Integrated geophysical approach for dam site investigations & dam health checks & monitoring

Modern major construction is inconceivable without high-level engineering explorations, which play a major role in increasing the economic efficiency of capital investments. For the design of structures it is indispensable to procure comprehensive high-quality information about the subsurface, within very short periods. The study of diverse natural conditions predetermines a variety of methods and technical means which can be used for carrying out exploratory work.

Engineering geophysics is an efficient means of subsurface investigation. The merit of application of this low cost aid lies in its ease of deployment and rapidity in providing a reliable knowledge of the underground over a large area, substantiating the requisite geotechnical evaluation studies thereby. Technological advancements and development of portable digital data acquisition instrument systems have increased the versatility in evaluating underground conditions and site characterization.

The state-of-the-art subsurface geophysical investigations are helpful towards minimizing involvement of the conventional direct exploration methods, aiding in accelerated and economical development of the construction projects, including new dams.

Dams in the country represent a major investment and huge benefits to population in terms of irrigation, power and flood control. Most of the big dams are very old and regular monitoring and maintenance of these dams is of utmost importance for continuing benefits. Unlike soil investigations, critical nature of dams, does not permit traditional invasive inspections by means of drilling, and such inspections are best avoided unless extremely important, and are done only when problem is too grave.

Although long recognized that dams need periodic inspection and monitoring, it has only been recently recognized that geophysical surveys can supplement the results of standard inspection and monitoring techniques. Geophysical surveys have been performed on a number of dams around the world, including India, and have yielded extremely useful insight into dam conditions. Seismic refraction surveys have been performed to provide a cross-section of the dam embankment and foundation materials in terms of seismic characteristics. Self Potential (SP) surveys have been performed to investigate seepage conditions within dam embankment and foundation materials, and abutment materials. Electrical Resistivity Tomography is also routinely performed on dams to determine internal saturation conditions in dams. ReMi is effectively used to determine shear wave velocities (Vs) in dams.

Present paper discusses various geophysical techniques with emphasis on their integration to provide unique solutions to subsurface challenges. The paper also presents few case studies where integration of two or more geophysical techniques helped resolve complex problems. The purpose of this paper is to demonstrate as to how these geophysical techniques are applicable to solve various issues related to proposed sites for new dams and existing dams. These geophysical surveys may find future use as monitoring tools applied to various dams.