Application of geophysical methods in a dam project: Life cycle perspective and Taiwan experience
Journal of Applied Geophysics 158 (2018) 82–92 Contents lists available at ScienceDirect Journal of Applied Geophysics journal homepage: www.elsevier.com/locate/jappgeo Application of geophysical methods in a dam project: Life cycle perspective and Taiwan experience Chun-Hun Lin a, Chih-Ping Lin b,⁎, Yin-Chun Hung c, Chih-Chung Chung d, Po-Lin Wu b, Hsin-Chan Liu b a Department of Marine Environment and Engineering, National Sun Yat-sen University, Kaohsiung, Taiwan Department of Civil engineering, National Chiao Tung University, Hsinchu, Taiwan Department of Urban Planning and Landscape, National Quemoy University, Kinmen, Taiwan d Department of Civil Engineering, National Central University, Zhongli, Taiwan b c a r t i c l e i n f o Article history: Received 28 March 2018 Received in revised form 27 July 2018 Accepted 27 July 2018 Available online 29 July 2018 Keywords: Dam safety Engineering geophysics TDR ERT Surface wave Seismic tomography a b s t r a c t There is a growing demand for using non-destructive geophysical techniques to internally image dam condition and facilitate the early detection of anomalous phenomena Near surface geophysical techniques have advanced significantly in the last few decades, and can play a significant role in the siting, construction, and operational safety and sustainable management of dams Application of engineering geophysics in site characterization during feasibility investigation phase is already part of the standard of practice This paper introduces newer applications of engineering geophysics during construction phase, dam safety assessment, and sustainable management, including quality control of compacted soils, investigation of abnormal leakage in an earth dam, evaluation of an aged concrete dam, geophysical health monitoring for a newly-constructed dam, and monitoring of sediment transport for sediment management The applications were presented with more emphases on the needs of dam engineering and adapting appropriate geophysical methods to make assessment more effective and consequential The collage of these case studies is to broaden the view of how geophysical methods can be applied to a dam project throughout a dam's life cycle and strengthen the linkage between geophysical surveillance and engineering significance at all stages © 2018 Elsevier B.V All rights reserved Introduction With growing population and higher demand for clean water, the number of dams has increased considerably during the last century In addition to the number of dams, increased heights and larger reservoir volume are common around the world The purpose of a dam is to retain water for societal benefits such as: flood control, irrigation, water supply, energy generation, recreation, and pollution control A great percentage of dams are located near densely populated areas Although many benefits are gained from dams, the potential threats to public safety and welfare cannot be ignored The failures of Spain's Puentes Dam in 1802, the U.S Teton Dam in 1976, and Brazil's Germano mine tailing dam in 2015 represent examples of the life threatening consequences resulting from unexpected or unrecognized dangers associated with dams, as well as serve as a reminder of the importance of a robust dam safety program These high-profile failures resulted in stricter, more prescriptive, regulatory procedures to better ensure safety during the dam's service life A dam project can be divided into three phases: feasibility and planning (Phase I), construction (Phase II), and operation (Phase III) For each phase, conceptual failure modes and risk ⁎ Corresponding author E-mail address: cplin@mail.nctu.edu.tw (C.-P Lin) https://doi.org/10.1016/j.jappgeo.2018.07.012 0926-9851/© 2018 Elsevier B.V All rights reserved assessment have been developed Site investigation during feasibility and planning study, quality control/assurance during construction, monitoring programs and regular safety evaluation during operation have been standardized to ensure public safety against risk of dam failure Nonetheless, engineering geophysics can supplement these safeguards by enhancing the technical and economical effectiveness of the resource management and safety throughout a dam's life cycle Application of engineering geophysics for Phase I site characterization was recognized as early as 1928, when I.B Crosby and E.G Leonardon used electrical methods to map high-resistivity bedrock for a proposed dam site (Burger et al., 2006) Since then, geophysical methods have become part of the investigation program for potential dam sites Further growing of geophysical applications on dam mainly focuses on the Phase III after the dam is completed Typical dam safety surveillance uses visual inspection, along with limited support from geotechnical measurements However, dams are massive structures and their internal hydraulic conditions may require attention before problems are detected by simple reconnaissance methods Visual inspections not provide information inside the dam, while the discrete monitoring instruments provide engineering parameters with limited spatial coverage of the dam There is a growing demand for non-destructive geophysical techniques to internally image the dam for early detection of anomalous phenomena and facilitating remedial actions 83 C.-H Lin et al / Journal of Applied Geophysics 158 (2018) 82–92 (Lee and Oh, 2018; Dai et al., 2017; Voronkov et al., 2004; Lum and Sheffer, 2005) Nonetheless, the linkage between geophysical surveillance and engineering significance needs further strengthening For Phase II during construction phase and sustainable management of Phase III, geophysical methods receive less attention All three phases in the life cycle of a dam are equally important, and geophysical methods can play an equally important role in all three phases Near surface geophysical techniques, such as time domain reflectometry, travel time velocity tomography, electrical resistivity tomography, and multi-station analysis of surface wave, have advanced significantly in the last couple of decades within the scientific community (Lin et al., 2015) Dam is a feat of engineering Application and adaptation of these methods on dams are of great interests to engineers A better understanding of the connections between geophysical results and engineering significance related to dam safety and sustainability can help engineers gain more useful information when employing these technologies This paper aims to broaden the view of how geophysical methods correct be applied to a dam project throughout a dam's life cycle and strengthen the linkage between geophysical surveillance and engineering significance at all stages A worldwide overview of geophysical applications on dams is first given It is followed by a collage of our studies in Taiwan to shed light on critical issues of future applications and creative developments from the perspective of dam's life cycle These studies are mostly newer applications of the engineering geophysical methods on dams, focusing mainly on construction and operation phases, including quality control of compacted soils, investigation of leakage in an earth dam, evaluation of an aged concrete dam, geophysical monitoring program in a newly-constructed dam, and geophysical monitoring for reservoir sediment management Worldwide overview of geophysical applications on dams Besides the site characterization in Phase I, the most frequent applications of geophysical methods in dam engineering are undoubtedly for dam safety assessments Many case studies from Asia, America, and Europe are gathered and listed in Table It is not meant to be comprehensive as many case studies were not published by dam owner's choice More cases were found in Asia simply because we have access to some project reports that are not published in journal or conference papers Nevertheless, Table shows the application trend and major problems to which geophysical methods can be applied Among these cases, it can be seen that abnormal seepages in earth dams draw the most attention of geophysical groups Internal erosion is the top safety concern in earth dams and abnormal seepage is the observable symptom as a result of it However, depending on the source and seepage path, not all the abnormal seepages are resulted from internal erosion Electrical methods such as electrical profiling, electrical resistivity tomography (ERT) and self potential (SP) method have been recognized as water-sensitive technologies and used to investigate the spatial distribution of wetted area and possible flowing paths (Song et al., 2005; Rozycki et al., 2006; Cho and Yeom, 2007; Panthulu et al., 2001; Sjödahl et al., 2005; Taiwan Power Company, 2009; Al-Fares, 2011; Engemoen et al., 2011; Moore et al., 2011; Karastathis and Karmis, 2012; Ikard et al., 2014; Lin et al., 2013; Mooney et al., 2014; Loperte et al., 2015; Camarero and Moreira, 2017; Dai et al., 2017; Yılmaz and Köksoy, 2017; Sentenac et al., 2018) The other major application is investigation of the cracks or voids in dams The cracks or voids in dams create preferential flowing paths susceptible to further erosion evolution Ground penetrating radar (GPR) and seismic tomography (ST) are popular technologies for such purpose If the voids or cracks are close to the surface, GPR may be an effective tool to quickly map their locations and depths (Xu et al., 2010; Li and Ma, 2011) On the other hand, if the voids or cracks are too deep, ST is a good alternative (Kepler et al., 2000) It is difficult to detect small voids or cracks by seismic methods The concept of applying ST here is not to directly locate them, but to search for low velocity anomalies caused by the diffraction around the voids or cracks The diffraction would increase the ray path and hence reduce the estimated velocity In Table 1, more applications of ST can be found in dealing with the strength of concrete dams (Hsieh et al., 2012; WRA, 2012) or seepage in dams (Karastathis and Karmis, 2012; Dai et al., 2017) Comparing to the application of engineering geophysics for site characterization in Phase I, the more complex conditions in dams, such as trapezoidal topography, zoned layers, and different targeting problems, inspire more creative and advanced applications For example, Cho and Yeom (2007) proposed a method named crossline resistivity tomography to investigate possible flowing path in a horizontal plan showing spatial distribution from upstream to downstream; Moore et al (2011) applied a trial and error inversion of SP to study possible vertical flowing path of seepage Furthermore, pushing geophysical Table World wild case studies of applying geophysical methods in dam safety assessments Area Name of the dam Aim of investigation Applied geophysical method Reference Asia Asia Asia Asia Asia Asia Asia Asia Asia Asia Asia Asia Asia America America America America America Europe Europe Europe Europe Europe Europe Europe Shuishe Dam Hsinshan Dam Wushantou Dam Shigang Dam Xishi Dam Sandong Dam Unkonwn Dam in Korea Afamia B Dam Som-Kamla-Amba Dam Nanshui Dam Sanqingting Dam S Dam Akdeğirmen Dam Dana Lake Dam Amistad Dam Barker Dam Avon Dam Cordeirópolis Dam Mornos Dam CHB Dam EI Tejo Dam Hällby Dam IJKdijk test Dam Monte Cotugno Dam Vitineves Dam Abnormal seepage in downstream shell Abnormal seepage in downstream shell Slips of slope in downstream shell Strength of concretes after chichi earthquake Aging concrete Abnormal seepage in dam abutment Abnormal seepage in downstream shell Abnormal seepage in dam foundation Abnormal seepage in downstream shell Voids inside dam body Cracks in dam body Seepage in dam foundation (grout curtain) Abnormal seepage in downstream shell Abnormal seepage in dam body Abnormal seepage in dam foundation Cracks in dam body Abnormal seepage in downstream toe Abnormal seepage in dam body Abnormal seepage in dam body Abnormal seepage in dam body Abnormal seepage in dam body Abnormal seepage in dam body Internal erosion in dam body Abnormal seepage in dam body Abnormal seepage in dam body 2D & 3D ERT Time-lapse 2D ERT MASW Seismic tomography Seismic tomography 2D ERT; SP 2D ERT EM; Electrical profiling; 2D ERT SP; Electrical profiling GPR GPR Crosshole ERT; Seismic Tomography 2D ERT SP; electrical profiling 2D ERT Seismic Tomography SP; 2D ERT 2D ERT 2D ERT; Seismic Tomography SP; 2D ERT SP 2D ERT monitoring SP; Acoustic emission 2D ERT monitoring EM; 2D ERT; SP Taiwan Power Company (2009) Lin et al (2013) Taiwan Chia-Nan Irrigation Association (2006) Hsieh et al (2012) Water Resource Agency (2012) Song et al (2005) Cho and Yeom (2007) Al-Fares (2011) Panthulu et al (2001) Xu et al (2010) Li and Ma (2011) Dai et al (2017) Yılmaz and Köksoy (2017) Moore et al (2011) Engemoen et al (2011) Kepler et al (2000) Ikard et al (2014) Camarero and Moreira (2017) Karastathis and Karmis (2012) Rozycki et al (2006) Rozycki et al (2006) Sjödahl et al (2005) Mooney et al (2014) Loperte et al (2015) Sentenac et al (2018) 84 C.-H Lin et al / Journal of Applied Geophysics 158 (2018) 82–92 methods from investigation to monitoring is another great effort to further bring geophysics into dam engineering, be it active or passive methods (Sjödahl et al., 2005; Lin et al., 2013; Mooney et al., 2014; Loperte et al., 2015) All the cases in Table are related to dam safety assessment of Phase III For Phase II during construction phase and sustainable management of Phase III, geophysical methods receive less attention Moreover, there is still a lot of room for further strengthening the linkage between geophysical surveillance and engineering significance In the following, a few of our recent studies in Taiwan are introduced to shed light on critical issues of future applications and creative developments from the perspective of dam's life cycle Geophysics in construction phase: quality control of compacted soils The compacted earthen dam is the most common type of dam worldwide Quality control and quality assurance of field compaction relies mainly on measuring the dry density and gravimetric water content of the compacted soil The well accepted procedure utilizes the oven-dry method [ASTM D2216, 2011] for water content measurements and the sand-cone method [ASTM D1556, 2011] for total density measurement Dry density is then calculated from the water content and total density, making the entire procedure highly time consuming The nuclear gauge later became available and more commonly used for making such measurements, because it is rapid and thus does not delay the construction activities However, due to its regulatory restrictions and concerns over the safety and overhead of using a device with a nuclear source, there has been increased efforts to find possible alternatives to the nuclear gauge for compaction quality control The compactness of a compacted soil is characterized by void ratio, which is equivalent to dry density considering specific gravity of soil grains is almost a constant The moisture content affects the compaction efficiency and soil structure, which has important implication on hydaulic conductivity In quality control of compacted soils, the goal is to be able to quickly measure water content and dry density (or void ratio) simultaneously Geophysical properties such as dielectric constant, electrical conductivity, shear wave velocity, and thermal conductivity are all related to the composition of compacted soils (Rathje et al., 2006) Among those, only the dielectric constant has strong relationship with volumetric water content of soils that is relatively independent of soil types At least one more measurement is required to simultaneouly determine the two parameters (gravimetric water content and dry density) Other parameters such as shear wave velocity, electrical conductivity has good correlation with void ratio or dry density However, this correlation is affected by water content and soil type, making quantitative estimation difficult (Rathje et al., 2006; Lin et al., 2012) Dielectric constants of soils in the field can be measured by time domain reflectometry (TDR) technique (Topp et al., 1980; Lin, 1999) It is based on transmitting an electromagnetic pulse through a leading coaxial cable to a sensing waveguide and recording reflections of the transmission due to changes in characteristic impedance along the sensing waveguide The sensing probe is designed such that there is an apparent impedance mismatch at the start and end of the probe TDR probes for laboratory and field measurements of compacted soils are shown in Fig The round-trip travel time of the EM pulse in the sensing waveguide of known length is determined from the arrival times of the two reflections Propagation velocity of the EM pulse can then be calculated that determines the dielectric permittivity of the material surrounding the probe The dielectric constant of a soil is dominantly affected by its volumetric water content For immediate application, TDR is used to accelerate current standard sand-cone method, which is usded to determine “total” density of top soils A new method named S-TDR method was proposed Combining the total density from sand-cone method and volumetric water content from TDR, the dry density and gravimetric water content can be measured from the S-TDR method Fig shows a typical result of field tesings at the construction site of Hushan earth Fig TDR probes and illustration of their associated electrical potentential distribution: (a) for measurements in compaction mold and (b) for field measurements dam in central Taiwan The testings were performed on the compacted silty sand during the construction of dam shell The result shows that the gravimetric water content and dry density obtained by the S-TDR method are both within 1% of the the standard conventional method (oven dry method for water content and sand-cone method for density) The difference is smaller than the expected variation of the standard conventional method, supporting the S-TDR mehtod as a quick alternative to the conventional method for compaction quality control Seismic surface wave testing has become a convenient method for measuring shear wave velocity profile non-destructively (Xia et al., 1999) Although shear wave velocity of a soil is affected not only by dry density, but also by water content (i.e., matric suction) and soil type (Cho and Santamarina, 2001), it can be used to quickly scan the compacted area for potential problematic spots of insufficient compaction for further quantitative testing by the S-TDR method Each compaction lift is typically 30 cm, accouting for the effective depth of compaction energy A mini surface-wave testing was experimented for obtaining shear wave velocity within top 30 cm of the compacted soil Successful results were obainted by a small cone-shape hammer as the impact source and a short geophone spread consisting of 12 4.5-Hz geophones with cm interval, as shown in Fig 3(a) and (b) The phase velocities of Rayleigh wave in the frequency range between 500 Hz and 1200 Hz were obtained by the dispersion analysis, as shown in Fig 3(c) Fig 3(d) shows the corresponding wavelengths between 10 cm and 25 cm are within the targeted compaction lift Shear wave velocity of the compaction lift can be directly estimated from the averaged Rayleigh wave velocity without inversion since it was relatively uniform The mini surface-wave testing was shown to be a convenient method for scanning the lateral variability of shear wave velocity for each compaction lift Research into broadband dieletric spectroscopy and multi-physical data fusion is currently pursuing an approach for determining water content and dry density simultaneouly and fully non-destructively Significant progress has been made for dielectric spectroscopy using practical TDR probes for both laboratory and field measurements (Lin et al., 2018) Dielectric spectra and shear wave velocities as a function of soil physical properites are under investigation 85 12 (a) +1% 11 10 -1% 8 10 11 12 Oven dry water content, % Measured dry density, g/cm3 Measured water content, % C.-H Lin et al / Journal of Applied Geophysics 158 (2018) 82–92 2.1 (b) 2.05 +1% 1.95 -1% 1.9 1.9 1.95 2.05 2.1 Dry density from conventional method, g/cm3 Fig (a) Comparison of S-TDR measured water contents in the field with oven dry water content; (b) Comparison of S-TDR measured dry densities in the field with dry densities from sand cone method Geophysics in operation phase: dam safety and management 4.1 Investigation of dam safety problems Uncontrolled seepage is one of the most concerned problem in earth dams Zoned drains are fundamental elements of earthen dams designed to control seepage through the embankment However, preferential flow paths may develop inside the dam that initiate abnormal seepage pathways Inappropriate treatment to an abnormal seepage may evolve into piping (i.e., internal erosion) of the embankment material, ultimately causing dam failure Several successful cases in applying electrical resistivity tomography (ERT) and self-potential method for seepage investigations have been reported (e.g., Oh et al., 2003; Sjödahl et al., 2005; Kim et al., 2007; Bièvre et al., 2017 among others) Most case studies show single temporal and spatial snapshot measurements with the ERT results used simply to qualitatively support a known situation, or provide an untested hypothesis for potential causes or scenarios Interpreting the results of ERT data in their correct context can be challenging, because earth resistivity is affected by many hydrophysical properties, including water content (or saturation), porosity, soil composition, and cementation (Lin et al., 2012) Although resistivity anomalies can be detected if they are of significant size and contrast relative to the background, it is often inconclusive regarding the engineering significance of these anomalies Furthermore, the topography and zonation of different materials may complicate the ERT survey and interpretation The resistivity of neighboring zone of different material and the topology change on the two sides of the survey line may cause 3D effects on the 2D inverted resistivity section right Fig (a) Receivers, (b) mini source, (c) seismogram in time-space domain, (d) dispersion image in frequency-velocity domain (white line showing the dispersion curve), and (e) dispersion curve in terms of wavelength vs phase velocity for a mini surface wave testing on compacted soils 86 C.-H Lin et al / Journal of Applied Geophysics 158 (2018) 82–92 beneath the line Therefore, this subsurface imaging tool should be used with caution acknowledging above factors Fig 4(a) shows an example of ERT surveys to investigate abnormal seepage in Hsinshan earth dam (Lin et al., 2013), whose main cross section is shown in Fig 4(b) The original dam has a sloping core with the crest at EL 75 m The original shell is composed of clayey sand with low permeability close to the core Therefore, the original dam is essentially a homogeneous dam with toe drain With increasing demand in water supply, it was raised by adding a core tipping further downstream and a vertical drain was added aside the new core and on top of the original downstream face A downstream shell was added to stabilize the new structure As the water level was raised over the old crest (EL 75 m), water was found seeping out the downstream face at several spots, as indicated Fig 4(b) From the result of ERT surveys (Line A on the dam crest and Line B on the downstream face along an access road) in Fig 4(a), two low resistivity zones were identified which are likely related to the underground pathways of abnormal seepage To further understand the possible mechanism, it is necessary to integrate geophysical results with geotechnical data According to the groundwater monitoring, the estimated phreatic line is still much lower than the identified low resistivity zones and the abnormal leakage spots on the downstream face Therefore, the steady-state seepage through the dam is not directly responsible for the abnormal seepage The results of leakage monitoring reveal high correlation between the precipitation and flow rate of abnormal leakage From these observations, it was hypothesized that some dirty layer (with lower hydraulic conductivity) may have existed and trapped the rainfall infiltration to cause the perched low resistivity zones The perched water migrates horizontally along the confining boundary to an exit point on the downstream face The one-time ERT surveys could not fully support the hypothesis since the resistivity values are affected not only by soil moisture but also by soil types The ERT investigation could be augmented by time-lapse measurements to provide the variation of subsurface resistivity in direct and unique response to change in soil moisture, whose relationship with reservoir water level and precipitation can be examined The ERT survey at Line B was repeatedly conducted once a month for one year Qualitatively, the inverted resistivity profiles at different times not show significant change and the results not seem to provide additional information However, more quantitative interpretation can be made by correlating the resistivity variation with its influencing factors (i.e the reservoir water level and precipitation) The Zone (with low resistivity) and Zone (with high resistivity) marked in Fig 4a were considered to represent an abnormal soaked zone and a normal permeable zone, respectively The time variation of average resistivity in these two areas is plotted in Fig to show its relationship with the reservoir water level and precipitation in terms of two-week accumulated rainfall prior to each ERT measurement The reservoir water level was relatively stable during the monitored period While there was a significant variation of resistivity value in the high resistivity zone in response to remarkable precipitation variation, the resistivity value in the low resistivity zone remained relatively constant The former is a normal behavior in a homogenous permeable shell, where rainfall infiltration seeps through the shell and drains to the filter beneath, causing the resistivity to decrease during the infiltration and then increase as the seepage drains out The latter further supports the hypothesis that the low resistivity zones are nearly saturated areas with perched water This example demonstrated that time-lapse ERT, together with monitored precipitation and water level, can provide additional strong information if the relationship between resistivity and hydrological factors is quantitatively analyzed Geophysical methods can be utilized to evaluate concrete dam as well Old concrete dams face different type of problems, as illustrated by an investigation conducted in northern Taiwan It is a concrete gravity dam with more than 90 years of service Schmidt hammer and uniaxial strength tests performed on cored samples from the downstream face indicated the strength of surface concrete is below regulatory limits The condition inside the massive dam body is unknown Seismic tomography (Lehmann, 2007) testing was used to assess the internal strength of the concrete dam Five P-wave travel-time tomography sections were conducted as shown in Fig Impact sources by rubber mallet were generated on the downstream face, and the generated waves were received by 28 Hz hydrophones attached to the upstream side Both seismic source and receivers were spaced at m interval L1-L3 are vertical cross sections, whereas H1 and H2 are horizontal ones slightly inclined to the downstream Fig shows that most P-wave (a) Line A Zone Line B Zone (b) Fig (a) ERT results of Line A at dam crest and Line B on downstream face; (b) The low resistivity zones from ERT and hydraulic heads from piezomters on the dam cross section near abnormal leakage spots (EL 46 m) and (EL 62 m) C.-H Lin et al / Journal of Applied Geophysics 158 (2018) 82–92 87 Fig Time-lapse resistivity in relation with reservoir water level and precipitation in (a) the low resistivity zone (Zone in Fig 4a) and (b) the high resistivity zone (Zone in Fig 4a) velocities inside the dam body were between 3.0–4.2 km/s Low velocity spots (below 3.0 km/s, which is ranked as in poor condition according to Whitehurst, 1951), concentrated mainly on the downstream face No obvious weak zone extended significantly into the dam interior This is a successful example showing geophysical exploration can play consequential role in the dam safety management 4.2 Geophysical monitoring of dam health The application of geophysical methods for dam safety can be extended from a single investigation survey to a regular monitoring program As shown in the previous case study, time-lapse geophysical measurements are appealing for process monitoring of the dam behavior Construction of a large embankment dam for the Hu-Shan Reservoir in Taiwan was completed in 2016, providing a unique opportunity for geophysical monitoring of the initial water filling phase of the reservoir as a baseline for future performance The reservoir consists of three zoned earth dams with 614.5 m, 393 m, and 648 m in length, respectively, and a maximum height of 75 m A geophysical monitoring program was devised for the new dam that includes electrical resistivity tomography (ERT), self potential (SP), and multichannel analysis of surface wave (MASW), as shown in Fig The dominant potential failure mode for an earth dam is seepage-related problems, justifying the use of ERT and SP MASW was used to measure the dynamic property (i.e shear-wave velocity) for the analysis of dynamic response and evaluating the strength condition of the stabilizing downstream shell Since the Fig (a) Field configuration of seismic tomography field testing at a concrete dam; (b) Cross-sectional schematic of source and receiver layout for L1~L3 88 C.-H Lin et al / Journal of Applied Geophysics 158 (2018) 82–92 Fig Fence diagram of the seismic tomography results purpose of the geophysical monitoring is to evaluate potential anomalies in any location of the dam, the whole extent of the dam in the longitudinal direction are covered by 2D ERT survey lines both on the crest (to cover mainly the core) and downstream shell Only one transverse survey line at the deepest section is planned to provide cross-sectional information on seepage behavior during water filling Other transverse investigation may be warranted should any anomalous spots on the longitudinal section be found 2D surveys assume the ground condition perpendicular to the survey line is homogeneous This assumption is apparently violated when conducting 2D ERT surveys in the longitudinal direction of the dam due to variation of topology and filled materials in the transverse direction The ability of 2D ERT to detect seepage anomalies under the influence of 3D effects has been investigated Dams are complex 3D structures Even the transverse survey line does not conform to the 2D condition due to abrupt elevation change of the valley 3D forward simulations were conducted for detailed planning of the survey and evaluating potential problems and resolution limitations of 2D ERT investigation on embankments The results show that the effect of change in reservoir water level can be so pronounced that the seepage anomaly is masked In order to constrain the effect of change in reservoir water level, we suggest ERT monitoring and time lapse analysis be performed under similar reservoir water level and environmental conditions (i.e., temperature and water salinity) The first stage of water filling began in May of 2016 Initial ERT measurements were collected before water filling as a baseline for future timelapse analyses The monitoring program provides a rare opportunity to make geophysical observation of the seepage process in a dam Before impoundment, the ERT and MASW surveys on the downstream shell were conducted under different weather conditions Of particular interest is the shear wave velocity variation after rainfall infiltration The largest rainfall event at the dam during the measurement period was during Typhoon Megi in 2016 Fig shows the initial shear wave velocity profile along L2 and the change of shear wave velocity after 200 mm of rainfall from the typhoon The decrease in shear wave velocity due to rainfall infiltration is rather significant and can reach over 40% comparing to the background values measured Fig The layout of geophysical monitoring program in the Hu-Shan Reservoir C.-H Lin et al / Journal of Applied Geophysics 158 (2018) 82–92 89 Fig (a) Shear wave velocity profile along L2 of Hu-Shan Reservoir and (b) the change of shear wave velocity after over 200 mm of rainfall after Typhoon Megi in 2016 during the dry season This has engineering significance in the context of dynamic response analysis The shear wave velocities used for the analysis were obtained during construction and after the dam completion before impoundment It is expected that shear wave velocity of the upstream shell will decrease due to impoundment Although the downstream shell is protected against seepage by the vertical drain, its shear wave velocity will also decrease due to rainfall infiltration This effect should be taken into account in the dynamic analysis to avoid underestimation of deformation during earthquake Geophysical methods can monitor the physical properties of a dam non-destructively Not only can the results be used for safety inspection, it can also provide more realistic parameter values for related dam analyses 4.3 Geophysical monitoring for sedimentation management While dam safety is the number one issue of dam management, a reservoir's sustainability relies on maintaining the storage capacity Unfortunately, erosion and landslides in many watersheds are aggravated due to geological weathering and climate change Sedimentation is becoming a serious problem in sustainable reservoir management worldwide Various actions are being taken to reduce the sedimentation rate in reservoirs, including watershed management to reduce incoming sediment yield, constructing bypass structures or low-level outlets for sediment pass-through to minimize sediment deposition, and removal of sediment from reservoirs by dredging Of all these measures, sediment sluicing or density current venting through low-level outlets is most cost-effective when hydrological conditions apply As illustrated in Fig 10, turbidity currents develop when water with a high sediment load enters a reservoir and plunges to the bottom, travelling through the original channel until settling near the dam in what is called a “muddy pool” (Morris and Fan, 1998) Density current venting involves the discharge of turbid sediment-laden water from a low-level outlet while surface waters remain clear or unchanged Management of these currents can drastically reduce sediment build-up at the base of a dam However, density current venting is seldom-used because density currents form only under certain hydrological conditions and the venting operation relies on surveying of a density current The monitoring of density current is where engineering geophysics can play a critical role in sediment management of reservoirs Commercial instrumentation for suspended sediment concentration (SSC) monitoring is limited by particle size dependency and measurement range A new technique based on time domain reflectometry Fig 10 Main questions defined for the SSC monitoring program in a reservoir 90 C.-H Lin et al / Journal of Applied Geophysics 158 (2018) 82–92 (b) Reflection coefficient, ρ (a) 0.2 -0.2 -0.4 -0.6 (c) x 10 10 20 30 40 30 40 -3 ρ' -1 -2 -3 Pulse 10 Pulse 20 Travel time, ns Fig 11 (a) Illustration of a TDR pulsing system; (b) Typical step-pulse waveform of the new coaxial TDR SSC probe; (c) the corresponding derivative of the waveform was developed to monitor SSC with the same basic principle of TDR water content measurement (Chung and Lin, 2011) As shown in Fig 11(a) and (b), the travel time between step-pulse reflections from the start and end of a sensing waveguide is related to the dielectric constant of turbid water, which is a function of SSC However, due to the common range of SSC in density currents, the required accuracy of SSC measurement is at least an order higher than that of water content measurement Travel time analysis in the time domain could not yield satisfactory results To determine the round-trip travel time of the EM wave in the sensing waveguide with high precision, a novel algorithm was developed with a concept borrowed from the surface wave dispersion analysis (Lin et al., 2017) By taking the derivative of the step-pulse waveform, the impulse waveform is obtained as shown in Fig 11(c) The impulse reflections from the start and end of the sensing waveguide are then extracted They can be treated as two propagating waveforms of two receivers spaced at a distance twice the probe length Applying Fourier transforms and calculating the phase shift between the two “receivers”, the frequency-dependent phase velocity can be determined At frequency higher than 100 MHz, the phase velocity was found independent of electrical conductivity and uniquely related to SSC An extensive SSC monitoring program for sediment management was recently implemented in the Shihmen reservoir, Taiwan (Wu et al., 2016) The Shihmen reservoir is one of the three major reservoirs in Taiwan that are facing serious threat of sedimentation By 2013, it has lost nearly 30% of its total storage capacity The monitoring program was initiated to understand the mechanism of sediment transport in the reservoir for planning remediation measures against sedimentation, and later expanded to provide all the information needed for sediment management, including total sediment income, total sediment discharge, the formation and characteristics of density currents, and evolution of muddy pool, as illustrated in Fig 10 Among all the monitoring stations, the most challenging are those that are mounted on floating platforms in the reservoir for capturing the behavior of density current Each monitoring station on the float consists of SSC sensing waveguides (or probes) at different water depths to survey the SSC profile The sensing waveguides are pulsed every 30 by a single TDR device on the floating platform through a multiplexer The TDR device and data acquisition system are powered by a solar panel and two batteries that last more than days without recharge Fig 12 gives an example of density current monitored by such a system during Typhoon Trami, 2013 The variation of SSC in ppm with Fig 12 SSC profile with time at several float stations during Typhoon Trami, 2013 C.-H Lin et al / Journal of Applied Geophysics 158 (2018) 82–92 depth and time was obtained at each monitoring station When data from all stations were assembled, a quasi 4D presentation of SSC distribution in the reservoir were generated, from which a great deal of realtime sediment information can be drawn The inflow to the Shihmen reservoir induced by Typhoon Trami was not particularly large The upstream monitoring stations on the right hand side show the increase in SSC at shallow depth in the early stage of the storm Later on, the formation and plunge of density current were observed The density current migrated downstream and banked up near the dam In the curved channel, the roll up of density current on the outside of bend was also observed The accumulated muddy water downstream settled slowly until the opening of the sluice tunnel that rapidly vented out the muddy pool The detailed field observation of the development and venting of density current was unprecedented This sediment monitoring system is valuable for effective sediment management It is currently realized by the TDR technique, but other more efficient waterborne geophysical survey may be possible Conclusions Near surface geophysical techniques have advanced significantly during the past few decades, including time domain reflectometry, electrical resistivity tomography, seismic travel-time tomography, and multi-station analysis of surface wave Safety and management issues in the context of dam's sustainability, and how engineering geophysics can play an important role in the decision-making process are corroborated by the case histories described in this paper The collage of these case studies is to broaden the view of how geophysical methods can be applied to a dam project throughout a dam's life cycle and strengthen the linkage between geophysical surveillance and engineering significance at all stages These case studies include quality control of compacted soils, identification of abnormal seepage pathways in an earth dam, quality evaluation of an aged concrete dam, geophysical health monitoring program for a newly-constructed dam, and monitoring of sediment transport for sedimentation management In current practice, geophysical results generally provide qualitative information More quantitative engineering interpretation and process monitoring were proposed in these case studies at various stages of dam engineering These case studies also demonstrate that, while promoting more uses of geophysical methods on dam, it is equally important to bring in the knowledge of dam engineering to make them optimally effective and consequential Acknowledgement Funding for this research was provided by the Water Resources Agency and Ministry of Science and Technology, Taiwan (NSC 100-2622-E-009-007) References Al-Fares, W., 2011 Contribution of the geophysical methods in characterizing the water leakage in Afamia B dam, Syria J Appl Geophys 75, 464–471 ASTM D1556, 2011 Standard Test Method for Density and Unit Weight of Soil in Place by the Sand-Cone Method, Annual Book of ASTM Standards ASTM International, West Conshohocken, PA ASTM D2216, 2011 Standard Test Methods for Laboratory Determination of Water (Moisture) Content of Soil and Rock by Mass, Annual Book of ASTM Standards ASTM International, West Conshohocken, PA Bièvre, G., Lacroix, P., Oxarango, L., Goutaland, D., Monnote, G., Fargier, Y., 2017 Integration of geotechnical and geophysical techniques for the characterization of a small earth-filled canal dyke and the localization of water leakage J Appl Geophys 139, 1–15 Burger, H.R., Sheehan, A.F., Jones, C.H., 2006 Introduction to Applied Geophysics: Exploring the Shallow Subsurface W.W Norton & Company (600p) Camarero, P.L., Moreira, C.A., 2017 Geophysical investigation of earth dam using the electrical tomography resistivity technique Rem Revista Escola Minas 70 (1), 47–52 Cho, G.C., Santamarina, J.C., 2001 Unsaturated particulate materials—particle level studies J Geotech Geoenviron 127 (1), 84–96 Cho, I., Yeom, J.-Y., 2007 Crossline resistivity tomography for the delineation of anomalous seepage pathways in an embankment dam Geophysics 72, G31–G38 91 Chung, C.-C., Lin, C.-P., 2011 High concentration suspended sediment measurement using time domain reflectometry J Hydrol 401, 134–144 Dai, Q., Lin, F., Wang, X., Feng, D., Bayless, R.C., 2017 Detection of concrete dam leakage using an integrated geophysical technique based on flow-field fitting method J Appl Geophys 140, 168–176 Engemoen, W., Mead, R., Hernandez, L., 2011 Evaluating the risks of an internal erosion failure at Amidtad dam Proceeding of 31st Annual USSD Conference, San Diego, California, pp 527–543 Hsieh, S.-H., Hsieh, C.-H., Hu, C.-H., Huang, J.-K., Lin, C.-M., 2012 Application of elastic wave tomography for dam safety Symposium on the Application of Geophysics to Engineering and Environmental Problems, 25th Annual Meeting SAGEEP 2012, Tucson, Arizona, USA Ikard, S.J., Revil, A., Schmutz, M., Karaoulis, M., Jardani, A., Mooney, M., 2014 Characterization of focused seepage through an earthfill dam using geoelectrical methods Groundwater 52 (6), 952–965 Karastathis, V., Karmis, P., 2012 Investigation of seepage and settlement problems at the Mornos earth dam, Greece, by geophysical methods Symposium on the Application of Geophysics to Engineering and Environmental Problems, 25th Annual Meeting SAGEEP 2012, Tucson, Arizona, USA Kepler, F.W., Bond, L.J., Frangopol, D.M., 2000 Improved assessment of mass concrete dams using acoustic travel time tomography Part II-application Construct Build Mater 14, 147–156 Kim, J.H., Yi, M.J., Song, Y., Seol, S.J., Kim, K.S., 2007 Application of geophysical methods to the safety analysis of an earth dam J Environ Eng Geophys 12, 221–235 Lee, B., Oh, S., 2018 Modified electrical survey for effective leakage detection at concrete hydraulic facilities J Appl Geophys 149, 114–130 Lehmann, B., 2007 Seismic Traveltime Tomography for Engineering and Exploration Applications (273 p) EAGE Publications Li, H., Ma, H., 2011 Application of ground penetrating radar in dam body detection Proc Eng 26, 1820–1826 Lin, C.-P., 1999 Time Domain Reflectometry for Soil Properties Purdue University (Ph.D Thesis, 229 p) Lin, C.-H., Lin, C.-P., Drnevich, V.P., 2012 TDR method for compaction quality control: multi evaluation and sources of error Geotech Test J 35 (5), 817–826 Lin, C.-P., Hung, Y.-C., Yu, Z.-H., Wu, P.-L., 2013 Investigation of abnormal seepages in an earth dam using resistivity tomography J GeoEng 8, 61–70 Lin, C.-P., Lin, C.-H., Wu, P.-L., Liu, H.-C., 2015 Applications and challenges of near surface geophysics in geotechnical engineering Chin J Geophys 58, 2664–2680 Lin, C.-P., Ngui, Y.-J., Lin, C.-H., 2017 A novel TDR signal processing technique for measuring apparent dielectric Spectrum Meas Sci Technol 28, 015501 Lin, C.-P., Ngui, Y.-J., Lin, C.-H., 2018 Multiple reflection analysis of TDR signal for complex dielectric spectroscopy IEEE Transactions on Instrumentation and Measurement (in print) Loperte, A., Soldovieri, F., Lapenna, V., 2015 Monte Cotugno dam monitoring by the electrical resistivity tomography IEEE J Sel Top Appl Earth Observ Remote Sens (11), 5346–5351 Lum, K.Y., Sheffer, M.R., 2005 Dam safety: review of geophysical methods to detect seepage and internal Erosion in embankment dams Hydro-Review 29 http://www hydroworld.com/articles/hr/print/volume-29/issue-2/articles/dam-safety-review html Mooney, M.A., Parekh, M.L., Lowry, B., Rittgers, J., Grasmick, J., Koelewijn, A.R., Revil, A., Zhou, W., 2014 Design and Implementation of Geophysical Monitoring and Remote Sensing During a Full-Scale Embankment Internal Erosion Test, 2014 ASCE GeoCongress, Atlanta, USA Moore, J.R., Boleve, A., Sanders, J.W., Glaser, S.D., 2011 Self-potential investigation of moraine dam seepage J Appl Geophys 74, 277–286 Morris, G.L., Fan, J., 1998 Reservoir Sedimentation Handbook: Design and Management of Dams, Reservoirs, and Watersheds for Sustainable Use McGraw-Hill Book Co., New York, New York Oh, Y.C., Jeong, H.S., Lee, Y.K., Shon, H., 2003 Safety Evaluation of Rock-Fill Dam by Seismic (MASW) and Resistivity Method, Proceedings of the 16th Annual Symposium on the Application of Geophysics to Engineering and Environmental Problems pp 1377–1386 Panthulu, T., Krishnaiah, C., Shirke, J., 2001 Detection of seepage paths in earth dams using self-potential and electrical resistivity methods Eng Geol 59, 281–295 Rathje, E.M., Wright, S.G., Stokoe II, K.H., Adams, A., Tobin, R., Salem, M., 2006 Evaluation of Non-Nuclear Methods for Compaction Control, Rep No.: FHWA/TX-06/0-4835-1 Center for Transportation Research, The University of Texas at Austin, Texas Department of Transportation http://www.utexas.edu/research/ctr/pdf_reports/0_4835_1 pdf Rozycki, A., Ruiz Fonticiella, J.M., Cuadra, A., 2006 Detection and evaluation of horizontal fractures in earth dams using the self-potential method Eng Geol 82, 145–153 Sentenac, P., Benes, V., Keenan, H., 2018 Reservoir assessment using non-invasive geophysical techniques Environ Earth Sci 77, 293 Sjödahl, P., Dahlin, T., Johansson, S., 2005 Using resistivity measurements for dam safety evaluation at Enemossen tailings dam in southern Sweden Environ Geol 49, 267–273 Song, S.H., Song, Y., Kwon, B.D., 2005 Application of hydrogeological and geophysical methods to delineate leakage pathways in an earth fill dam Explor Geophys 36, 92–96 Taiwan Chia-Nan Irrigation Association, 2006 The Plan of Remediation Program of Wushantou Reservoir (1st Year) (In Chinese) Taiwan Power Company, 2009 Technical Report of the Third Safety Assessment of Sun Moon Lake Reservoir (In Chinese) Topp, G.C., Davis, J.L., Annan, A.P., 1980 Electromagnetic determination of soil water content: measurement in coaxial transmission lines Water Resour Res 16, 574–582 92 C.-H Lin et al / Journal of Applied Geophysics 158 (2018) 82–92 Voronkov, O.K., Kagan, A.A., Krivonogova, N.F., Glagovsky, V.B., Prokopovich, V.S., 2004 Geophysical methods and identification of embankment dam parameters: In Proceedings of the 2nd International Conference on Geotechnical and Geophysical Site Characterization, 593–599 Water Resource Agency, 2012 Technical Report of Application of Geophysical methods in Dam Safety Evaluation (In Chinese) Whitehurst, E.A., 1951 Soniscope tests concrete structures J Am Concr Inst 47, 433–444 Wu, I.-L., Lin, C.-P., Chung, C.-C., 2016 Technical Report on Sediment Transport Monitoring Facility Maintenance and Flood Event Observations in Shihmen Reservoir Water Resource Agency, Taiwan (In Chinese) Xia, J., Miller, R.D., Park, C.B., 1999 Estimation of near surface shear wave velocity by inversion of Rayleigh waves Geophysics 64, 691–700 Xu, X., Zeng, Q., Jin Wu, D.L., Wu, X., Shen, J., 2010 GPR detection of several common subsurface voids inside dikes and dams Eng Geol 111, 31–42 Yılmaz, S., Köksoy, M., 2017 Application of electrical resistivity imaging and self-potential methods to determine leakage pathways in an earth fill dam Pamukkale University Journal of Engineering Sciences 23 (6), 799–803 ... methods in dam safety assessments Area Name of the dam Aim of investigation Applied geophysical method Reference Asia Asia Asia Asia Asia Asia Asia Asia Asia Asia Asia Asia Asia America America America... 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