Dams are constructed by making huge investment for development of water resources towards socio-economical development of a country. In spite of taking proper care in planning, design and execution stages, there are incidences of distress in dams. As such, it becomes essential to diagnose dam distress and causes to ensure dam safety by using several possible techniques to arrive at the most suitable controlling measures to rehabilitate the dam. There are reported incidences of catastrophic dam failures all over the globe. To reduce the risk of failure regular monitoring and analyzing defects / distresses by applying advanced methods helps in deciding appropriate and economical remedial measures can be decided pertaining to the type of structures.
Central Water & Power Research Station (CW&PRS), Pune, is a premier hydraulic research institute offering wide range of R&D services in problems related to dam safety and rehabilitation. For the past few decades, CW&PRS has developed expertise in providing cost effective and viable solutions towards dam safety and rehabilitation by conducting field and laboratory investigations. Mathematical modeling is being also used for the purpose in some situations. In the year 1979, under UNDP aided project Tracer Hydrology (presently named as Isotope Hydrology) division was established. In the initial phase of its functioning, this division had contributed to water resources sector through radio isotope studies. With the application of state of art equipment and techniques this division has also built up the capability for providing rapid, economical and more accurate solutions to problems related to various civil engineering structures.
Hydraulic structure(s) viz. dams, canals, hydro-power & nuclear power stations etc. are basically engineering structures that need safety measures to be considered post commissioning. Although it’s an inherent function in the planning, designing, construction, maintenance and operation of hydraulic structure(s), many of these structures show signs of distress and failures, in spite of taking utmost care in planning, design and different stages of execution. Distresses might lead to seepage or leakage causing extreme conditions like failure of the structure. Suitable remedial measures need to be taken after comprehensive understanding of the cause and extent of seepage. There are several methods of which the non-destructive testing (NDT) is widely adopted to ascertain the exact path and source of seepage. Borehole logging and Tracer studies are aptly integrated to decipher the source and extent of seepage and also to decipher mechanical properties towards strengthening of the dams. An integrated approach of these two investigation methods can depict the in-situ engineering properties, potential seepage pathways, lithological variations, solution activity and interconnectivity of seepage path. Consequently, a most suitable remedial measure(s) can be adopted to rehabilitate the same.
An overview on Dams: Dams are constructed to store large amount of water for the use of irrigation, power generation, flood control, etc. A huge amount of investment is required in planning, designing, construction, operation and maintenance of dams for storage of water to meet the needs of water supply, irrigation and hydropower for socio economic development. Safety of dams is very important for safeguarding the national assests and the benefits derived. Most of the dams in India were constructed during the period when knowledge about dam construction and technology was limited. Now guidelines have been developed based on state of art technology, in terms of design, construction and maintenance of dams; need arises to rehabilitate dams following a check of their compliance with latest standards and also due to distresses resulting from ageing, foundation failure, seepage, heavy floods, earthquakes, etc.
Undertaking dam safety and rehabilitation measures is a highly technical task. This should be based on sound background of study and analysis as regards to the occurrence of distress, its location and amount. It is necessary to identify the defects in dams which are susceptible for threatening the safety of the structure by applying advanced and integrated methods. The costs of these investigations are meager amount as compared to the total cost of repair of the structure. Multidisciplinary techniques such as geological and geotechnical methods, dam instrumentation, geophysical methods, tracer techniques, nuclear logging and mathematical modeling for monitoring, detecting, analyzing distresses in dams should be effectively made use of in order to arrive at an optimum solution for dam rehabilitation problems.
The probable causes of deterioration of the structure should be understood before taking up the repair / restoration work failing which either the repairs will become ineffective resulting in wastage of money or sometimes lead to further damage necessitating additional expenditures. Usually deficiency in design and construction are the two prime factors responsible for distress. The dam safety procedure is followed by a general plan of physical inspection of dam, monitoring dam with instrumentation and surveillance, external and internal examination of dams, measurement of displacement settlement, seepage, uplift pressure water level etc.
This article is aimed at providing comprehensive information on nonconventional techniques mainly involving Borehole logging and tracer studies carried out at CW&PRS, Pune towards the investigations for dam safety and rehabilitation.
Tracer Technique: Tracer technique is basically adopted by doping or injecting a predetermined quantity of tracer into the medium (soil/water) either through Nx-size (76 mm dia) borehole located in the near vicinity or on the banks of a canal or in the water logged area of the canal. The movement or dilution of tracer is observed in boreholes drilled, either on the same side of the canal or in the same well. Similarly, tracer is injected in the reservoir or in the boreholes drilled in the body of the dam and the dilution of tracer is either observed in same borehole or the same is monitored at seepage location(s) in the downstream to ascertain its interconnectivity.
Borehole logging Methods: Borehole logging technique is increasingly used in seepage investigations of dams for identifying cracks/voids and permeable zones. The techniques are also applied for detecting engineering parameters of dam like bulk density, sonic velocities and mechanical properties of the rock and body of the dam. It is basically a study that involves lowering of a sensing device in a borehole and obtaining a continuous record of depth measurement (vs) variation in certain physical properties of rocks/lithology in the bore hole. The subsurface geological conditions and engineering characteristics can be determined directly or indirectly from the properties measured by these techniques. Borehole logs can be interpreted to determine the lithology, geometry of the formation, resistivity, bulk density, porosity, compressional and shear wave velocities (Vp & Vs), moisture content, water bearing strata and movement of water.
Presently, Isotope Hydrology division is equipped with borehole logging unit manufactured by M/s Robertson Geologging Ltd., U.K, which consists of a winch with a 200 m long multi-core cable, a Micro logger data acquisition system with high-speed data link to connect to a laptop and various probes VIZ. Electrical, Nuclear (consisting of Natural Gamma, Gamma-Gamma Density, Neutron), Caliper, Full waveform Sonic, Focused Electric, Temperature and Conductivity and a notebook PC along with data acquisition and processing software’s. The division is also equipped with a Laboratory fluorometer manufactured by M/s Turner Design, USA, which consists of light emitting diodes, sensitive photodiodes, solid glass filters, and electronics to ensure reliable and repeatable readings with no instrument drift.
(a) Borehole Logging Unit:
- Micro Logger: It is a portable single data acquisition unit with an interface that communicates with sonde and host notebook PC or a tough book. It is powered at 12 V, ideally with a battery and designed to attach to a winch.
- Winch: The winch is battery operated and includes a sophisticated speed controller and a depth indicator. It has a 200 m long multi conductor armored cable that supplies electric signal to the probe and the recorder to ensure synchronism to actual depth of borehole and is displayed on PC.
- Data Acquisition Software: A tough book with an acquisition software (i.e. WINLOGGER) designed to function with the Micro-logger is used for acquisition of data and creates the logs and header files. This software provides recording and replay of the data, on-screen display of log-curves during data acquisition, data output using selectable calibration files, real time printer support and data export facilities. The obtained density logging data is then analyzed for computing the in-situ density of the subsurface and delineating of weak zones susceptible for seepage in hydraulic structures.
A schematic representation of Borehole logging study and different types of probes available for measuring different parameters is shown below.
(b) Laboratory fluorometer:
- The Trilogy Laboratory Fluorometer is a multifunctional laboratory instrument that can be used for making fluorescence measurements using the appropriate snap-in Optical Module.
- Fluorescence Modules are available for discrete sample measurements of various fluorescent materials including Rhodamine, Fluorescein etc.
Measurement of fluorescent dye tracer using laboratory fluorometer and the resultant plot(s) is shown in the schematic diagram below.
Over the past few decades, numerous projects and research studies pertaining to dams seepage, canals seepage, foundation structural strength and underwater logging for piling strength etc., have been undertaken using these NDT studies. In mostly all the projects, both these techniques are used in integration as they complement each other in analysis and interpretation.
Nuclear and caliper logging along with tracer studies were carried out to find the source of leakage in damaged portion of the tail channel and also identification of weak zones in Bhama Askhed Dam, Maharashtra. The results, enabled in the identification of weak zone through interconnectivity between the dam and Energy Dissipater Appurtenances (EDA) section. Similarly, nuclear logging comprising gamma-gamma, neutron and caliper along with tracer studies were conducted at Dimbhe masonry Dam, Maharashtra in order to identify weak/permeable zones; and to delineate the path of seepage. The results indicated the variation in the density and borehole diameter indicating the presence of cracks /voids with respect to depth. The results of the tracer studies established the interconnection between reservoir through cracks and voids.
Distresses in the form of seepage and leaching in the foundation gallery was observed in Massanjore dam, Jharkhand. Nuclear logging studies were carried out to determine the in-situ density of the dam at different monoliths. The results of the nuclear logging studies showed variation in density of the masonry which helped in deciphering the weak zones. A glimpse of few field studies highlighting borehole logging and tracer technique is illustrated below.
References
- CWPRS Technical Memorandum on “Seepage Mapping Methods in Dams and Canals”, 2015, pp 1-65.
- Gaspar E and Onescu M (1972), Radioactive Tracers in Hydrology, Elsevier Publication, BUCURESTI.
- Darwin V.Ellis (1987), Well Logging for Earth Scientist, Elsevier Science Publication, New York pp 1-520.
- Keys W S., (1990) Borehole Geophysics Applied to Groundwater Investigations, USGS.
Authored by –
Dr. Rolland Andrade* Amol Chunade and Archana K Pund
Central Water & Power Research Station, Pune-411024
*E-Mail: andrade.rolland@gmail.com
About Authors
Dr. Rolland Andrade
He is presently working as Scientist C in Central Water & Power Research Station, Khadakwasla, and Pune – 24, in Isotope Hydrology Division. Dr. Rolland has nearly 16 years of experience in the field of hydrogeology, geophysical exploration and other R&D studies related to seepage problems through hydraulic structures. He has worked upon several projects related to sustainable groundwater management strategy development, Earthquake precursory studies using hydrological parameters, Geohydrological characterization of nuclear waste dumping sites etc. He has nearly 38 research and conference papers and more than 17 technical reports to his credit. Dr. Andrade was adjudged with best technical paper and was awarded with Gold Medal by the Society of Exploration Geophysicists, for his paper presentation, in a technical session in 26th SPG International Conference.
Amol Chunade
He is presently working as Asst. Research Officer in Isotope Hydrology Division in Central Water & Power Research Station, Khadakwasla. He has nearly 14 years of experience in the field of Borehole Logging and Tracer studies and other R&D studies related to seepage problems through hydraulic structures. He has undergone specialized training in ‘Radiation safety aspects of nucleonic gauges from BARC, Mumbai. He is associated with nearly 14 client sponsored projects and has nearly 13 technical reports & 6 publications to his credit.
Archana K Pund
She obtained her Masters Degree in Science (Physics) from Nagpur University, Nagpur, Maharashtra in 2006. She joined Central Water & Power Research Station, Khadakwasla, Pune – 24 as Research Assistant in 2011. She has undergone specialized training in ‘Radiation safety aspects of nucleonic gauges from BARC, Mumbai. She is associated with about 10 client-sponsored projects and 5 technical reports of different Projects and 5 publications published in international and national journals and conferences.