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The Potential of Borehole Geophysical Logging and Tracer Techniques in Dam Seepage Investigations

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water resource management
G. A. Panvalkar, Scientist ‘B’
Isotope Hydrology Division, Central Water & Power Research Station
N. V. Deshpande, Scientist ‘C’
Isotope Hydrology Division, Central Water & Power Research Station

Introduction

Effective water resource management demands that specific plans be provided to focus on dam safety. Many dams have shown signs of distress and failures despite taking due care in planning, design and execution stages. The likelihood of dam failures has been further aggravated by the fact that a number of dams are ageing and have lacked the supervision and maintenance needed for guaranteeing their structural safety and the operational integrity to prevent possible failures. Therefore dams need to be reassessed for their safety through modern approaches and standards. The growing demand of safety awareness has stimulated the development of several monitoring techniques capable of detecting events at an early-stage which can prevent dams from major failures. As such, seepage in hydraulic structures is a major parameter that needs to be investigated in assessing rehabilitation of dams.

Dams experience some seepage as the impounded water seeks the path of least resistance. Seepage generally occurs through the body of the dam, the structure-foundation interface or through geological in-homogeneities present in the vicinity of the structure. The efficiency and speed with which the seepage source can be located may be the difference between a timely remediation and a catastrophe (Kamble et al., 2011). Therefore, to ascertain dam safety, it is essential to identify as precisely as possible the areas of water loss and seepage entry points in these hydraulic structures. This information is vital for adapting the remedial measures necessary to reduce or prevent the water loss and thereby provide a cost effective solution. In the recent past, multidisciplinary techniques like geological and geotechnical methods, dam instrumentation, geophysical methods, tracer techniques and mathematical modeling studies are being adapted for detecting, positioning and mapping seepage in dams. Among these, ‘Geophysical Borehole Logging’ and the non-destructive ‘Tracer Techniques’ are cost effective tools in assessing the sustainability and safety of structures and also to evaluate the efficacy of remedial measures and are discussed here.

Causes of Distress in Dams

Many existing dams in India are thirty to sixty years old and are ageing. The decay and deterioration in the dam normally occurs due to ageing, unsuitable geological foundations, design and construction deficiencies, lack of monitoring and maintenance. These factors are usually a cause of dam seepage. The development of seepage through body and subsoil of a dam provides basic information on the state of a hydraulic structure and on the possibilities of its safe operation. Therefore, seepage through or under a hydraulic structure can be considered as one of the most important aspects in structural safety.

The causes of seepage and dam failure vary depending on its types. Seepage in Earthen Dams mainly occurs through the embankment, foundation, and abutments probably due to lack of sufficient filter protection and improper filter design, washing away of particles or clogging of drains, poor compaction, open seams, cracks caused by earth movement, etc. In ‘Masonry Dams’, failure is mainly due to the seepage path through the body because of the moisture absorption by the weak zones, temperature effects, leaching, excessive uplift pressure, construction deficiencies etc. In ‘Concrete Dams’, seepage causes include construction deficiencies, disintegration, scaling, erosion, spalling, pop-outs, cracks etc. Transition between the masonry/concrete dam and earth dam requires special attention and detailing during designing and construction phase, as it constitutes an area of discontinuity in the material properties, and if left unattended may lead to dam failure (CWPRS Technical Memorandum, 2015).

Seepage Detection Techniques

Dam seepage is a complex geotechnical engineering problem as the hydraulic structures are heterogeneous, non-linear, non-conservative and anisotropic. Hence, as a primary step, any seepage study should always include periodical physical inspection of the dam and detailed study of all geological and hydrogeological information of the affected area. Further, in general, a single technique is not sufficient to detect seepage in dams and often a combination of techniques result in successful leakage detection. Among these geophysical borehole logging and tracer techniques are being increasingly used as cost effective tools in identifying susceptible seepage zones and so also to evaluate the efficacy of the adopted remedial measures. These techniques often play a catalytic role by supplementing conventional techniques of seepage detection from planning and designing up to the operational stage.

 

 

Geophysical Borehole Logging Techniques

Borehole logging can be defined as “The science of making measurements in boreholes or wells for determining the in-situ properties of soils and rocks, the fluid contained in the rocks and the construction of the borehole”. It is adapted by lowering sensing devices in boreholes and recording physical parameters, which may be interpreted in terms of characteristics of subsurface formations. Although this technique evolved in the oil exploration industry, the parameters measured are finding increasing applications in designing the foundation of superstructures, stability analysis, seepage studies, as well as in strengthening and rehabilitation of hydraulic structures.

Advantages

The main purpose of borehole logging is to obtain more information about the sub-surface than can be obtained from drilling, sampling and testing (US Army Corps of Engineers, 1995). It provides a continuous quantitative set of data and the volume of material investigated by most logging sondes far exceeds that of core samples. Logs can be run in all boreholes including those cased with metal or plastic casing and filled with water, brine, mud or air. This technique has an added advantage that log data is repeatable over varying periods of time and comparable when measured with different equipments.

These features that make the borehole logging technique very important and useful tool in rehabilitation of dams and can also provide information about the health of the hydraulic structure. Borehole logs can be interpreted to determine the lithology, resistivity, bulk density and porosity, compressional and shear wave velocities (Vp & Vs), moisture content, water bearing strata and movement of water (Scott Key, 1971). Based on the parameter to be measured, borehole logging is classified into various types as tabulated below (Table-1).

Electrical Resistivity Logging

An electric log is a continuous record of the electrical properties of the material in and around the borehole. Electric logging is performed in the uncased portion of a borehole by passing electric current through electrodes in the probe, or sonde and out into the borehole and the geologic medium. Other electrodes located on the surface or in the borehole complete the circuit to the source and recording device. Electric logging surveys are efficient and cost effective because the process is automated and several electrical properties are measured simultaneously by integrating several electrode configurations in the same tool. A single electrical resistivity probe can thus simultaneously record i) Self Potential (SP) Log, ii) Single Point Resistance (SPR) Log, iii) Short Normal Log and iv) Long Normal Log. Electric logging techniques can be used in geotechnical investigations to assess the geological materials and associated fluid variation as a function of depth. Electric logs from two or more boreholes are used to correlate and determine the continuity of geologic strata or zones which have similar electrical properties.

Nuclear Logging

Gamma-Gamma (Density) Logging

Gamma-gamma logs are records of the intensity of gamma radiation generated from a gamma source in the probe after it is back scattered and attenuated within the borehole and surrounding medium. The gamma-gamma probe consists of a 50 mCi (milli curie), 137Caesium gamma source and a thallium activated Sodium Iodide crystal as detector. When the probe is lowered into the borehole, gamma rays collide with the electrons in the formation and loose energy by a phenomenon commonly known as Compton Scattering. These back-scattered gamma rays are detected and recorded. Gamma radiation attenuation is considered to be proportional to the bulk density of material it passes through which is measured (Keys, 1990).

Neutron-neutron (Porosity) Logging 

Neutron logs are used principally for delineation of porous formations and determination of its porosity. In neutron logging, neutrons are artificially introduced into the formation and the effect of the environment on the neutrons is measured. The neutron interaction with the subsurface material measures the amount of hydrogen present, which is a direct indication of water content (Keys, 1990). The neutron probe has a 1 ci 241Am-Be source and 3He detector. Fast neutrons emitted by the source, loose energy by elastic collision with the nuclei of formation material and are captured by atoms of chlorine or hydrogen which is thus an indication of porosity.

Caliper Logging

Caliper logs have three arms, separated by 120° to each other and provide a continuous record of borehole diameter. This log is thereby essential to guide in interpreting other logs affected by changes in borehole diameter and therefore detect the presence of caving, if any, in the borehole. (Keys, 1990).

Acoustic / Sonic Logging 

The acoustic or sonic logging is used to determine the compressional and shear wave velocities of the formation adjacent to the borehole. Acoustic logging is undertaken in water filled borehole or below the water table in the borehole. The sonic probe uses dual-transmitter dual-receiver array to provide high quality data. The slowness, which is the reciprocal of the velocity, is actually measured by the sonde, from which the velocity of propagation of the compressional waves of the body of the dam formation can be estimated. The dynamic properties of the material surrounding the borehole are obtained from nuclear density and sonic logs. The P-and S-wave velocities obtained can be used directly to calculate the Poisson’s ratio, Young’s modulus, and Shear modulus that are useful in adapting measures for strengthening of dams.

Tracer Techniques

The non-destructive tracer method is widely used as a cost effective technique to detect seepage zones, seepage path, seepage loss, movement of water etc. qualitatively and quantitatively. The major objectives of using tracer technique in geotechnical studies is to determine, seepage in dams, location of seepage entry zones, delineating seepage path, assessing the efficiency of remedial measures, etc. Tracers are broadly classified into two groups: (i) Conventional tracers and (ii) isotope tracers including stable isotopes viz 13C, 2H, 18O, environmental isotopes viz. tritium, and unstable isotopes like 3H, 51Cr, 60Co etc. (Gaspar et. al, 1972). Tracer techniques help to identify the part/group of molecules as they pass through the system by injecting a predetermined quantity of tracer into the borehole or suspected entry points in the upstream face of the dam and monitoring its arrival in the leakage points towards the downstream side of the dam. The choice of tracer is important as it must behave like the material to be traced but still distinguishable from it for purposes of detection (Gasper et al, 1972). Among fluorescent tracers, Sodium fluoroscein (Uranine) and Rhodamine are widely used. A combined use of these techniques, give a better understanding of the sub-surface, thereby enabling the application of suitable remedial measures to reduce seepage and thus enhance dam safety.

Instrumentation

In Central Water and Power Research Station (CWPRS), Pune, India, borehole logging is carried out using a well logging unit imported from UK, which consists of a high speed data acquisition unit (micro logger), winch, gamma-gamma, neutron-neutron and caliper probes, a notebook PC with data acquisition and processing software. The micro logger is an interface that communicates with sonde and host notebook PC. The WINLOGGERTM software designed to function with the micro logger is used for acquisition of data. The VIEWLOGTM software can later aid in processing the data.

In CWPRS, the tracer studies are conducted using a laboratory fluorometer, TD-700, imported from USA. In this device, the ultraviolet light from a lamp is passed through an excitation filter, which transmits light of a specific wavelength to the sample compound being measured. The light passes through the sample, which emits light proportional to the intensity of the exciting light. The emitted light is then passed through an emission filter, which selects the appropriate wavelength which can then be detected by a photo-multiplier tube, as a readout device which indicates the presence of tracer material.

CWPRS, Pune, India is one of the pioneering organizations in the country that has been conducting comprehensive assessment of the distress in hydraulic structures, its diagnosis and suggesting repair measures and methodology for repairs/ strengthening of damaged/distressed dams. Using a multidisciplinary approach, an integration of methods has been successfully applied in various projects to strengthen /rehabilitation of distressed hydraulic structures in India and some of the published success stories are depicted below.

One such example where these studies were conducted was at Bhama-Askhed Dam, Maharashtra (Kamble et al, 2011). Nuclear Logging and tracer studies were conducted to delineate the weak zone and ascertain the seepage path responsible for oozing of water and damage to the chute channel portion. The results of nuclear logging enabled in the successful identification of weak zones in foundation corresponding to the presence of red breccia. Interconnection between the red breccia in the dam foundation and oozing of water in chute channel portion was further confirmed by tracer studies. Suitable remedial measures were recommended for treatment of the red breccia zone and it was also suggested to provide drainage holes in the chute channel area to release uplift pressure for its proper functioning.

Another example of Kolkewadi Dam, Maharashtra depicts the use of nuclear and acoustic logging studies to determine the insitu dynamic properties namely – in-situ bulk density, compression and shear wave velocities, Poisson’s ratio, shear and Young’s modulus of different UCR proportions in the masonry of dam. They were utilized for adopting strengthening of the dam.

Thus, the application of these techniques described here enables the assessment of health of hydraulic structures economically, before implementation of expensive remedial measures. These studies help to delineate zones of excessive seepage through foundation and the structure. Different parameters measured by these techniques serve as input parameters for planning dam safety and rehabilitation. As such, conducting these said studies prior to taking up of remedial measures can pinpoint the area / zone of treatment and thus provide cost effective remedial measures. In addition, these techniques also have the added advantage of being used to study the efficacy of the remedial measures adopted.

References

  • CWPRS Technical Memorandum on “Seepage Mapping Methods in Dams and Canals”, 2015, pp 1-65.
  • Gaspar E, Onescu M (1972), Radioactive tracers in hydrology. Elsevier Publications, Bucuresti.
  • Kamble R. K., Panvalkar G. A. and Chunade A.D. (2009),” Evaluation of Dynamic Properties of the Masonry By Sonic And Nuclear Density Logging At Kolkewadi Dam”, 7th International R & D Conference on Development and Management of Water and Energy Resources, CBIP, Orissa.
  • Kamble R. K., Panvalkar G. A. and Chunade A.D. (2011), Mapping seepage in the tailrace channel, Bhama-Askhed dam: A case study, Bulletin of Engineering Geology and Environment, Vol. 70, No. 4, 643-649
  • Keys W.S., 1990, Borehole Geophysics applied to groundwater investigations USGS.
  • Scott Key W., Mac Cary L.M., 1971, Application of Borehole Geophysics to Water Resources Investigations, USGS, pp 124.
  • US Army Corps of Engineers 1995 Geophysical Exploration for Engineering and Environmental Investigations, ASCE Press, 1998, pp 204.

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