Study with the several resources on Docsity
Earn points by helping other students or get them with a premium plan
Prepare for your exams
Study with the several resources on Docsity
Earn points to download
Earn points by helping other students or get them with a premium plan
geotechnical course about tunnel
Typology: Schemes and Mind Maps
1 / 48
This page cannot be seen from the preview
Don't miss anything!
Under the aegis of: In association with: Organized by:
Central Water Commission Central Board of irrigation & Power The World Bank International Commission on Irrigation and Drainage
IBRD IDA WORLD BANK GROUP
AF Academy
Training Notes
Latest Trends in Inspection & Investigations of Dams
DAM SAFETY WORKSHOP
May 30-31, 2019, New Delhi, India
Guidelines on
Geophysical Investigation of Dams
PREFACE
Dams in the country represent a major investment and huge benefits to population in terms of irrigation, power and flood control. Most of the big dams are very old and regular monitoring and maintenance of these dams is of utmost importance for continuing benefits. Unlike soil investigations, critical nature of dams does not permit traditional invasive inspections by means of drilling, and such inspections are best avoided unless in extreme cases, when the problem is too grave.
Although long realized that dams need periodic inspection and monitoring, it has only been recently recognized that geophysical surveys can supplement the results of standard inspection SOPs and monitoring techniques. Geophysical surveys have been performed on a number of dams around the world, including in India, and have yielded extremely useful insight into dam conditions. Seismic refraction surveys have been performed to provide a cross-section of the dam embankment and foundation materials in terms of strength characteristics. Self-Potential (SP) surveys have been performed to investigate seepage conditions within dam embankment and foundation materials, and abutment materials. Electrical Resistivity Tomography is also routinely performed on dams to determine internal saturation conditions in dams. ReMi/MASW is effectively used to determine shear wave velocities (Vs) in dams. Cross-face and longitudinal section seismic tomography has been found to yield high resolution images of dam interior.
The present guideline document aims at presenting various possible applications for geophysical techniques for investigations of different types of dams. Various case studies have also been included in the document to illustrate the results that can be expected from these non-destructive investigations.
Dr. Sanjay Rana Chairman, AF Academy
In the event of a dam failure, the economic loss as well as the potential hazard to life and property could be enormous. Typical dam safety surveillance consists of visual inspections supported by limited instrumentation. However, the problems in dams can become quite advanced before the problem is detected by these means. Recently, interest has grown regarding the use of non-intrusive geophysical techniques to facilitate early detection of anomalous zones leading to seepage, piping, internal erosion and other degradation issues.
Geophysical methods are sensitive to contrast in the physical properties in the subsurface. Different methods respond to different physical properties, like material strength, material conductivity (linked to water saturation), fluid movement (seepage), change in density etc. The application of geophysical methods to dams enables detection of problems in early stages and hence can become part of dam safety surveillance program.
Geophysical techniques, by virtue of their non-invasive and non-destructive nature, offer an excellent solution for investigation or regular monitoring of dams, and detection of anomalous conditions which might snowball into major problems if left unattended and untreated.
Various geophysical methods are available to investigate following problems of earthen, masonry, concrete or composite dams:
x Leak path detection x Internal Erosion x Identification of zone of water accumulation x Cavity/ sinkhole x Concrete degradation
Applied geophysics uses physical methods, such as seismic, gravitational, magnetic, electrical and electromagnetic at the surface of the Earth to measure the physical properties of the subsurface, along with the anomalies in those properties. It has been extensively used, historically, for oil and gas exploration, closely followed by mineral exploration. The depths explored are typically in hundreds and thousands of meters. Engineering geophysics is often used in construction sites to solve a variety of geological, geotechnical, or quality control problems and helps fill gaps in geotechnical data. The basic task of engineering geophysics is to apply physics theories and methods to determine subsurface conditions for building foundations and test quality of man-made structures, like bridges, roads, dams, etc.
Geophysical methods are sensitive to contrast in the physical properties in the subsurface. Different methods respond to different physical properties, like material strength, material conductivity, change in density etc. Geophysical techniques, by virtue of their non-invasive and non-destructive nature, offer an excellent solution for site investigations. No single geophysical technique can uniquely solve the problem due to a large overlapping of physical properties in various subsurface materials. That is the reason why it becomes important to use a combination of geophysical methods to uniquely resolve the problem. Choosing the right tool/technique to address to a specific problem is critical for success of a geophysical program.
the planners, engineers and contractors, concentrates primarily on the aspects of dam issues and geophysical investigative methods, which are important to existing dams. The concepts and procedures presented here are applicable worldwide with appropriate adaptation for local conditions and methods.
The purpose of this document is to provide dam engineers with a basic knowledge of geophysical methods for planning a dam investigation program. The document is intended to provide Engineers with tools that will assist them in the use of suitable geophysical methods to evaluate various physical properties of dams.
The document also has a section on investigations of stilling basins.
3. GEOPHYSICAL METHODS
Applied geophysics can be divided into the following general methods of exploration:
x Magnetic x Electrical x Electromagnetic x Seismic x Gravitational x Radioactivity x Well logging
Each geophysical method can be used for many different applications (e.g., mining exploration, oil and gas exploration, engineering, and environmental). The division of each method is based on the physics that governs it; therefore, geophysical techniques (e.g., refraction) within each method(i.e., seismic) are designed primarily for applications of the method to a given problem (e.g., rock quality & depth).
Most of the geophysical methods listed above have corresponding IS, BS or ASTM codes, standardizing their application.
Geotechnical geophysical, non-destructive testing and non-contact tools are very similar and in some instances are identical. Physical property models are generated for specific parameters by all these types of instruments.
The major difference between these is that geophysical tools are used to investigate the earth, whereas NDT methods are used to investigate manmade structures like buildings wherein non- contact tools are used for measuring temperature, thermal imaging etc. without coming in contact with the object.
Geophysical investigations are carried out preferably prior to geotechnical investigations and are of immense value in selection of borehole locations. Such investigations, in addition, will provide reliable information about variability of sub-surface between two boreholes.
Selection of most appropriate geophysical method is a two stage process. In stage-1 potentially useful geophysical methods are identified on the basis of engineering problem, which can be done by utilizing the information available on the methods and later on in stage-II, the most suitable geophysical tools are selected based on the site specific criterion such as depth of target, required resolution, site accessibility and cost.
4. GEOPHYSICAL METHODS FOR DAM INVESTIGATION
No single geophysical technique can uniquely solve the problem due to a large overlapping of physical properties in various subsurface materials. To date, the use of geophysical methods to investigate dams has produced mixed results, partly because the application of these methods is not well-understood and partly because false positives cannot be tolerated. That is the reason why it becomes important to use a combination of geophysical methods to uniquely resolve the problem. Following table presents a range of choices available to address to various issues and concerns:
One of the methods being increasingly used in geological exploration is the electrical resistivity imaging (also termed as electrical resistivity tomography). The purpose of electrical resistivity imaging survey is to determine sub-surface resistivity distribution by taking measurements on the ground. From these measurements true resistivity of sub-surface can be estimated. Fig 1 provides an example of 2D section derived from electrical resistivity imaging survey.
The resistivity is related to various geological parameters, like, mineral and fluid content, porosity and degree of water saturation in rock. Electrical resistivity surveys have been used for many decades in geotechnical, hydrological and mining investigations. Other applications of electrical resistivity imaging test are;
i. To correlate data from resistivity survey with those obtained from borehole and trial pit logs. ii. To delineate weak formations, faults and dykes, if any and to identify locations of steeply dipping contacts between different rock types and earth material. iii. To rapidly explore the sub surface conditions to locate ground water, thickness of over burden, depth of different rock types and stereographic features. iv. Assessment of ground water potential, quality and determination of aquifer characteristics.
GEOPHYSICAL METHODS
ISSUES AND CONCERNS CONCRETE DAM EARTH EMBANKMENT DAMS MASONRY DAMS CRACKS DEGRADATION WATERLEAKS LANDSLIDE (^) HOLESSINK WATERLEAKS STRENGTH Electrical Resistivity Streaming Potential Georadar Radar Tomography Seismic Tomography Seismic Refraction ReMi
Sea Water 0. Minerals Galena 3x10 -3^ - 3x10^2 Bauxite 2 x 10 2 -6 x 10 2 Cuprite 10 -3^ - 300 Hematite 3.5 x10-3^ -10^7 Magnetite 5 x 10 -5^ - 5.7 x 10^3 Quartz 4 x 10 10 – 2x10 14 Uraninite 1 – 200 Calcite 2 x 10 12 Rock Salt 30 – 10^13 Diamond 10 – 10^14
Fig 2: Resistivity imaging instruments Fig 3: Data collection in progress
4.1.1. Basic Principle of Electrical Resistivity Imaging
Measurement of ground resistivity involves passing an electrical current into the ground using a pair of steel or copper electrodes and measuring the resulting potential difference within the subsurface using a second pair of electrodes. These are normally placed between the current electrodes. Fig 2 provides a glimpse of resistivity imaging equipment and Fig 3 shows a typical field setup for data acquisition. Figure 4 shows basic working principle of electrical resistivity imaging with data points corresponding to electrode arrangement.
Fig 4: Arrangement of electrodes
Unlike conventional resistivity sounding and lateral profiling surveys, 2D resistivity imaging is a fully automated technique that uses a linear array of number of electrodes connected by multicore cable. The current and potential electrode pairs are switched automatically using a laptop computer and control module connected to a ground resistivity meter (that provides the output current).
In this way a profile of resistivity against depth ('pseudosection') is built up along the survey line. Data is collected by automatically profiling along the line at different electrode separations. The computer initially keeps the spacing between the electrodes fixed and moves the pairs along the line until the last electrode is reached. The spacing is then increased by the minimum electrode separation (the physical distance between electrodes which remains fixed throughout the survey) and the process is repeated in order to provide an increased depth of investigation.
The maximum depth of investigation is determined by the spacing between the electrodes and the number of electrodes in the array. For a 64 electrode array with an electrode spacing of 5m this depth is approximately 60m. However, as the spacing between the active electrodes is increased, fewer and fewer points are collected at each 'depth level', until on the final level only 1 reading is acquired. In order to overcome this, the array is 'rolled-along' the line of investigation in order to build up a longer pseudo section.
The raw data is initially converted to apparent resistivity values using a geometric factor that is determined by the type of electrode configuration used. Many 2D resistivity imaging surveys are carried out using the Wenner Array. In this configuration the spacing between each electrodes is identical. Once converted the data is modelled using finite element and least squares inversion methods in order to calculate a true resistivity versus depth pseudo section.
4.1.2. Key Applications of Electrical Resistivity Imaging
Key applications of electrical resistivity imaging are as follows: i) To determine the underground water resources ii) To determine bedrock quality and depth measurements iii) Mineral prospecting iv) Dam structure analysis v) To determine landfill characteristics vi) Contamination source detection
4.1.3. Key Advantages of Electrical Resistivity Imaging
Main advantages of electrical resistivity imaging are as follows: i) Excellent 2-dimensional display of ground resistivity. ii) Delineation of small features like cavity, contamination plumes, weak zones in structures like dams etc. iii) The technique is extremely useful for investigations of important sites to get information on weak zones or buried channels, under the rock interface, which goes undetected in seismic refraction, which terminates at rock interface. iv) For deep penetration seismic refraction techniques requires use of explosives, which are not always feasible to deploy especially in sensitive areas. In such cases resistivity imaging can be effectively used to get detailed information of deeper layers. v) Resistivity imaging can also be effectively used to determine rock profile across high current shallow rivers where deployment of hydrophones is not possible restricting use of seismic refraction.
4.1.5. 3-D Resistivity Imaging Surveys
All geological structures are three dimensional in nature, hence a 3-D resistivity survey using 3-D interpretation model gives the most accurate result. With the development of multi-channel resistivity meters which enables recording of more than one measurement at a time and availability of computers the inversion of very large data sets comprising more than 8000 data points and survey grid of greater than 30m x 30m can be enabled. The pole-pole, pole-dipole and dipole-dipole arrays are frequently used for 3-D surveys because other arrays have poor data coverage near the edge of survey grid. Fig 8is an example of a slice from a 3D model, from a work carried out for cavity detection in sandstone:
Fig 8-3D Resistivity Interpretation Model
4.1.6. Use of ERI for Dam Investigations
The direct current resistivity method has well-established data acquisition and interpretation techniques for standard survey configurations. The method uses pairs of electrodes to inject current into the ground and measure the resulting electrical potential distribution. Its application to dam seepage investigations is two-fold. The method may be used to monitor spatial and/or temporal variations in electrical resistivity in response to changing soil conditions caused by internal erosion and anomalous seepage. The method also may be used to characterize the electrical resistivity of the subsurface for the purposes of interpreting SP data.
The method can also be used to detect changes in resistivity with time, which may be linked to the development of internal erosion in the core of the embankment.
The results of electrical surveys carried out on the crest of a dam are presented as vertical sections showing the electrical properties of the dam materials. Electrical currents travel along preferential pathways in the most conductive materials such as dam core composed on fine grained materials. The method provides picture of internal resistivity distribution of the dam structure, identifying areas of water saturation in the dam body, and thus identifying the zones of water accumulation and wetting.
2D Resistivity Imaging uses an array of electrodes (typically 64) connected by multicore cable to provide a linear depth profile, or pseudosection, of the variation in resistivity both along the survey line and with depth. Switching of the current and potential electrode pairs is done automatically using a relay box. The computer initially keeps the spacing between the electrodes fixed and moves the pairs along the line until the last electrode is reached. The spacing is then increased and the
z
f
process rep resistivity a
The modell Fig. 9. Blue the plot illu electrode sp
The method
The results (joint interp
Figure
Interpretati correspond zone centre seismic velo section S further conf
peated in or against dept
led results es represent ustrates the pacing are i
Figure 9
d can be app
istivity inve m, and can b ng-term resi embankme dying deviat etition of m wever, the m l as long-ter
s can also b pretation wi
e 10. Electri
on Exampl ding sonic L ed at aroun ocities in th was condu firmed by s
rder to prov th ('pseudos
are displaye t areas of lo gradual red increased.
9. Electrical
plied in two
estigations a be used to in istivity mon ent to detec tions from t measurement monitoring rm instrume
be interprete ith results o
ical Resistiv
le: “The li L-section. Th nd 300m. T his zone. An cted at this ection S5”
vide an inc section') is b
ed as scaled ow resistivi duction in t
Resistivity
o ways.
as a onetim nvestigate su nitoring ma ct anomalie the time-var ts provides approach is entation.
ed with oth f seismic to
vity Section Saturat
ne shows l he lowest re This was th nother low r s location.
creased dep built up alon
d resistivity ity whilst re the amount
Section Sho
me survey ma uspected str ake use of th s not only riation patte additional s also more
her geophys omography)
from dam c tion (Green
lower resis esistivity sho he location resistivity zo Third low r
Guidelines o
pth of inves ng the surve
y-depth pse eds are relat of data acq
owing Zone
ay detect sp ructural wea he seepage- in space, b ern. The sec evaluation e demandin
sical techni )
crest of a m &Blue)
stivity in th owing maxi where S one is obser resistivity z
on Geophysic
stigation. In ey line.
udosection tively highe quired as th
e of Saturati
patially anom aknesses. -induced se but more im cond approa possibilitie g as installa
ques as sho
masonry dam
he zone ha imum moist was carrie rved centre zone is cent
cal Investiga
n this way
as illustrate er. The wed he current an
ion (Blue)
malous zon
easonal vari mportantly ach is more es for seepa ations are n
own in exa
m Showing Z
aving low v ure level is ed out, conf d at around tred at 217
tion of Dam
a profile o
ed below in dge shape o nd potentia
nes along the
iation inside in time, by powerful as age analysis necessary as
mple below
Zone of
velocities in observed in firming low d 168m, and 7m, which is
s
f
n f al
e
e y s s. s w n n w d s
4.1.8. Limitations of ERI for Dam Investigations
The primary limitation of a 2D survey is that when the resistivity image is calculated, it makes an assumption that the layers continue sideways (perpendicular to the survey line) in the same way they were measured. For example, if the survey line goes north to south and an anomaly is spotted in the cross section, the algorithm assumes the anomaly expands infinitely to the east and west. This, of course, is not the case, but the method does often give a good picture of the cross-section. Absolute resistivity values, however, are not considered for interpretation and rather relative change is resistivity values forms basis of interpretation.
The streaming-potential/ self-potential (SP) method is unique among geophysical methods applied to embankment dams in that it offers an opportunity to detect seepage directly. When immersed in water, most minerals develop an electrical double layer, with negative ions bound to the surface of the mineral. Details of the formation and properties of this double layer are given in references such as MacInnes (1961), Mitchell (1976), Morgan (1989), and Morgan et al. (1989). A cloud of positive ions, attracted by the bound negative ions, surrounds the bound negative ions. Some of these positive ions are “mobile” (i.e., free to move), and can be swept downstream by flowing water. This process causes negative charge accumulation at the upstream end of a seepage flow path and a positive charge accumulation at the downstream end.
The streaming potential along the seepage flow path generates an electric current flow in the earth that produces SP variations at the surface. Typically, negative SP variations are seen above the upstream portion of a subsurface seepage flow path, or above an area of seepage inflow; and positive SP variations are seen above the downstream portion of a flow path, or at areas of seepage outflow. An SP null is located at some point along the flow path, between the negative upstream inflow area and the positive downstream outflow area.
For purposes of dam investigations, any SP variation due to a non-seepage source is considered to be noise. Noise variations generated by sources such as buried metal and electrical grounds can be of considerably greater amplitude than seepage-related SP anomalies and can mask the effects of concentrated seepage flow.
Much of the effort of SP interpretation consists of separating the desired seepage anomalies from the superimposed noise variations. SP anomaly amplitudes tend to increase with increasing reservoir elevation and seepage flow, while the intensity of most noise sources is not related to reservoir elevation. Therefore, comparison of SP survey data sets measured at different reservoir elevations can be used to help differentiate seepage-related variations from noise.
Fig-13: Development of SP due to seepage from reservoir floor
Fig-14: Development of SP due to seepage through dam
An onshore SP survey is conducted by mapping the electric potential field between an arbitrary base station and a series of points on the surface of the earth. The results are presented in the form of profiles and/or contours of the measured electric field. Three basic survey techniques may be used to perform the measurements: the fixed-base, gradient, and multiple-electrode methods.
The comparison of various configurations is as under:
x Fixed-base- Long wire, less sensitive to surface anomalies x Gradient- Short wire, extremely sensitive to noise x Multiple-electrode methods- Repeated surveys, expensive
Major objectives of SP investigations at dam sites include:
x Response to an observed problem (sinkhole, spring development, increased seepage flow rates, evidence of piping by appearance of fines in seepage outflow, slides, etc.);
The results are generally qualitative (though quantitative results are possible through modelling) and used in conjunction with electrical resistivity imaging results to differentiate between ‘saturated only’ and ‘saturated with flow’ zones, as shown in example below.
Fig-17: Joint interpretation of SP and ERI data
The upper image is result of electrical resistivity imaging on a masonry dam showing low resistivity zones (blue and green). The lower SP image shows negative SP development across the blue zone, confirming flow across it. Across green zones there is no SP development confirming that though these zones are having higher water saturation (getting weakened/ deteriorated), there is no flow across these zones at the time of measurement.
It is always advisable to conduct a ground penetrating radar surveys to detect any buried metallic pipe, before conducting the SP survey.
The combination of ERI and SP provide invaluable information on seepage. Electrical resistivity measurements of the subsurface serve a number of purposes. Electrical resistivity information complements SP data, in that SP responds to seepage flow but not to structure or saturation levels, while the inverse is true for resistivity. The information provided by a resistivity survey may include geologic structure, porosity, depth to the phreatic surface, estimates of pore water salinity, and levels of pore water saturation.
Sonic method’s principle is based on generation of elastic energy using various sources which is propagated through the investigated structure following different modalities and tracks depending on elastic properties variations of particular material crossed. The elastic waves are recorded by specific sensor in the form of electric signals.
Velocity an distance be processing parameters
Sonic tomo transversal (reservoir s geophones have to be mapping of
Sonic tomo distribution process sta reciprocally Tomograph paths acqui
The acquisi different re processing determined
In processin are calcula dimensions between co process ado investigated
nalysis con etween the t of data se with elastic
ography can section on side) while t installed on processed f downstrea
ography surv ns of elastic arts from tim y cross eac hy resolution ired and wit
ition proces eceiver poin phase. As d by an appr
ng process ated for ea s are chosen onsecutive t opted but is d materials,
Fig-
nsists of an transmitter-r et to calcul c properties
n be applie n dam bod the receiver n dam galler with inclin m receiver
vey allows c waves an me measuri ch other w n and then t th their angu
ss is realize nts. In this previously ropriate geo
the investig ch single n n in order to transition an justified by , having len
8 : Basic pri
n estimation receiver cou late sonic v s of investig
ed between dy, operatin r point plac ry roof and nometer log points (as d
to obtain pa nd their atte ing of long within the a the final res ular coverin
ed through t s way is po mentioned metry mapp
gated sectio node, assum o be compar nd receiver y the assum ngth differen
inciple of se
n of time n uple. There velocity dis gated area.
boreholes ng with tra ed on down source ham g surveys ( done case of
articular ma enuation of gitudinal wa area delimi sults accura ng.
the acquisiti ossible to r d, every rec ping.
on is divided ming biline red either w points. Suc mption that s nt by zero,
Guidelines o
eismic tomog
needed by fore the sec stributions
or between ansmission nstream side mmer on dam in case of f dams).
aps (called t crossed m aves along itated by tr acy are stric
ion of each realize the ceiver and t
d by a recta ear velocity with signal w ch discretiz sonic waves in a finite w
on Geophysic
graphy
the elastic cond step co and to est
n 02 faces. points loca e. L-section m top. Tom boreholes)
tomograms) material alon high numb ransmitter tly connecte
signal tran data grid n transmitter
angular grid y variation wave length zation is als s mediate th width portio
cal Investiga
impulse to onsists of tim timate a tig
It can be ated on up ns can be re mography me or with a
) that show ng plane se er of ray tr and receiv ed with num
nsmitter poin necessary t points coo
d cells wher along the h and relativ so aimed to he character on (first Fre
tion of Dam
o cover the me-distance ghtly linked
done along stream side ecorded with easurements topographic
the velocity ctions. This racks which ver position mbers of ray
nt related to o next data rdinates are
re velocities cells. Cel ve distances o calculating ristics of the esnel Zone)
s e e d g e h s c y s h
n. y
o a e
s ll s g e ).
