seismic modelling. Results of surveys performed at the Uma Oya Multipurpose Development Project in Sri Lanka using this technology are discussed. Here, the headrace tunnel of 15 km length is under construction using a shield TBM. Although installing pre-cast segments behind the cutter head, seismic investigation become still manageable and provide an important impact on logistic optimization that could end up in an economic and consequentially time-and cost-effective practice of prediction ahead.