Application of geophysical methods in a dam project: Life cycle perspective and Taiwan experience

Application of geophysical methods in a dam project: Life cycle perspective and Taiwan experience

Application of geophysical methods in a dam project: Life cycle

perspective and Taiwan experience

Chun-Hun Lina, Chih-Ping Linb,⁎ , Yin-Chun Hungc, Chih-Chung Chungd, Po-Lin Wub, Hsin-Chan Liub

aDepartment of Marine Environment and Engineering, National Sun Yat-sen University, Kaohsiung, Taiwan

bDepartment of Civil engineering, National Chiao Tung University, Hsinchu, Taiwan

cDepartment of Urban Planning and Landscape, National Quemoy University, Kinmen, Taiwan

dDepartment of Civil Engineering, National Central University, Zhongli, Taiwan

a b s t r a c t

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

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 dur-ing feasibility investigation phase is already part of the standard of practice This paper introduces newer appli-cations 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 surveil-lance and engineering significance at all stages

© 2018 Elsevier B.V All rights reserved

Keywords:

Dam safety

Engineering geophysics

TDR

ERT

Surface wave

Seismic tomography

1 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

conse-quences 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

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 fail-ure Nonetheless, engineering geophysics can supplement these safe-guards 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 characteriza-tion 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 in-spections do 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-de-structive geophysical techniques to internally image the dam for early detection of anomalous phenomena and facilitating remedial actions

⁎ Corresponding author.

E-mail address:cplin@mail.nctu.edu.tw (C.-P Lin).

https://doi.org/10.1016/j.jappgeo.2018.07.012

Contents lists available atScienceDirect

Journal of Applied Geophysics

j o u r n a l h o m e p a g e :w w w e l s e v i e r c o m / l o c a t e / j a p p g e o

(Lee and Oh, 2018;Dai et al., 2017;Voronkov et al., 2004;Lum and

Sheffer, 2005) Nonetheless, the linkage between geophysical

surveil-lance 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

reflec-tometry, travel time velocity tomography, electrical resistivity

tomogra-phy, and multi-station analysis of surface wave, have advanced

significantly in the last couple of decades within the scientific

commu-nity (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

geo-physical methods correct be applied to a dam project throughout a

dam's life cycle and strengthen the linkage between geophysical

sur-veillance and engineering significance at all stages A worldwide

over-view 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

fu-ture applications and creative developments from the perspective of

dam's life cycle These studies are mostly newer applications of the

en-gineering geophysical methods on dams, focusing mainly on

construc-tion and operaconstruc-tion 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

2 Worldwide overview of geophysical applications on dams

Besides the site characterization in Phase I, the most frequent

appli-cations of geophysical methods in dam engineering are undoubtedly for

dam safety assessments Many case studies from Asia, America, and

Europe are gathered and listed inTable 1 It is not meant to be

compre-hensive 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 1shows 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 symp-tom 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 to-mography (ERT) and self potential (SP) method have been recognized

as water-sensitive technologies and used to investigate the spatial dis-tribution 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 sus-ceptible to further erosion evolution Ground penetrating radar (GPR) and seismic tomography (ST) are popular technologies for such pur-pose If the voids or cracks are close to the surface, GPR may be an effec-tive 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 anom-alies 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 exam-ple,Cho and Yeom (2007)proposed a method named crossline resistiv-ity 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 ver-tical flowing path of seepage Furthermore, pushing geophysical

Table 1

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 Shuishe Dam Abnormal seepage in downstream shell 2D & 3D ERT Taiwan Power Company (2009)

Asia Hsinshan Dam Abnormal seepage in downstream shell Time-lapse 2D ERT Lin et al (2013)

Asia Wushantou Dam Slips of slope in downstream shell MASW Taiwan Chia-Nan Irrigation Association (2006)

Asia Shigang Dam Strength of concretes after chichi earthquake Seismic tomography Hsieh et al (2012)

Asia Xishi Dam Aging concrete Seismic tomography Water Resource Agency (2012)

Asia Sandong Dam Abnormal seepage in dam abutment 2D ERT; SP Song et al (2005)

Asia Unkonwn Dam in Korea Abnormal seepage in downstream shell 2D ERT Cho and Yeom (2007)

Asia Afamia B Dam Abnormal seepage in dam foundation EM; Electrical profiling; 2D ERT Al-Fares (2011)

Asia Som-Kamla-Amba Dam Abnormal seepage in downstream shell SP; Electrical profiling Panthulu et al (2001)

Asia Nanshui Dam Voids inside dam body GPR Xu et al (2010)

Asia Sanqingting Dam Cracks in dam body GPR Li and Ma (2011)

Asia S Dam Seepage in dam foundation (grout curtain) Crosshole ERT; Seismic Tomography Dai et al (2017)

Asia Akdeğirmen Dam Abnormal seepage in downstream shell 2D ERT Yılmaz and Köksoy (2017)

America Dana Lake Dam Abnormal seepage in dam body SP; electrical profiling Moore et al (2011)

America Amistad Dam Abnormal seepage in dam foundation 2D ERT Engemoen et al (2011)

America Barker Dam Cracks in dam body Seismic Tomography Kepler et al (2000)

America Avon Dam Abnormal seepage in downstream toe SP; 2D ERT Ikard et al (2014)

America Cordeirópolis Dam Abnormal seepage in dam body 2D ERT Camarero and Moreira (2017)

Europe Mornos Dam Abnormal seepage in dam body 2D ERT; Seismic Tomography Karastathis and Karmis (2012)

Europe CHB Dam Abnormal seepage in dam body SP; 2D ERT Rozycki et al (2006)

Europe EI Tejo Dam Abnormal seepage in dam body SP Rozycki et al (2006)

Europe Hällby Dam Abnormal seepage in dam body 2D ERT monitoring Sjödahl et al (2005)

Europe IJKdijk test Dam Internal erosion in dam body SP; Acoustic emission Mooney et al (2014)

Europe Monte Cotugno Dam Abnormal seepage in dam body 2D ERT monitoring Loperte et al (2015)

methods from investigation to monitoring is another great effort to

fur-ther 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 inTable 1are related to dam safety

as-sessment of Phase III For Phase II during construction phase and

sus-tainable 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

signifi-cance In the following, a few of our recent studies in Taiwan are

intro-duced to shed light on critical issues of future applications and creative

developments from the perspective of dam's life cycle

3 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

con-tent of the compacted soil The well accepted procedure utilizes the

oven-dry method [ASTM D2216, 2011] for water content

measure-ments 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

restric-tions and concerns over the safety and overhead of using a device with a

nuclear source, there has been increased efforts to find possible

alterna-tives 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

con-stant, electrical conductivity, shear wave velocity, and thermal

conduc-tivity 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

den-sity) Other parameters such as shear wave velocity, electrical

conduc-tivity has good correlation with void ratio or dry density However,

this correlation is affected by water content and soil type, making

quan-titative estimation difficult (Rathje et al., 2006;Lin et al., 2012)

Dielectric constants of soils in the field can be measured by time

do-main reflectometry (TDR) technique (Topp et al., 1980;Lin, 1999) It is

based on transmitting an electromagnetic pulse through a leading

coax-ial cable to a sensing waveguide and recording reflections of the

trans-mission 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 1 The round-trip travel time of the EM pulse in the sensing

wave-guide 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

ac-celerate current standard sand-cone method, which is usded to

deter-mine “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

gravimet-ric water content can be measured from the S-TDR method.Fig 2shows

a typical result of field tesings at the construction site of Hushan earth

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 den-sity) The difference is smaller than the expected variation of the stan-dard 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 tion for further quantitative testing by the S-TDR method Each compac-tion 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 5 cm interval, as shown inFig 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 inFig 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 pursu-ing an approach for determinpursu-ing 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

Fig 1 TDR probes and illustration of their associated electrical potentential distribution: (a) for measurements in compaction mold and (b) for field measurements.

4 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,

pref-erential 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

mate-rial, 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., 2017among others)

Most case studies show single temporal and spatial snapshot measure-ments 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 to-pography and zonation of different materials may complicate the ERT survey and interpretation The resistivity of neighboring zone of differ-ent material and the topology change on the two sides of the survey line may cause 3D effects on the 2D inverted resistivity section right

1.9 1.95 2 2.05 2.1 1.9

1.95 2 2.05 2.1

8 9 10 11

12

-1%

+1%

Oven dry water content, %

Dry density from conventional method, g/cm3

-1%

+1%

Fig 2 (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.

Fig 3 (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)

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

sec-tion is shown inFig 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

essen-tially 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

orig-inal 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 indicatedFig 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) inFig 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

leak-age 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

af-fected not only by soil moisture but also by soil types The ERT

investiga-tion 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

do not show significant change and the results do not seem to provide additional information However, more quantitative interpretation can

be made by correlating the resistivity variation with its influencing fac-tors (i.e the reservoir water level and precipitation) The Zone 1 (with low resistivity) and Zone 2 (with high resistivity) marked inFig 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 inFig 5to show its relationship with the reservoir water level and precipitation in terms of two-week accumu-lated 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 for-mer is a normal behavior in a homogenous permeable shell, where rain-fall 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 ad-ditional 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 grav-ity dam with more than 90 years of service Schmidt hammer and uni-axial 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 sec-tions were conducted as shown inFig 6 Impact sources by rubber mal-let 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 1 m interval L1-L3 are vertical cross sections, whereas H1 and H2 are horizontal ones slightly inclined to the downstream.Fig 7shows that most P-wave

(a)

(b)

Zone 1

Zone 2

Line A

Line B

1

Fig 4 (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

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

con-sequential 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 oppor-tunity 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, re-spectively, 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 sur-face wave (MASW), as shown inFig 8 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 evaluat-ing the strength condition of the stabilizevaluat-ing downstream shell Since the Fig 5 Time-lapse resistivity in relation with reservoir water level and precipitation in (a) the low resistivity zone (Zone 1 in Fig 4 a) and (b) the high resistivity zone (Zone 2 in Fig 4 a).

Fig 6 (a) Field configuration of seismic tomography field testing at a concrete dam; (b) Cross-sectional schematic of source and receiver layout for L1~L3.

purpose of the geophysical monitoring is to evaluate potential

anoma-lies in any location of the dam, the whole extent of the dam in the

lon-gitudinal 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

lon-gitudinal 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

longitudi-nal 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

limita-tions 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 measure-ments were collected before water filling as a baseline for future time-lapse 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 down-stream shell were conducted under different weather conditions Of particular interest is the shear wave velocity variation after rainfall infil-tration The largest rainfall event at the dam during the measurement period was during Typhoon Megi in 2016.Fig 9shows the initial shear wave velocity profile along L2 and the change of shear wave ve-locity 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 7 Fence diagram of the seismic tomography results.

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

comple-tion 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-destruc-tively 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

aggra-vated 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 incom-ing sediment yield, constructincom-ing 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

inFig 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 dis-charge of turbid sediment-laden water from a low-level outlet while surface waters remain clear or unchanged Management of these cur-rents can drastically reduce sediment build-up at the base of a dam However, density current venting is seldom-used because density cur-rents 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 measure-ment range A new technique based on time domain reflectometry Fig 9 (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.

Fig 10 Main questions defined for the SSC monitoring program in a reservoir.

was developed to monitor SSC with the same basic principle of TDR

water content measurement (Chung and Lin, 2011) As shown inFig

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

measure-ment 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 disper-sion analysis (Lin et al., 2017) By taking the derivative of the step-pulse waveform, the impulse waveform is obtained as shown inFig 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 “re-ceivers”, the frequency-dependent phase velocity can be determined At frequency higher than 100 MHz, the phase velocity was found indepen-dent 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 inFig 10 Among all the moni-toring 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 8 SSC sensing waveguides (or probes) at different water depths to survey the SSC pro-file The sensing waveguides are pulsed every 30 min 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 bat-teries that last more than 5 days without recharge

Fig 12gives an example of density current monitored by such a sys-tem 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.

-0.6

-0.4

-0.2

0

0.2

-3

-2

-1

0

1x 10

-3

Travel time, ns

(a)

(b)

(c)

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.

depth and time was obtained at each monitoring station When data

from all stations were assembled, a quasi 4D presentation of SSC

distri-bution in the reservoir were generated, from which a great deal of

real-time 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

for-mation and plunge of density current were observed The density

cur-rent 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

monitor-ing system is valuable for effective sediment management It is currently

realized by the TDR technique, but other more efficient waterborne

geo-physical survey may be possible

5 Conclusions

Near surface geophysical techniques have advanced significantly

during the past few decades, including time domain reflectometry,

elec-trical 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

corrobo-rated 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

signifi-cance 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

monitor-ing 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

engineer-ing 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

Integra-tion of geotechnical and geophysical techniques for the characterizaIntegra-tion 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:

Explor-ing the Shallow Subsurface W.W Norton & Company (600p).

Camarero, P.L., Moreira, C.A., 2017 Geophysical investigation of earth dam using the

elec-trical tomography resistivity technique Rem Revista Escola Minas 70 (1), 47–52.

Cho, G.C., Santamarina, J.C., 2001 Unsaturated particulate materials—particle level

stud-ies J Geotech Geoenviron 127 (1), 84–96.

Cho, I., Yeom, J.-Y., 2007 Crossline resistivity tomography for the delineation of

anoma-lous seepage pathways in an embankment dam Geophysics 72, G31–G38.

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 Characteri-zation 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 measur-ing 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 elec-trical resistivity tomography IEEE J Sel Top Appl Earth Observ Remote Sens 8 (11), 5346–5351.

Lum, K.Y., Sheffer, M.R., 2005 Dam safety: review of geophysical methods to detect seep-age 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 mo-raine 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 Depart-ment 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 geo-physical 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 (1 st 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 con-tent: measurement in coaxial transmission lines Water Resour Res 16, 574–582.