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Geodetic Deformation

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  • ANCOLD 2006 Conference Page 1

    OVERVIEW OF GEODETIC DEFORMATION MEASUREMENTSOF DAMS

    Dr. J. M. RegerSchool of Surveying and Spatial Information Systems

    University of New South WalesUNSW SYDNEY NSW 2052

    [email protected]

    ABSTRACT

    After a brief review of the origin and early days of the technique, the present role of geodeticdeformation measurements is discussed. The design of geodetic measurement schemes is thenconsidered, followed by a review of geodetic measurement, analysis and reporting techniques. Anoverview of the important discussions, that need to take place between engineers and surveyors in thedesign phase, follows. This covers the definition of the engineering needs and the resolution ofsurveying issues.

    1 INTRODUCTION

    To get a better understanding of the termgeodetic deformation measurements, it is usefulto have a brief look at the origin and history ofthis dam monitoring method. In Switzerland,the construction of (mainly concrete) waterstorage dams (for electricity generation) startedin earnest in the 1920s. Since the safety ofdams is very important for the people livingdownstream, it is understandable that the damengineers wanted to know more about thebehaviour of dams than the earlier monitoringmethods (levelling, clinometers (tiltmeters) andoptical alignment) could provide. Thestructural deformation caused by changes inreservoir (water) levels and (air) temperaturegive an excellent insight into the quality and, inconsequence, the safety of a dam. Beginning in1921, Swiss National Mapping was contractedby a number of dam owners to determine thedeformation at a number of points distributedover the dams (Lang 1929). The increasedinterest in dam deformation measurements afterthe failure of the St. Francis Dam (12 March1928) in the USA lead to the publication byLang on the geodetic method developed inSwitzerland.

    The geodetic method was proposed by H. Zlly,then chief of geodesy at Swiss NationalMapping. Originally, two to three referencepoints (survey pillars, downstream) were used tointersect the object points (survey marks placedin the downstream face of the concrete dams).The stability of the reference points

    (observation pillars) themselves was checked byresection from close (relocation points) anddistant targets. Horizontal directions weremeasured with (precise vernier) theodolites.Only one distance was taped. The deformationswere obtained by semigraphic means, using thedifferences of values measured in twoconsecutive epochs. The settlement of thestructure was either monitored by levelling runsacross the crest and along the base of the damor, less often, by zenith angles from thereference points (pillars) to the object points(targets on the dam).

    Figure 1 (after Fig. 18 in Lang 1929) shows atypical example of one of the early measuringschemes for the Schrh-Dam in the WgitalValley, about 40 km NE of Zrich(Switzerland). This concrete gravity dam wasbuilt in 1924, is 112 m high, 156 m long and at900 m above sea level. 19 object points(targets) were installed in four rows (horizontalprofiles) and six columns (vertical profiles).The measurements were taken from the threereference points (observation pillars). (Pillarmovements of 0.7 mm were noted in theseearly measurements.) The points A to C, E toH in Figure 1 were all used to check the stabilityof the pillars (by graphical resection). Themarks A, B, F, G, H are relocation marks atclose range. The more distant marks C, E, J (aswell as the rays to the other pillars) were usedto orientate the arcs of directions on the threepillars (using weighting according to distance).Further information on the dam may be foundon the world wide web at www.swissdams.ch.

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    Figure 2 (after Fig. 37 in Lang 1929) shows athree-dimensional representation of the lines ofhorizontal and radial deformations in sixvertical and four horizontal profiles of theSchrh-Dam in the Wgital Valley (Switzerland)between the zero measurement in May 1925and second filling and the second emptying ofthe dam. The axonometric diagram assumes avertical plane for the initial measurements inMay 1925 (reservoir at 860 m above sea level)and zero deformations at the abutments. The14 mm maximum deformation in October 1928before the emptying of the dam was the same asthat of the initial filling (in October 1926).After lowering the reservoir in March 1929 by40 m, the dam moved a maximum of 2 mmupstream. This means that the first fillingcaused an irreversible deformation of amaximum of 12 mm. Subsequently, the damshowed an elastic behaviour (2 mm for a 40 mchange in water level). Such diagrams can onlyvisualise the structural movements between two(or, as here, three) epochs of measurements.

    A number of features of the early geodeticmeasuring schemes in Switzerland and elsewherehave withstood the test times: The Freiberger ball (16.53 mm diameter,according to Lang (1929)) centring ofinstruments and targets and the correspondingbrass centring bolts (with protective cover) arestill used on some dams. (Since about 1973, theKERN pillar centring plates with a diameter of158 mm were used on many new dams.) The basic layout of monitoring targets (in agrid pattern on the downstream face ofconcrete dams) has found wide acceptance, forexample with six vertical profiles and fourhorizontal ones. Marks near the abutments areimportant if abnormal behaviour is to bedetected. The measuring precision of directions (in twofaces) achieved in the early surveys wasexcellent with better than 1" in most cases andhas not changed since then.

    Figure 1: Typical example of an early geodetic deformation measuring scheme: Schrh GravityConcrete Dam in the Wgital Valley, about 40 km NE of Zrich (Switzerland). Two of the 19 objectpoints are labelled (1a, 6a). Pillars 1 to 3 are the reference points, the marks A, B, F, G, H arerelocation marks and the marks C, E, J are (distant) targets used for orientation purposes (after Fig. 18in Lang 1929)

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    The (up to four) relocation targets nearpillars for the check of the stability of thepillars are not always installed on newerschemes even though they are useful to monitorthe pillar behaviour against the surroundingground. The use of concrete pillars (0.5x0.5x1.15 mat the time) was very successful. Today oneprefers round double skin concrete pillars whichminimise mechanical damage of the inner core.The inner pillar is shaded and, thus, moves lesswith the sun. Lang (1929) noted that 20 to 30 rays can bemeasured within the same arc of horizontaldirections because of the good stability providedby concrete pillars. Each reference point should have clear sightsto at least four orientation marks. He alsosuggested that, in an arc of horizontaldirections, 50 % of rays to reference pointsshould be measured first, followed by all objectpoints and, then, the rest of reference points. Two arcs of directions should always bemeasured (for full geodetic measurements). The

    early Swiss experience was that measuring 4 arcsdoes not improve the results.

    Some features of the early schemes howeverchanged over the years: The (two to three) observation pillars wereoriginally placed at about mid height of thedam. Today, more observation pillars areinstalled up- and downstream of the dam. Thisrequires that some pillars are higher than thedam. The distant targets (used originally for theorientation of arcs on the pillars) have beendropped in newer schemes. The targets on the dam featured whitevertical lines (straight or conical) on a blackbase. Later, this was changed to concentriccircles (see (4) in Fig. 20). The targets that were inserted into thecentring bolts of pillars originally carriedvertical lines. Later designs featured concentrictarget patterns (see (3) in Fig. 20) or, evenbetter, brightly coloured spherical balls (Fig.27).

    Figure 2: Three-dimensional (axonometric) representation of the lines of horizontal and radialdeformations in six vertical and four horizontal profiles of the Schrh-Dam in the Wgital Valley about40 km NE of Zrich (Switzerland) between the zero measurement (May 1925) and the measurementsimmediately before (October 1928) and after (March 1929) the first draw down of the dam (after Fig.37 in Lang 1929)

    For the early geodetic deformationmeasurements in the 1920s, National Mappingof Switzerland tested a number of ways on howto present the results of geodetic measurementson dams. A number of them are still being usedtoday. Particularly useful and visual are therepresentations of deformation lines. Figure 2

    gives an example (after Lang 1929) of ana x o n o m e t r i c 3 - D r e p r e s e n t a t i o n .Unfortunately, such diagrams can only visualisethe structural movements between two (or,possibly, three) epochs of measurements. Thelateral and tangential movements have to beplotted separately.

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    The early publication by Lang (1929) containsmany other useful suggestions for the design ofmonitoring networks and the execution ofsurveys. The author warns of lateral refraction(affecting lines of sight close to ground orstructures) and the huge refraction problemsexperienced when measuring along the crest(caused by the bending of the up- ordownstream winds over the crest and the strongtemperature gradients associated with it). Thisis a reason why optical alignment on the crest isnot considered suitable for highest precision. Ifoptical alignment is to be used, Lang (1929)suggests to place (on the abutments) theinstrument and reference target higher than thecrest.

    2 ROLE OF GEODETICMEASUREMENTS IN THEMONITORING OF DAMS

    According to the Swiss Commissioner for DamSafety (Biedermann 1996, 1997), the geodeticdam monitoring techniques have become lessattractive over the years, firstly, because theyrequire skilled personnel and, thus are expensiveand time consuming and, secondly, becausedirect mechanical measuring devices such aspendulums and wire alignment systems (that canbe operated by less skilled dam based staff) havebecome available. The geodetic techniques arestill very important since they produce absolutedata and connect the localised dam basedmeasuring devices to the dam's foundations andthe area surrounding the dam (and, possibly,slopes along the reservoir). These days, thegeodetic measurement scheme provides thefoundation for the measurements in the case ofan abnormal behaviour of the dam and ismeasured infrequently.

    To assure the safety of a dam, three elementsare necessary (Biedermann 1996): safe state-of-the-art design of the construction, monitoringof the structure and an emergency concept. Itcould be argued that, in line with the firstrequirement, the dam should be measured duringthe first filling and emptying to test if theactual deformations agree with the expecteddeformations. The monitoring must be able todetect damages, constructive deficiencies andthreats to safety so that an abnormal event canbe detected and responded to (Biedermann1996).

    The Swiss Commissioner for Dam Safety(Biedermann 1996) suggests that themonitoring of dams be carried out as follows: Visual Inspection ( once a week). Since notall threats to the safety of a dam and reservoircan be captured by measurements, a visualinspection by persons familiar with thestructure is essential. The visual inspectionmust cover the dam, its surroundings and, ifnecessary, the slopes along the reservoir Measurements ( once per month) of keyindicators of the behaviour of the dam, itsunderground and its surroundings (includingslopes along the reservoir, if necessary). This isacceptable since abnormal behaviour ofstructures and terrain usually develop slowly.The knowledge of the radial displacement atone or more points along the crest is sufficientfor this purpose. On concrete dams, thesefrequent measurements are typically carried outby on-site personnel with direct measuringdevices, such as pendulums, wire alignmentsystems, clinometers, extensometers, etc. Suchmeasurements are very precise, simple to makeand cost effective. Automatisation of this typeof measurements and on-line recording andinspection is easily possible, if so desired orrequired. Periodic Safety Examination (large dams:every 5 years, small dams: when required).Biedermann (1996) suggests that a reducedmeasuring program of the installed geodeticnetwork be measured during these five yearlysafety checks of the dams. Ideally that shouldbe at full reservoir and at the same time of theyear since seasonal effects are often morepronounced that changes caused by water levelin the reservoir. It is further suggested tomeasure the complete geodetic network every15 years when the reservoir is empty.

    It follows that the Swiss authorities see the roleof geodetic measurements mainly as ameasuring base from which more specialisedmeasuring schemes can be developed should anabnormal behaviour of the dam be detected byother means (visual inspection, direct frequentmeasurements of key indicators). A three-dimensional network is required to determineradial and tangential movements, settlementsand rotations of the dam as well as deformationof the surrounding terrain. In the case of anemergency, measurements are taken morefrequently and the monitoring scheme might beextended. Important is that a fundamentalgeodetic network, covering all relevant parts ofthe reservoir (dam, abutments, foundation,surroundings, and, if required, slopes along the

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    reservoir, unstable rock masses and (in otherparts of the world) unstable glaciers) has beeninstalled, is being maintained and measuredinfrequently (e.g. every 15 years) to provide areference (zero-epoch).

    If the dam experiences snow fall in winter, thedesign of the full geodetic measuring schemeshould ensure that the measuring points of the

    network are accessible in winter as far aspossible. Naturally, dams with galleries atdifferent levels (ideally extending into theabutments on both sides and equipped with wirealignment or survey precision traverses andlevelling networks) and vertical shafts (withpendulums, ideally extended into the foundationwith inverted pendulums) are best suited formeasurements under snow cover.

    Figure 3: Principle of a geodetic deformation measurement network for concrete dams. Inside thedam the network comprises three pendulums and one inverted pendulum as well as three surveyingtraverses in galleries at three levels. The interior network is in a vertical plane. The exterior surveynetwork is essentially in a horizontal plane at crest level with a number of targets on the crest(connecting the pendulums to the exterior net) and a number of reference points up- and down-stream.The exterior network is measured by directions, zenith angles, distances and/or GPS. Some levellinglines at the crest level are also shown, as is an additional survey network at a lower level. (AfterBiedermann 1985)

    Figure 3 (after Biedermann 1985) shows thebasic layout of a geodetic deformationmeasurement network for straight or curvedconcrete dams. It features measurements in twoplanes. In a vertical plane (may be curved), thenetwork comprises three pendulums and oneinverted pendulum as well as three surveyingprecision traverses in galleries at three levels.If the dam is straight, a wire alignment systemcan replace the surveying traverses in thegalleries. Ideally, the galleries should extendinto the abutments to provide an additionalabsolute reference. As shown, an absolutereference is provided in the interior network byan inverted pendulum reaching into theunderground.

    The basic exterior survey network in Fig. 3 is ina horizontal plane at crest level with a number

    of pillars on the crest (connecting thependulums to the exterior net) and a number ofreference pillars up- and down-stream. Theexterior network is measured with electronictacheometers (by directions, zenith angles anddistances) and/or GPS. Some levelling lines atthe crest level are also shown, as is anadditional survey network at a lower level.Essential is that the interior and the exteriornetworks are interconnected as shown by thelarge circles in Fig. 3. The interior network(vertical plane) and the exterior network(horizontal plane) are connected at three pointson the dam's crest. The levelling lines areconnected to the reference points of the othersurvey measurements. As mentioned before,the network might have to be extended if thereference points shown are still in the influencezone of the dam and/or if the ground near the

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    dam and/or the slopes along the reservoir needto be monitored too. With the GlobalPositioning System (GPS) the network caneasily be extended to cover a wider area withoutrequiring inter-station visibility.

    It follows from the earlier considerations in thissection that at least one pendulum and theinverted pendulum be measured every month,the interior network (pendulums, traverses inhorizontal galleries) every five years and thefull network every fifteen years or so.

    Figure 4: Wire alignment system installed inthe parapet of a gravity dam. Pulley and weightare seen on the right. The measuring point ison the left. The measuring microscope isattached to the big bolt seen above the wire.After Biedermann (1997, Fig. 2.10).

    Since most dams in Switzerland are concretedams, the discussion above (and Fig. 3) aretailored for this type of dam. Embankmentdams (deck type or fill type) and small concretedams do not have internal galleries and verticalshafts. This means that the interior networkshown in the vertical plane in Fig. 3 needs to beconfigured as an equivalent grid of object pointson the downstream face of the dam. Ratherthan run traverses along berms, the objectmarks on the downstream face are connected tothe reference points by standard surveyingmeasurements (horizontal directions, slopedistances and zenith angles) or, possibly, bysatellite measuring techniques (e.g. GPS). Thatposes no problems for the (reduced) geodetic

    measurements every five years and the fullmeasurements every 15 years.

    The remaining problem with embankment dams(or, generally, dams without galleries or shafts)is the simple measurement of criticalparameters at monthly intervals. Biedermann(1997) suggests a wire alignment system alongthe crest. This is possible for straight concretefaced rockfill dams with crest/parapet wall, forexample (if the settlements are not too large!).Since wire alignment systems are rarely seen inpublications, the system installed within theparapet of the Rempen gravity dam inSwitzerland is shown in Fig. 4. (Moreinformation on the dam can be found atwww.swissdams.ch.) The wire is tensioned by aheavy weight through a pulley system. Whennot in use, the measuring points and the weightare protected by panels.

    Figure 5a: Simple measurement of anglesfrom two reference points (Pillars 1, 3) toobject points (42, 44, 46) on the crest of theChapfensee/Parmort (gravity) dam inSwitzerland. The upstream-downstreammovements of the object points 42, 44, 46 arecomputed from the changes of the measuredangles (and, as a check, from the measuredangles ). After Egger & Walser (2005, Fig.2.06-1).

    Alternatively, the optical alignment of one ormore crest point(s) is feasible or themeasurement of some distances from a pillardownstream to some object points on thedownstream face of the dam or the intersectionof one or few crest points by simple anglemeasurement or c l inometer / t i l tmeterobservations on the upstream face. Anarrangement of simple angle measurements isshown in Fig. 5a for the Chapfensee (Parmort)

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    Dam in Switzerland. The reservoir is located 4km west of Sargans and features two dams.Since only the northern one (120 m long and20 m high) is listed at www.swissdams.ch, it isassumed that Fig. 5a refers to the northern dam.Further information on the dam may be foundon the world wide web at www.swissdams.ch.

    On slim and strongly curved arch dams (with nopendulums) simple vertical angles (or zenithangles) can be measured to bull's eye (circular)targets on the dam that face downwards. SeeFigure 5b. Because of the quasi-vertical line ofsight, a change in the zenith angle is a measureof an upstream/downstream displacement of thedam (after multiplication with the heightdifference (assumed known) from the pillar tothe mark). One-second theodolites withdiagonal eyepiece are used for the purpose.According to Egger & Walser (2005), qualifiedpersonnel is required for this type ofmeasurement that is simple in principle but notin practice.

    Figure 5b: Simple measurement of verticalangles from an observation pillar at the base ofthe dam to object points (11 to 16) on thedownstream face of slim double curvature archdams. The upstream-downstream movementsof the object points 11 to 16 are computedfrom the changes of the measured verticalangles (and the known distance from thepillar). After Egger & Walser (2005, Fig. 2.06-2).

    Some of these alternatives require some trainingin surveying of dam staff. The problemsassociated with optical alignments were alreadyknown in the 1920s. Biedermann (1997) citessome tests by the Swiss Federal Institute ofTechnology (ETH) in Zrich with opticalalignment that exhibited errors of ten seconds

    of arc (10", 14 mm/300m) on a Swiss dam.Given the reduced precision (compared topendulum observations in concrete dams) of themonthly simple observations on dams withoutgalleries or shafts, Biedermann (1997)recommends geodetic measurements (of thedam face) at least once a year (rather thanevery 5 years).

    3 DESIGN OF GEODETICDEFORMATION SCHEMES

    The design and measurement of geodeticdeformation schemes have been described manytimes. The reader may refer to the followingpublications for a more detailed discussion, forexample: Swiss National Committee on LargeDams (1997, 1993, 1985), Egger & Keller(1976), Keller (1978), Kern (1971), Untersee(1951, 1975).

    Based on his extensive experience withdeformation measurements, Egger (1997, 1993)gives some sound advice on terrestrialmeasurements of deformations. Themonitoring scheme must be designed for a longservice life of more than 50 years. Thegeodetic network must be as complete aspossible and allow for later extension (in case ofabnormal behaviour or new construction work).Close collaboration between engineers andsurveyors is essential and should begin as earlyas possible in the preliminary design phase sincethe location of pendulums, galleries, wirealignment systems, etc. must allow connectionsto the geodetic measurement scheme. Themeasuring scheme should be flexible enough toallow the adoption of new measuring techniqueslater.

    The reference points must be close to the dam,both upstream and downstream, outside theinfluence zone of the structure, must haveinter-visibility and, ideally, must be accessibleall year round. Experience shows that one totwo points in each scheme become unreliablewith time or get destroyed (e.g. by constructionactivity, rock fall, avalanche). Egger (1997,1993) considers four reference points anabsolute minimum. The reference pointsfeature normally deeply anchored double-walledconcrete pillars with a forced centring devicethat is protected from the elements andvandals. Brackets for the attachments ofumbrellas are helpful. In exposed locations, thepillar must be inside a concrete shelter that

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    protects from possible rock falls, avalanches,land slides and the like. Four relocation marksat close range are useful to check for pillarmovements (by resection, against thesurrounding terrain). If satellite measurements(e.g. using GPS) are planned, then there shouldbe no obstructions above an elevation angle of15-20 degrees and no reflecting surfaces nearby.Unfortunately, the water in the reservoir is aperfect microwave reflector.

    Figure 6: Connection of the horizontalposition of the pendulum to the exteriorgeodetic network. The pendulum is 'measuredin' from 'C'. The point 'B' is establishedvertically above 'C' by optical plumbing. Afterthat, 'B' can be measured in from the pillar 'A'.

    The object points are placed so that theyprovide information on the behaviour of thestructure (and its surroundings, if required). Theobject points can be pillars on the crest of thedam (with forced centring system), brackets(with forced centring system), bolts (withforced centring system) in the ground. Objectpoints must be able to be surveyed withconventional surveying techniques. Since theseinclude distance measurements, EDM reflectorsmust be able to be fitted (temporarily) to theobject points and the object points should beaccessible all year round. If object points mustbe established on the downstream face of

    concrete dams (because the dam lackspendulums and galleries), they are usuallycircular targets on brass bolts and observed bythe traditional intersection method. Egger(1997, 1993) does not favour inaccessiblereflectors in dam walls since they become dirtyand the reflecting surfaces deteriorate withtime, particularly in a humid climate. Usually,the deformation measurement scheme alsoincludes some object points ('bench marks') thatare only determined in height (by levelling).

    Figure 7: Observation platform on thedownstream face of the Gigerwald arch dam inSwitzerland. (Photo by Kern & Co Ltd Aarau,Page 62, Swiss National Committee on LargeDams 1985)

    Object points located inside the galleries (ofconcrete dams) provide the most accuratevertical and horizontal movements. Unless thetraverses in the galleries extend into theabutments, they must be connected to theexterior network to provide absolutedeformation information. The preferredmethod is to connect the gallery traverses tothe pendulums and the top (anchor points) ofthe pendulums to survey pillars on the crest.Figure 6 shows an early version of thisapproach (Egger & Keller 1976). Theconnection of the pendulums to the outernetwork can be simplified if the anchor points

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    of the pendulums are at (or near) the crestlevel.

    Egger (1997, 1993) considers the alternativemethod of connecting the galleries to the outernetwork through openings (see Fig. 7) in thedownstream face less desirable. As in tunnelwork, the measurements from the outside to theinside of the dam usually suffer from largehorizontal refraction effects.

    Figure 8: CERN forced centring system withcentral 30 mm bore. Fits (directly) reflectorsand targets (shown) fitted to Taylor-Hobsonspheres as well as the DISTINVAR invarmeasuring device. Standard surveyingequipment can be attached (and locked!) in thecentring device with adaptor plates fitted withthe customary 5/8 inch Whitworth thread ontop (see Fig. 25). The Taylor-Hobson targetwith concentric circles shown in the figure isnot part of the device proper.

    For the terrestrial measurements, theodolites,electronic distance meters and levellinginstruments are typically used, the former twooften combined in the form of electronictacheometers. Typical theodolite precisionsare 0.7" for directions, 1" for zenith anglesand (0.1 mm + 0.7 ppm) for the KernMekometer ME5000 precision distance meter.Egger (1997, 1993) quotes levelling with 0.1mm/station. For the internal traversingnetworks, invar wires are/were often used,recently in the form of the DISTINVARoriginally developed by CERN (EuropeanOrganization for Nuclear Research). Accordingto Egger (1997, 1993) the DISTINVARachieves 0.02 mm in routine measurements.

    The DISTINVAR requires the CERN centringsystem (with a 30 mm diameter centringcylinder) which is commercially available in itsor iginal or s implif ied forms (seewww.geodesie.com). The original CERNcentring system (in three parts to allow tomake the central cylinder vertical) is made outof aluminium alloys and shown in Figure 8(after CERN 1974).

    Measurements with motorised electronictacheometers with automatic target recognitionallow to measure under computer control toreference and object points that are equippedwith EDM reflectors. If instruments areinstalled permanently (in weather proof andvandal proof observation buildings), continuousmeasurements are possible. Remote control isavailable if the communication links areinstalled. Continuous monitoring systems arevery costly since they require substantialinstallations and maintenance and permanentlytie down an expensive instrument. Egger(1993, 1997) notes that, ' in certaincircumstances, this is entirely justifiable'.

    During the measurement of the network (calledone epoch of measurements), a predeterminedobservation plan should be followed and theobservations should be carried out according tothe rules of good professional practice as itapplies to precision surveys. The observationprocedures have to be designed in such a waythat obvious errors are detected on site and thatthey can be remedied on the spot. That mightrequire some preprocessing of data in theevenings. (Should errors be found later in theoffice during processing, it is too late to takeadditional measurements.) It is customary, touse always the same equipment (in the sameorientation) on the same reference and objectpoints since this eliminates some systematicerrors in the deformations that are beingdetermined. If electronic data recording is beingused, daily back-up of the data to otherrecording media should be carried out.

    Irrespective of the type of equipment used,some aspects are always important (Egger1997, 1993): height of instruments, targets, reflectors,antennas above mounting plate atmospheric parameters (temperature,pressure, humidity), weather, reservoir level Misalignments, scale errors and eccentricitiesof instruments, targets, reflectors, staffs,antennas

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    Figure 9: Geodetic deformation network of the Mattmark embankment dam in Switzerland (height:120 m, length: 770 m). The points marked 210 (and 600, 650) are concrete pillars from which themeasurements to the object points (marked by circles) are taken. The reference points 2,4,7 and 9 arepresumed to be outside the stress zone. Some targets (11 -21) mounted on rock faces were originallyused for the orientation of arcs. The rows of points (numbered 651 to 958) were used to investigatesome abnormal behaviour and are not part of the continuing observation scheme. After Kgi (1978,Fig. 1).

    avoidance or elimination of other systematicerrors affecting the measurements.

    Ideally, the reservoir level should be keptconstant during the measurements of one epochof data. Even so, daily readings of all directmeasurement devices (e.g. pendulums) and ofwater level should occur. Simultaneously withthe geodetic measurements, all other types ofmonitoring devices should also be read to allowthe correlation of the results.

    Having discussed some important principles ofgeodetic deformation measurements, it isappropriate to look at some practical examples.Figure 9 depicts the inner (on the dam) and theouter geodetic networks of the Mattmarkembankment (fill) dam high in the Swiss Alps.The dam is in the Canton Valais, 17 km east ofZermatt and 23 km ENE of the famousMatterhorn mountain. It has a height of 120

    m, a length of 770 m and a volume of 10.4million cubic metres. The maximum reservoirlevel is at 2197 m above sea level.Observations (directions, zenith angles and,these days, distances) are taken from the points210 (double-walled concrete pillars). (Thepillars 600 and 650 were only usedtemporarily.) Four of the nine observationstations (2, 4, 7, 9) are reference points on thesides of the valley and likely stable. Pillar 10 ison the dam to improve the geometry of thenetwork. It is subject to deformations like theother points on the dam. Object points are onboth faces of the dam and on the crest. Thoseon the upstream face are often under water.According to Gilg (1985), there is a total of 98object points on the dam. Not all of them areshown in Fig. 9. An additional 11 object pointsare in the drainage gallery in the base of thedam and not shown. There are also 35(levelling) bench marks in the monitoring

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    scheme. (Only some are shown in Fig. 9.)More information on the Mattmark Dam canbe found in Kgi (1978), Gilg et al. (1982), Gilg(1985) and at www.swissdams.ch. Biedermann(1997, Fig. 2.8) gives an updated plan of theMattmark network: There is now a secondobservation pillar on the dam, the targets onthe rock faces are no longer shown nor are therows of dense points for the temporary specialobservation scheme.

    The modern internal network of a gravityconcrete dam in Switzerland is shown in Figure10. The Panix (Pigniu) Dam was commissionedin 1989, is 53 m high and 270 m long. Thereservoir is 32 km east of Chur in the Cantonof Grisons and at an elevation of 1450 m abovesea level. This dam monitoring scheme followsthe recent trend and has no object points on thedam's faces. There are only three survey pillarson the crest to connect the interior net (of

    Figure 10: Interior geodetic deformation network of the Panix (Pigniu) gravity dam in Switzerland. 1:survey pillars on crest to connect the interior to the exterior network. 2: wire alignment, 350 m long.3: pendulums. 4: inverted pendulum. 5: alignment reading points (one in each block and in theabutments). 6: pendulum reading points. 7: floating intermediate deformation network of the Panixgravity dam in Switzerland. After Biedermann (1997, Fig. 2.7).

    Figure 11: Interior measuring scheme of the Gigerwald Dam in Switzerland showing the four normaland four inverted pendulums as well as the three galleries with the traversing surveys. The lowerdiagram in Fig. 11 shows the uppermost gallery and its traverse stations at 32 m intervals (length ofinvar wires). Three pillars ('A') on the crest connect the uppermost traverse to the exterior network asdo three pillars ('B') on little balconies in the downstream face. After Biedermann (1997, Fig. 2.6).

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    pendulums and wire alignment) to the exteriorone. The up/down stream deformations aremeasured by the pendulums and the wirealignment, the sideway movements by thependulums. It has to be assumed that thevertical movements are monitored by levellingin the gallery and/or on the crest.

    For completeness, the geodetic monitoringnetwork of the Swiss Gigerwald double curvaturearc dam is shown in Figures 11 and 12 eventhough the layout has been shown many timesbefore (Keller 1978, Egger & Keller 1976,Egger 1993 1997, Biedermann 1997 1993).The Gigerwald Dam was commissioned in 1976,is 147 m high and 430 m long. It is about 15km SW of Chur, Switzerland, and at 1335 mabove sea level. Figure 11 depicts the internalmeasuring scheme of four normal and four

    inverted pendulums as well as three gallerieswith traversing surveys (originally: angles bytheodolite and distances with invar wires). Thelower diagram in Fig. 11 shows the uppermostgallery and its traverse stations at 32 mintervals (length of invar wires). The traversestations in the galleries consist of steel bracketsattached to the walls carrying a simplifiedversion of the CERN centring system (Fig. 8).

    The DISTINVAR measuring device fits theCERN centring directly. The rest of the surveyequipment uses appropriate adaptors. Keller(1978) shows most of the equipment used onthe dam in the late 1970s. The internalnetwork is connected to the external network(shown in Fig. 12) by three pillars (see Fig. 13)on the crest (connecting to three points ofthe uppermost traverse using the procedure

    Figure 12: 'Complete' exterior measuring scheme of the Gigerwald Dam in Switzerland (Scale: Distance2-3 = ~395 m). 1: Observation stations. 2: reference points (presumed stable). 3: Pillars on crest (603,611, 621). 4: Pillars on platforms in the downstream face (207, 211, 215). 6: Rock monitoring points(1F to 5F). 7: Close range relocation marks. 8: point number and elevation. 9: reciprocal and one-waymeasurements of directions, zenith angles and distances. Tamina is the name of the river in the valley.After Biedermann (1997, Fig. 2.6). Since 1997, the network was extended by two stations about 550 mupstream. (Refer to Fig. 4 in Egger & Walser 2005)

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    outlined in Fig. 6) and by three pillars onplatforms in the downstream face (Fig. 7 showsone of these connection pillars denoted by B inFig. 11.)

    Figure 13: KERN Mekometer ME5000precision distance meter on Pillar 603 of theGigerwald Dam (Aeschlimann 1988, Kern1988). The top of the pillar with the KERNpillar centring plate is normally protected by asteel cover fitting onto the steel ring visible onthe outer pipe.

    Figure 12 shows the 'complete' exteriormeasuring scheme of the Gigerwald Dam inSwitzerland. (Fig. 4 in the 2005 publication ofthe Swiss National Committee on Dams showsthat, since 1997, the Gigerwald Dam networkwas extended by two reference stations, about550 m upstream from the dam.) The dark linesindicate the primary geodetic network betweenthe four reference points (1, 2, 3, 4) and thethree pillars on the crest. The additional raysconnect the three pillars in the downstreamface (207, 211, 215) and the rock monitoringpoints (1F to 5F). Measurements are onlytaken to the rock monitoring points but notfrom them. According to the photos in Keller(1978) all marks of the outer network on theGigerwald arch dam are equipped with the Kernpillar centring plates of 158 mm diameter (seeFigs. 13 and 24). The rock monitoring pointsfeature centring plates that are attached toheavy duty steel brackets. Keller (1978) givesalso the diagram of a 'reduced' exteriormeasuring scheme of the Gigerwald Dam, with aselection of measurements from Pillars 211 and1. This example of a complete and a reduced

    measurement scheme is useful when selectingreduced schemes on other dams.

    4 RESULTS OF GEODETICMEASUREMENTS: ANALYSISAND REPORTING

    In the early days of geodetic dam deformationmeasurements (in the 1920s), pocketcalculators, and computers in general, were notavailable; least squares adjustments had to becarried out by hand using logarithm tables. Nowonder that the analysis of dam deformationmeasurements was carried out with semi-graphicmeans that was much faster and sufficientlyaccurate. Today, most survey data are recordeddigitally. The subsequent processing of the datais done on personal computers.

    Whereas the early geodetic measurements ondams involved only horizontal direction,levelling and, possibly, zenith angleobservations, today's data are likely to includeelectronic (and/or invar wire) distancemeasurements and, increasingly, GPSmeasurements. EDM measurements requiresome preprocessing to account for ambienttemperature, pressure and humidity. Also, theearlier derivation of the deformations from thechange (between measuring epochs) of the rawobservations was replaced by the execution ofleast squares network adjustments of two epochsand the derivation of the deformations fromthe change in coordinates between epochs.This evolution from simple differences ofmeasurements to changes in coordinates(determined by least squares networkadjustments) has some serious consequences,that should not be overlooked.

    As with all geodetic network adjustments, allobservations must be reduced to a commondatum or coordinate system before the networkcan be adjusted. In the case of dam deformationnetworks, a local x-y coordinate is usuallyadopted, typically with a false origin to avoidsimilar x and y coordinates in the project area.Essential is that the reference elevation of thehorizontal coordinate system is clearly definedand that all distance measurements are reducedto this reference elevation (being a sphericalsurface). The reduction of measured slopedistances to the reference elevation is nottrivial; it can be based on measured zenithangles or the elevation of the terminals of thelines (Reger 1996).

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    Since surveying instruments are levelled at eachset-up (so that the direction of vertical axiscoincides with that of the plumbline), anydeviation of the local vertical (plumblinedirection) from the normal to the datum surfacemust be considered. In alpine areas these'deviations of the vertical' can be significantand, if ignored, can affect the coordinates andprecisions obtained by network adjustments. Itmight be necessary to model (in the leastsquares adjustment) the deviations of thevertical or to correct the measurements beforethe adjustment.

    In most cases, the horizontal measurements areadjusted (in the local x-y coordinate system)separately from the height (difference)measurements. Typically, levelling data andheight differences (computed from zenithangles and slope distances) are used in the latter.So, essentially, the adjustments of the dataoccur in 2+1 dimensions.

    If the geodetic deformation measurementsinclude GPS data, then the network adjustmentbecomes more involved. Schneider & Wiget(1997) suggest combined terrestrial-GPSnetwork adjustments to obtain realistic results.These authors report the use of a true 3Dhybrid adjustment in a Cartesian (or geocentric)coordinate system and the (additional) solutionof the two components of the deviation of thevertical at each point.

    To evaluate if the dam has experienceddeformations between two successive epochs,the two epochs are adjusted in one commonadjustment. This adjustment will have to berepeated a number of times to statistically testthe stability of the reference points and toidentify the object points that have movedstatistically between epochs. All statistical testsshould be carried out at 95% confidence level.Software for the rigorous deformation analysiscan be obtained from a number of sources. Theresults of a rigorous deformation analysis arebest displayed in 2+1 dimension. Then, the95% confidence ellipses of the horizontal (2D)deformation vectors can be plotted togetherwith the deformation vectors (using the sameorigin for ellipse and vector). Any vector thatcrosses the ellipse is significant. Figure 14 givesan example (Welsch et al. 2000). The verticalmovements can be shown in a similar diagramtogether with the error bars at 95% confidencelevel.

    The reporting of the results is described in detailby Egger (1997). This author states that 'themain aim must always be the production of anobjective and complete record of the survey'and mentions that results that are difficult toexplain are often the first sign of an abnormalbehaviour of the dam. Egger (1997) suggeststhat the horizontal and vertical pointmovements (presumably between twoconsecutive epochs) are presented in graphicalas well as in tabular form. It should be addedthat the diagrams and tables should clearlyindicate the movements that are statisticallysignificant and those that are not. As shown inFig. 14, it might be appropriate to show onlysignificant vectors in the graphicalrepresentation of the epoch-to-epochmovements. Separate diagrams of the longterm movements might also be useful orrequired. Here, it might be better to show allepoch-to-epoch changes since small(insignificant) changes can add up to significantones with time.

    Figure 14: Deformation vectors of a damdeformation network, with 95% confidenceellipses of the vectors. Only the significantvectors are shown. (after Welsch et al. 2000,Fig. 9.3-2)

    Figures 15 to 17 give examples of the graphicalrepresentation of the long term deformation ofa large embankment (fill) dam in Switzerland.All results shown are based on geodeticmeasurements. The dates and reservoir levelsof the epochs shown in the figures arereproduced in Table 1. As mentioned above,the Mattmark Dam is located in the Swiss Alps,23 km ENE of the famous Matterhornmountain. The dam has a height of 120 m anda length of 770 m. The maximum reservoirlevel is at 2197 m above sea level. Theconstruction was completed in 1967 and thefirst filling occurred in 1969. The surveynetwork is shown in Fig. 9.

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    Epoch Date Water Level (m)

    22 6.1971 213423 10.1971 219424 6.1972 213225 9.1972 219026 6.1973 214327 10.1973 219628 6.1974 213529 10.1974 219330 6.1975 214331 10.1975 219752 6.1977 215853 10.1978 219655 6.1980 214056 10.1981 219658 6.1983 2145

    Table 1: Mattmark Embankment (Fill) Dam,Switzerland. Dates and corresponding reservoirlevels of the measurements shown in Figs. 15 to17. (after Gilg 1985)

    Figure 15 shows the horizontal movements offive points on the crest. Evidently, the crestpoints move towards the centre of the dam aswell as downstream. Also, some plasticmovement continues 16 years after completionof the dam. An elastic behaviour of about 2 cm

    between full and empty reservoir can beobserved.

    Figure 16 (after Gilg 1985, Fig. 4.3.3-5) showsthe vertical and downstream displacements ofthe Mattmark Embankment (Fill) Dam,Switzerland, from June 1971 to June 1983. Thedata are from geodetic measurements of theobject points (306 to 312) in the central profileM. Note that the construction was completedin 1967 and the first filling in 1969. Since theobject points on the upstream face can only bemeasured at low water levels, they exhibit acontinued settlement and downstreammovement. (Point 309 moves 248 mm downand 92 mm downstream over 12 years.) Themarks on the downstream face show anadditional reversible horizontal movement ofabout 20 mm between full and empty reservoir.

    Figure 17 (after Gilg 1985, Fig. 4.3.3-6) depictsthe vertical movements (settlements) of thesurvey marks in the drainage gallery of theMattmark embankment (fill) dam, Switzerland,from June 1964 to June 1983. (Construction:1961 - 1967, first filling: 1965 - 1969.) Gilg

    Figure 15: Mattmark Embankment (Fill) Dam, Switzerland. Horizontal displacements (from geodeticmeasurements) of the object points (110 to 510) on the crest from 1971 to 1983. The constructionwas completed in 1967 and the first filling occurred in 1969. See Fig. 16 for the movements in thecentral Profile M. (after Gilg 1985, Fig. 4.3.3-4)

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    (1985) reports that the maximum settlementreached 1.85 m by January 1967. The firstcomplete filling of the reservoir (by thenorthern autumn of 1969) caused a furthersettlement of 0.30 m. The following 13 fillingcycles added another settlement of 0.20 m. In1985, the drainage gallery settled at about 10mm/year with no end of the settlements insight.

    The report on geodetic measurements mustsatisfy a disparate group of readers, such as theadministrators, technical and legal staff of theowner of the dam as well as civil engineers, damspecialists and chief surveyors (Egger 1997).The findings and conclusions should beexpressed in such a way that they aremeaningful for all targeted readers.

    Following mainly Egger (1997 1993), theessential components of the report on thegeodetic measurements (of an epoch) can belisted as follows:(a) date and times of survey(b) external conditions (reservoir level, airtemperature, weather)c) condition of the installed equipment, anydeterioration or damage to the reference orobject points, any new modification orextensions

    (d) instruments used, names of the observers

    Figure 17: Mattmark Embankment (Fill)Dam, Switzerland. Settlement of the objectpoints in the drainage gallery in the base of thedam from 1964 to 1982. The construction wascompleted in 1967 and the first filling occurredin 1969. 1: alluvium, 2: bedrock (after Gilg1985, Fig. 4.3.3-6)

    Figure 16: Mattmark Embankment (Fill) Dam, Switzerland. Vertical and downstream displacements(from geodetic measurements) of the object points (306 to 312) in the central profile M from 1971 to1983. (See Fig. 15 for the location of the profile.) The construction was completed in 1967 and thefirst filling occurred in 1969. (after Gilg 1985, Fig. 4.3.3-5)

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    (e) number of measurements taken andprogress of observations with time (observationprogram)(f) operating methods(g) results of measurements and their accuracy(incl. 95% confidence intervals)(h) geodetic analysis (interpretation) of theresults (incl. significant deformations since thelast epoch at 95% confidence level)(i) appendices

    Egger (1997 1993) adds that items (d), (e) and(f) are mainly for the benefit of the surveyorsand assist the planning and execution of themeasurements of the following epoch.

    5 GEODETIC MEASUREMENTTECHNIQUES: PAST,PRESENT AND FUTURE

    The hardware and methodology of geodeticdeformation measurements have changedconsiderably since the first intersectionmeasurements of the 1920s (see Section 1).Because of the long service life of dams, thetype of survey equipment, the observationprocedures and the analysis techniques are likelyto change over the lifetime of the structure.Accordingly, the geodetic network has to bedesigned as flexible as possible.

    For an up-to-date listing of all geodeticmeasuring tools currently considered for damdeformation measurements, the readers arereferred to a recent and detailed publication ofthe Swiss Committee on Dams (2005). Thesummary given here is derived from thispublication as well as from Welsch (2000) andothers.

    TheodolitesThe theodolites used in geodetic deformationmeasurements have changed over the years butthe precision of the directions and zenith anglesmeasured with them very little. The Wild T3precision theodolite was already foreshadowedin Lang (1929) and was a marked improvement(for the measuring comfort) on the verniertheodolites used earlier. (According to Dedual(2005), Series 1 was in production from 1927to 1934 and Series 2 ('NT3') from 1935 to1957.) This theodolite allowed the removal ofthe bottom plate of the tribrach and theattachment of a centring ball (Freiberger ballcentring, diameter ~16.5 mm) underneath fordirect centring in the pillar bolts that are

    referred to as 'Wild Bolts' by ANCOLD (1983,Fig. 4.1.2 e). Wild started the manufacture ofthe precision electronic theodolite Wild T2000in 1983. The Wild T3000 was manufacturedbetween 1989 and 1997 and the Leica T2002from 1988 to 1996 (Dedual 2005). Thestandard versions of these instruments do notfeature the Freiberger ball centring for the('Wild') bolts used since the 1920s. Theseinstruments are not more accurate than theearlier Wild T3 or Kern DKM3, but theyfeature dual-axes level sensors (important forsteep sights) and digital recording. Some ofthese instruments are available in motorisedversions with or without automatic pointing (toEDM prisms only). Leica is advertising the(motorised) electronic theodolite TM5100 asthe replacement of the earlier T3000. Thesetwo instruments feature the same telescopes.The angle encoders of the TM5100 areaccurate to 0.5" and the servo motors have apositioning accuracy of 0.7" (Leica 1997). It isnot known if the TM5100, as the T3000 itreplaces, can be accurately attached to the Kernpillar centring plates using the system shown inFig. 24.Sadly, the motorised video theodolites (KernE2SE and Wild TM3000V), that were availablein the 1990s (Reger 2003) and, theoretically,permitted automatic measurements to circulartargets on downstream dam faces, are no longeravailable. Since at least one electronictacheometer is presently (2006) sold with abuilt-in CCD camera, there may be hope thatinstruments capable to measure to non-reflective targets such as the target bolts shownin ANCOLD (1983 Fig. 4.1.2 d), do emerge(again). Egger & Walser (2005) quote a typicalprecision of directions measured with precisiontheodolites as 0.7" and a typical centringprecision of smaller than 0.1 mm.

    Electronic Distance MetersThe first small electro-optical distance meters(precision 10 mm) became available in 1969(Reger 1996). The first precision distancemeter (Kern Mekometer ME3000, see Fig. 7)entered the market in 1973. The newer KernMekometer ME5000 (see Fig. 13) was releasedin 1986. Both types of Mekometers were usedwidely for the deformation measurements of(concrete) dams. Sadly, none of the dedicatedprecision distance meters is still in production.Stand-alone electronic distance meters measureslope distances only. The complementaryangle measurements must be obtained with atheodolite. No wonder that today's surveyorsprefer to use instruments that can measure in

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    three dimensions (horizontal directions, zenithangles and slope distances). These combinedinstruments are called electronic tacheometers(or, sometimes, 'total stations').Egger & Walser (2005) quote a typicalprecision of slope distances measured withprecision distance meters as (0.1-0.2 mm + 1ppm) and centring precisions of less than 0.1mm. The ppm term is likely to include theuncertainty of the temperature, pressure andhumidity measurements that are required tocorrect the measured distances. In the geodeticdeformation network of the Mangrove CreekDam (NSW), Reger (1995) demonstrated thatthe ppm term can be significantly reduced if thelocal scale parameter method is used (and thetemperature, pressure and humidity areignored).

    Figure 18: Red acrylic reflectors (roaddelineator, 80 mm diameter) on UNSW carrierwith rotating vertical axis fitting the 5/8 inchthread of the object points on the downstreamface of the Mangrove Creek Dam in NSW.(Reger 1994, Reger & Sippel 1994)

    To achieve their rated precision, distancemeters (as well as manual and robotic electronictacheometers) must measure to the (glassprism) reflectors suggested by the respectivemanufacturers. However, there are cases wherethe full precision is not required. In such cases,cheaper reflectors might be used once theirperformance with the intended distance meterhas been tested. (Investigate errors in anglemeasurement and change of reflector constantwith distance AND with angular mispointing ofreflector, horizontally and vertically.) The(red) acrylic reflector shown in Fig. 18 wassuccessfully used by UNSW with theinstrumentation shown in Fig. 19 and someother Wild electronic tacheometers of thattime.

    Electronic TacheometersElectronic tacheometers have been around sincethe early 1970s. The early instrumentsrecorded data on telex paper tape or audiocassette tapes. The second generation ofelectronic tacheometers followed in 1977/8(Reger 1996). By the mid 1980s, allmanufacturers concentrated on this type ofinstrument. The Wild TC2000 wasmanufactured from 1983 to 1987 and the WildTC2002 from 1990 to 1997. Currently, Leicaoffers the TCA1800, TCA2003, TC2003 andthe TDA/TDM5005 for more precise work.(The TDA and TCA models are motorised andcan point automatically to reflectors.) Datarecording is now slowly changing from PC-cards(PCMCIA-cards) to 'CompactFlash' cards (CF-cards).

    Figure 19: Early type of surveying robot(Wild TM3000D and DI3000, owned andoperated by UNSW) on a reference point of theMangrove Creek Dam (NSW), automaticallymeasuring to other reference points and theobject points (Fig. 18) on the downstream face.Note the (stainless steel) pillar centring systemwith 5/8 inch Whitworth thread that allowsdirect mounting of most (if not all) surveyinginstruments presently on the market. (Reger1994, Reger & Sippel 1994)

    Robotic Electronic TacheometersRobotic electronic tacheometers are equippedwith servo motors on the two axes and withautomatic pointing systems. Some types canfind reflectors autonomously all over thehorizon, others need training (or an equivalentdata file) to find the reflectors. Fine pointing isautomatic in both cases. For the monitoring ofdams, the robotic instruments that require aninput data file with the approximate pointingsis appropriate. This ensures that the

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    observation routine is the same in all epochs.Figure 19 shows one of the earlier roboticelectronic tacheometers used by UNSW withstudents for deformation measurements on theMangrove Creek Dam (NSW). Note the(simple) pillar centring with 5/8 inchWhitworth thread known from surveyingtripods.Today, the robotic instruments are smaller,faster and more accurate, particularly as far asthe measurements of directions and zenithangles are concerned. Leica (1997) quotes theprecision of the automatic target recognitionsystem (ATR) of the TCA2003 and TDA5005as 1 mm for reflectors closer than 200 m and2-3 mm for reflectors at 500 m. Theseinstruments can be operated remotely if theappropriate software and communication linksare available. Permanently installed roboticinstruments are used for the continuousmonitoring of dams on tectonic fault lines, inearth quake zones or on sites that are at riskbecause of tunnelling underneath the dam andreservoir (Brker 2006, Chrzanowski et al.2006, Duffy et al. 2001, Wilkins et al. 2002).Krickel et al. (2001) measured the referencepoint network of a German dam (Dreilgerbach)with a robotic electronic tacheometer Zeiss EltaS10 twice, once with manual pointing and oncewith automatic pointing. The measurementswith automatic pointing ('FineLock') were atleast as accurate as the manual measurements.The automatic direction measurements(automatic pointing and software for automaticdirection measurements) reduced the puremeasuring time at instrument stations by 70%.Total field time at the dam was reduced by 20%using robotic measurements.

    Levelling InstrumentsFor more accurate work in dam deformationmeasuring schemes, only precision levellinginstruments with their corresponding invarstaffs should be used. The original Wild N3precision spirit level was in production from1929 to 1970 and the updated version from1973 to 1996 (Dedual 2005). The 10 mminvar staffs are usually 3 m long; 2 m longversions were produced for tunnels and dams aswell as shorter ones for the levelling of pillarson dams, for example. The first automatic(compensator) levelling instrument (Ni2) wasreleased by Zeiss (Oberkochen) in 1950.Special observation routines must be followed toovercome some aspects of these instruments.Pre-1984 automatic levels might be affected bythe Earth's gravity field (Reger 2006). Themost sophisticated automatic level was the

    Zeiss (Jena) NI002. Digital levels appeared in1990. By 1992, the first precision digital level(WILD NA3000) and the first invar bar codestaffs were available. The Zeiss DiNi12 ispresently the most accurate digital level ifinherent systematic errors are considered.Users of digital levels for dam deformationsurveys should be well aware of the errors ofdigital AND automatic levels since all digitallevels are also automatic levels. Theillumination of staffs in galleries remains aproblem. Egger & Walser (2005) note that it isessential that levelling runs be carried outindependently forward and backward sinceredundancy is always poor. These authors alsonote that precision levelling on dams, thoughsimple in principle, is full of surprises.

    Satellite Signal ReceiversSatellite signal receivers using GPS(USA),Glonass(Russia) and/or Galileo(Europe) satellitesignals can achieve a precision of about 10-20mm for the coordinate vector between stationsirrespective of observation and processingmethods (Egger & Walser 2005). Betterprecisions can be achieved but require a muchlarger measuring effort and more sophisticatedprocessing softwares (e.g. Bernese). In smalldeformation networks, extending 1-2 km inboth (horizontal) coordinates, GPS canpresently not achieve the precisions oftraditional techniques because of a number ofsystematic errors that cannot easily be modelled(Egger & Walser 2005). Satellite techniqueshave, however, a number of advantages:stations do not have to be inter-visible and themethod is largely independent of weather andday-time. Satellite techniques are suitable toextend the dam monitoring networks toreference points further away in geologicallystable ground and to monitor unstable terrainnear and along the reservoir. Swiss NationalMapping started using GPS in 1989 mainly forthe extension of older dam monitoringnetworks. Schneider & Wiget (1997) giveexcellent advice on the procedures to befollowed for this purpose. Reflecting surfaces(including the water surface in the reservoir)near satellite antennas are a particular difficultproblem. Schneider & Wiget (1997) discuss 3Dand 2+1D adjustments of combined terrestrial-satellite data and demonstrate the precision ofhorizontal GPS vectors (maximum length about4 km) of 1.5 mm (vertical component: 3.0mm) in the combined adjustment withhorizontal directions (1"), zenith angles(1.3") and slope distances (0.3 mm + 0.5ppm). Since satellite techniques provide the

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    results in a 3D geocentric Cartesian coordinatesystem and the terrestrial data are essentially inlocal 2D+1D coordinate systems, the deviationsof the vertical, the difference betweenellipsoidal and geoidal heights and the scaledifference between terrestrial distancemeasurements and the satellite measurementsmost be taken into account. For these reasons,combined ('hybrid') adjustments require experts.

    PhotogrammetryTypically, aerial photogrammetry providesprecisions of 0.01% and 0.015% of the flightpath above ground of horizontal and verticalcoordinates, respectively (Flotron 1997). Withmuch overlap, a precision (in cm) of 0.05% ofthe photo scale can be achieved, that is 2 cmfor a photo 1:4000. Usually, photogrammetryis too expensive (and often not accurateenough) for the monitoring of the dam proper.However, photogrammetric techniques are idealfor the monitoring of terrain movements nearor along the reservoir. If centimetre precisionis required, monitoring points have to bemarked. If precisions of decimetres aresufficient, no monitoring points need to bemarked. Naturally, some (marked) referencepoints around the area of interest must beavailable.Fryer & Barlett (1989) reported on the use ofterrestrial photogrammetry for the monitoringof the Chichester Dam in NSW. The dam is254 m long and 43 m high. A precision of 3-8mm was achieved in all components of theobject point coordinates.

    Laser ScannersLaser scanners became available about 10 yearsago. They are instruments that can measuredistances, zenith angles and direction in a gridpattern over structures and terrain (Reger2003). The measurements are fast and do notrequire reflective targets on the structure orterrain. Reflectors must be placed on a fewreference points so that the data can betransformed later into the dam coordinatesystem. The maximum range of these devicesvaries greatly between brands and models,typically 100 m to 350 m. Precision gets worsewith distance: 5 mm/50 m and 25 mm/200 mare quoted (Reger 2003). Since laser scannersprovide 3D models of the area surveyed, theyhave similar applications as photogrammetry.Schulz & Zogg (2006) discuss laser scannermeasurements (with a Leica HDS 3000) of aconcrete dam in Switzerland. The instrumentwas set up 100 m from the dam. The dam wasscanned (in 30 minutes) with a point density of

    about 0.20 m. These authors measured fourepochs. The plots of the coordinate differences(i-th epoch minus reference epoch) show valuesof up to 5 cm near the abutments. The authorsattribute these differences to measuring errors(and not to dam deformations). They quoterelatively long distance, humidity or wetness ofthe concrete, angle between the measuring beamand the structure (ideally 90) and inaccuracy ofthe reference system used as likely sources ofthe discrepancies. They conclude that, for thetime being, the conventional measurements areto be preferred for the monitoring of the damproper.Laser scanners can be replaced by the cheapermotorised reflectorless electronic tacheometers,which may have longer range and betterprecision but are much slower. They need to beprogrammed to allow scanning. Because of thetime involved, such instruments usually scan thestructure with a much lesser density of points.

    Laser TrackersLaser Trackers contain a tracking laserinterferometer as well as angle encoders on themirror that keeps the laser beam on the prism(Reger 2003). The tracker follows an EDMprism mounted inside a steel (Taylor-Hobson)sphere as it is moved by an assistant from onemeasuring point to the next. If survey marksare to be measured, they have to have a conicaltop so that the sphere can be placed in it. (TheCERN centring system has this facility built in.)Starting point for measurements is a referencepoint on the instrument. The 3D accuracy isabout 0.03 mm per 10 m. The range of thetrackers is typically 35 m. Some trackersfeature an additional precision distance meteron board so that a measurement sequence doesnot have to be restarted after a beaminterruption. Laser trackers are used inmetrology but could be used to replacetheodolites and the DISTINVAR in the gallerytraverses in concrete dams. More likely thannot, this would require some changes to thetraversing networks.

    Hanging and Inverted PendulumsSince pendulums are the measuring instrumentsof choice inside concrete dams, they are brieflymentioned, even though they are not usuallyoperated and installed by surveyors. Thesedevices are used to measure horizontaldisplacements. The accuracy is 0.2 mm. Thetension in the wires varies between 200 N and2000 N. Manual and automatic read-outdevices are available. Some good advice oninstallation and operation can be found in the

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    recent publication of the Swiss Committee onDams (2005). Both types of pendulumequipment are commercially available (e.g. seewww.huggenberger.com, under 'inclination').

    Wire AlignmentThe alignment with horizontal steel wiresallows to detect the horizontal movements ingalleries and, if it is the only option, on thecrest of (concrete) dams. Jakob (1969) andMilev (1985) summarised Bombcinsij'sdevelopment of a wire alignment system for theKuibyschew Dam in the former USSR. A 1 mmdiameter wire was used over 600 m, at 600 Ntension, with an alignment precision of 0.3mm. At least three floating supports (see Milev1985 for photo) were used. The offsets weremeasured at 20 positions. Deumlich (1976)reported on a wire alignment over 825 m in theformer USSR (Krasnojarsk), with float supportevery 60 m, a wire diameter of 1.2 mm, atension of 2000 N (200 kgf) and an alignmentprecision of a few 0.1 mm. For the Kirov Damin Kirgiskaya the wire was supported by floatsin a water channel along the dam. Thiseliminates sag, reduces the influence externalinfluences on the wire and increases accuracy to0.1 mm (Deumlich 1976).The Swiss Committee on Dams (2005) quotesthe accuracy of wire alignment as 0.2 mm andstates that spans of 200 m are achievable withsags of less than 0.20 m. In Switzerland, thewire is tensioned at one end through a pulleysystem. Longer lengths require intermediatesupports that allow free lateral movements.Float suspension as in inverted pendulums canbe used.The measuring precision is independent oflength and refraction; automatic reading anddata transfer are possible. Air currents ingalleries should be minimised since they canaffect the wire's position. No commercialequipment seems to be available. Butinstallations in Switzerland (4 gravity concretedams) date back many years. The dam('Schrh') shown in Figs. 1 and 2 was retrofittedwith a wire alignment system on the crest in1973. The system on the crest of another dam('Rempen') is shown in Fig. 5. Photos of thekey components of a mechanical alignmentsystem in a dam gallery can be found in thepublication by the Swiss Committee on Dams(2005) together with advice on installation anduse.CERN in Geneva is developing a new generationof a wire alignment system for the newCompact Linear Collider (CLIC). They use a60 m long carbon fibre wire of 0.32 mm

    diameter to measure horizontal and verticaldisplacements in real-time, using capacitivepick-offs. Tension is applied by 6 kg weights(60 N). The repeatability is 1 micrometre. Inits present form (see www.fogal.fr) this device isnot rugged enough, and does not have a largeenough measuring range for dam measurements.However, the potential of measuring twocomponents (also replacing levelling andhydrostatic levelling) in real-time is attractiveif the instrumentation can be transplanted fromclean laboratories to not so clean and dry damsites.

    Hydrostatic LevellingHydrostatic levelling has been in use for a longtime. Commercial manual and automaticequipment for deformation measurements isavailable from Freiberger Przisionsmechanik(ww.fpm.de), for example. The resolution ofthese systems is 0.01 mm. According to theSwiss Committee on Dams (2005), themeasuring units have to be connected by threetubes (3-4 mm diameter for the tubes carryingthe water and 6-7 mm for the air tubes). Thewater used has to be degassed. Unequaltemperatures along the tubes are anotherproblem. Inside the dam, hydrostatic levellingcan replace precise levelling if properlyhandled. It has the advantage, that the readingscan be made automatically and becommunicated to a control centre elsewhere.In particle physics laboratories, hydrostaticlevelling systems with resolutions at themicrometre level have become very popular.(See www.emp-winterthur.ch and www.fogale.fr,for example.)

    Invar Distance MeasurementThere are two modern distance measuringinstruments that are based on invar wires. TheDISTINVAR was developed in the 1960s by theEuropean Organization of Nuclear Research(CERN) in Geneva. It is commercially available(including an electronic readout version) fromthe company G e o d e s i e I n d u s t r i e l l e(www.geodesie.com). The Distometer wasdeveloped by the Swiss Institute of Technologyand marketed through KERN & Co Ltd. (I donot know if the Distometer is still availablecommercially.) Both devices are depicted inKeller (1978), use wires of 1.65 mm diameterand can measure lengths from 1 to 30 m with ameasuring range of 50 mm (DISTINVAR) and100 mm (Distometer). The instrumentsmeasure only the changes of length betweenepochs. Since the lengths of the wires are fixed,the point spacing (in dam galleries for example)

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    must be standardised to avoid the need to buymany different lengths. For absolutemeasurements, a calibration of the system isrequired. It is advisable to have 2-3 wires of thesame length and to measure a number of stablereference lines near the dam before and afterthe dam measurements. (Fig. 9 shows thelocation of a tunnel with a calibration facilityfor invar wires.) Further advise on themeasurements with these devices can be foundin Egger & Walser (2005).

    Optical AlignmentThe optical alignment (across the crest of adam) qualifies as a simple technique. Dedicatedequipment is available from the firm FreibergerPrzisionsmechanik in Freiberg, Germany(www.fpm.de). As expected, this equipmentdoes use the Freiberger Ball centring for thealignment telescope and the fixed target (onboth ends of the alignment axis). The balldiameter is, however, larger than the 16.53 mm(Lang 1929) of the traditional centring boltsadvertised (in the past) by Wild and Kern.Freiberger Przisionsmechanik (www.fpm.de)also supplies special marks for the (road on the)crest and an adjustable target that allows tomeasure the offset of ground marks from theline. The accuracy of the method depends onthe length of the alignment axis and thehorizontal refraction errors. As mentionedbefore, refraction errors of 14 mm at 300 mhave been found experimentally (Biedermann1997). Egger & Walser (2005) suggest thatoptical alignment is obsolete and is betterreplaced by other techniques.

    Optical PlummetsThe accuracy of optical plummets is given as0.5 to 1 mm by Welsch et al. (2000, p. 59).Although surveying instrument manufacturersdo sell specialised zenith (up) and/or nadir(down) plummets, plumbing upwards can bedone equally (if not more) precise withtheodolites that have a diagonal eyepiece. Ifthe dam is equipped with vertical shafts, thenwire pendulums are the preferredinstrumentation for many reasons. Opticalplummets (or theodolites or electronictacheometers fitted with diagonal eyepieces)may have to be used to connect the top galleryto the crest (if the pendulum attachment pointis not at crest level). Such a case was discussedin connection with Fig. 9.

    Centring SystemsFreiberger Ball Centring (~16.5 mm diameter)

    In the 1920s, the Swiss dams were equipped withbrass centring bolts with a bore diameter to fitthe Freiberger Ball centring (diameter 16.53mm) of the theodolite 'without constraint butwith a tight fit' (Lang 1929). The Hildebrand(vernier) theodolites supported this pillarcentring as did the Wild T3 precisiontheodolites afterwards. (The T3 owned byUNSW has a centring ball of 16.52 mmdiameter.) Wild supported this pillar centringkit (Fig. 20) at least until 1986. The centringrepeatability is given as 0.02 mm when usedwith the Freiberger Ball attachment oftheodolites (or tribrachs). Untersee (1951,1975) shows how the Wild T3 fits the pillarbolt (1) shown in Fig. 20. According to theLeica-Geosystems web page (September 2006),the products shown in Fig. 20 are no longeravailable.

    Figure 20: Special Wild equipment formonitoring and deformation surveys: Wildpillar centring bolt (1), with ~16.5 mm bore andprotective cap, spot bubble for levelling boltduring attachment to pillar (2), bullseye targetfor centring bolts (3), bullseye target forvertical dam faces (4). (after Wild 1986)

    For all other Wild theodolites available in 1986(e.g T2, T2000, T2000S, TC2000) Wildsuggested a special tribrach for centring onpillars with the Freiberger Ball (diameter ~16.5mm) centring (Fig. 21). This arrangement issubject to two random centring errors, onebetween the instrument and the dish of thetribrach and one between the ball and the bolt,and a systematic one (ball in centre of tribrachdish). Naturally, this centring system alsoallows to fit most non-Wild/Leica equipmentonto pillar bolts. Sadly, the equipment shownin Fig. 21 is no longer listed in Leica's 2000'accessories' booklet nor on the current Leicaweb page. There is some concern about therotational stability of the set-up shown in Fig.21 when modern motorised theodolites areinserted; the angular accelerations anddecelerations of these robotic instruments

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    introduce torsional forces that may causeslipping of the footscrews' disks on the pillar.This rotational slipping would depend on theweight of the instrument, the torque applied bythe servo motor (around the vertical axis) andthe smoothness of the pillar surface.

    Kern supported the same bolt system (diameter~16.5 mm) with the pillar bolt system shown inFig. 22. The centring bolt has a protecting cap.The set includes three brass bolts for the feet ofthe trivet (Fig. 23). Two of these have flatsurfaces, one has a groove to provide torsionalstability. For example, the reference points inthe network shown in Figure 9 are fitted withthe centring kit of Fig. 22.

    Figure 21: Wild GDF24 Freiberger ballcentring device (diameter ~16.5 mm) withdetachable base plate and three discs for thefoot screws. A small scale allows to measurethe height of the tribrach over the bolt. Withthis tribrach, all instruments that fit the Wildtribrach dish can be mounted on the bolt shownin Fig. 20. (after Wild 1986)

    Figure 22: Centring bolt and three supportbolts (for the feet of the trivet) concreted intothe top of an observation pillar. Central boreof bolt fits 16.53 mm diameter Freiberger ballcentring. (after Kern 1971)

    To use the Freiberger ball centring bolts withKERN instruments, the trivet shown in Fig. 23had to be mounted over the bolt. The trivetfeatures a spot bubble (1) to level the top plate,a short centring rod (2) that can be lowered intothe bolt and a lever (3) to lock the ball and

    socket top. Again, the height of the trivetabove bolt can be read off a scale. The centringprecision of the trivet was better than 0.1 mm(Kern 1971). The trivet shown in Fig. 23 canbe used to mount all those instruments onFreiberger ball centring pillars, that fit the Kerncentring system. This includes the precisiondistance meter Kern Mekometer ME5000.

    Figure 23: Kern trivet for centring onFreiberger ball bolts of ~16.5 mm diameter.The trivet features a spot bubble (1) to level thetop plate, a short centring rod (2) that can belowered into the bolt and a lever (3) to lock theball and socket top. The height of the trivetabove bolt can be read off a scale. (after Kern1971)

    Kern Pillar Centring PlatesAbout 1973, Kern introduced an alternativepillar centring system with their 158 mmdiameter aluminium pillar centring plates.These plates were permanently attached(screwed) to observation pillars and, sometimes,object points. Important is that the top of theplate is horizontal after mounting (and that thepillar does not go off level with time). TheKern pillar plate is shown in Fig. 13 (and, later,in Fig. 24). The exterior network of theGigerwald Dam shown in Figs. 11 and 12 isequipped with the Kern pillar plates.Having a number of monitoring schemes basedon the Kern pillar plates lead to the problem offixing other makes of instruments onto them.One way was to use a special Wild tribrach, thatfeatures the Kern centring at the base. ThisWild GDF25K tribrach (Fig. 24) featured onefixed 'footscrew' and two adjustable ones so thatthe height of the tribrach dish did not changefrom one set-up to the next (provided that thetribrach is always mounted in the sameorientation).In addition, the central disk in the base of thistribrach can be removed so that the (10 mm)

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    ball centring extension for the theodolitesT2000, T2000S and T3000 can be used. Thesetheodolites (like the T3 before) carry a centralthread underneath that allows the attachmentof the ball centring extension to the instrumentbody. This centring rod then goes directly intothe central bore of the Kern centring plate andcentres the theodolite (not the tribrach) to 0.2mm over the pillar plate. This tribrach wasavailable in 1986 (Wild 1986) but not anylonger in 2000 (Leica 2000).

    Figure 24: Wild GDF25K tribrach with onefixed 'footscrew' and two adjustable ones. The(10 mm diameter) ball centring extensionshown on the left is explained in the text. Thistribrach allows to fit any Wild-tribrachcompatible instruments to the Kern pillar plateor (Kern tripods for that matter). (after Wild1986)

    Centring on fixed 5/8 inch Whitworth ThreadMost current surveying instruments featureWild-type tribrachs, which carry a female 5/8inch Whitworth thread (with 11 turns over 25.4mm, after British Standard BS 84) underneathfor the attachment to tripods. Therefore, itsuggests itself to equip the top of pillars with asecure (stainless steel) 5/8 inch Whitworththread, usually centred on a solid and smoothstainless steel plate. The pillars oftrigonometric stations in NSW are so equippedas are the observation pillars (and downstreamface points) on the Mangrove Creek Dam inNSW, for example. Figures 18 and 19 showexamples of the latter.I do not know of any investigations into therepeatability of the centring of instruments onsuch pillars or marks. The centring precision iscertainly worse than the 0.02 mm of theFreiberger Ball centring and somewhat worsethan the 0.1 mm centring precision of theKern trivets (Fig. 23) on the pillar bolts (Fig.22). This simple pillar centring system mightbe entirely appropriate where centring errors ofa few tenths of a millimetre can be tolerated,

    namely where no short rays are involved andwhere no equipment of highest precision isbeing used.Since embankment dams typically show largerdeformations than concrete dams, this simplepillar centring might be more appropriate forthe former. Galleries in new concrete damsmay have to be equipped with the CERNcentring system discussed below (since the 16.5mm bolts and the Kern pillar centring plates(158 mm diameter) are no longer commerciallyavailable). For robotic instruments with fastacting servo motors (large rotationalaccelerations) this simple centring systemmight provide more torsional stability than thesystem shown above.

    Figure 25: Adaptor for instrument tribrachswith 5/8 inch Whitworth thread that fits (andlocks in) the 30 mm CERN centring system(see Fig. 8, from www.geodesie.com, 'UniversalFixation for Wild')

    CERN (30.00 mm) Centring SystemThe system shown in Fig. 8 is the currentversion of the 30 mm cylindrical CERNcentring system. It was redesigned to allow thecentring cylinder to be levelled (e.g. madevertical). The centring accuracy is at the 0.01mm level. Taylor-Hobson spheres (which cancontain circular targets or EDM prisms) can beput into the conical surface at the top with aheight repeatability of 0.02 mm (CERN 1974).This is of advantage if laser trackers are to beused. The CERN centring system is very sturdysince it has to serve the CERN DISTINVAR,which applies a tension of 150 N to the invarwire (against the centring cylinder).In deformation measurements, the CERNcentring system has been used in galleries ofconcrete dams for the traversing network. TheCERN centring system is commerciallyavailable (from Geodesie Industrielle in Geneva,

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    Switzerland, www.geodesie.com). Figure 25shows how standard surveying equipment can beattached (and locked!) in the CERN centringdevice with adaptor plates fitted with thecustomary 5/8 inch Whitworth thread on top.The likely centring errors between tribrach andthe 5/8 inch Whitworth thread have beendiscussed in the previous section. It is worsethan the 0.01 mm centring between the CERNsocket (Fig. 8) and the adaptor shown in Fig.25.

    6 DEFINING ENGINEERINGNEEDS

    As early as possible in the design phase of adam, the engineering needs should becommunicated to the surveyor. Earlydiscussions are essential since the positions ofshafts for pendulum systems and the locationsof galleries inside the dam have to interfacewith the geodetic deformation measurementscheme, for example.

    1D, 2D or 3D?Although the actual radial (upstream-downstream) movements as well as the up-downmovements (settlements) are of primaryinterest, the lateral (tangential) componentsalso need to be known since an abnormal dambehaviour can include rotations and block shifts.The scheme must also cover any potentiallyunstable terrain around the dam and unstableslopes or rock formation along the reservoir.Since, the geodetic measuring scheme has toprovide the reference for any future specialisedmeasuring schemes established in response toabnormal behaviour, it should be as redundantand wide ranging as possible. The geodeticnetwork on large dams should provide 3Dinformation in an absolute sense, that is againststable terrain outside the influence zones of damand reservoir.

    Magnitude of Anticipated DeformationsThe engineers must indicate the expecteddeformations and displacements of criticalpoints on the planned dam for the differentstages of the dam that need to be measuredsince the required measuring precision mustrelate to them. Measuring stages of interestare: total long-term deformation before and after first filling (possiblyadditional measurements at the 1/2 or 1/3 and2/3 points of filling)

    before and after the first emptying (possiblyadditional measurements at the 1/2 or 1/3 and2/3 points of emptying) seasonal changes (summer, winter;temperature often dominant influence)Once the expected deformations for the aboveevents are defined, the required surveyingprecision can be estimated with a rule-of-thumbdesign equation. From the expecteddeformations of the listed events above, thesmallest one is the minimal movement (dy) ofobject points that the geodetic measurementshave to be able to resolve. The measuringprecision (sy, in one epoch, one standarddeviation) can be estimated conservatively as(Welsch et al. 2000 Eq. 1.3-3, Pelzer et al.1987, Eq. 2.1-3)

    sy dy/5 (1)This formula considers that two epochs ofmeasurements are needed to obtain a change inthe position of an object point and thatchanges larger than dy must be significant at95% confidence level. Pelzer et al. (1987) alsogive other rule-of-thumb advice that is usefulwhen designing deformation networks. (ForEnglish translation, see Reger 1997).Based on Eq. (1), the surveyor can selectappropriate measuring instruments andtechniques and design a monitoring network.Using least squares adjustment programs, theexpected precision (95% confidence errorellipses) of the deformation vectors betweentwo epochs can be simulated (without actualobservations). The 95% confidence ellipses ofthe critical object points must all be smallerthan the (dy) specified. During the preliminaryanalysis of the geodetic network, the reliabilityand redundancy of the network design must bechecked. The suggestions by Carosio & Dupraz(1997) are helpful in this regard. The potentialloss of one or two reference points must befactored in.

    Density of Object PointsRecent trends show that, wherever possible, themonitoring points are established inside thedam, where galleries are interfaced withpendulums in vertical shafts. Ideally, thependulum measurements connect to horizontalwire alignments (rather than to surveyingprecision traverses) in galleries or along wallson the crest. So, ideally, no object points willbe placed on the outside of dams other than onthe crest. This approach suits gravity concretedams and some concrete arch dams. See Figs. 6,10, 11 and 12. Since concrete dams are pouredin blocks, it is often required to place the

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    internal points in such a way that each block ismonitored.For embankment dams, the traditionalplacement of survey marks in rows (horizontalprofiles) and columns (vertical profiles) datesback to the 1920s and is well established andproven. Six vertical and four horizontal linesof object points (e.g. see Figs. 1 and 2) on thedownstream face of dams with no galleries andno pendulum shafts are a starting point for theconsiderations. Naturally, the number of objectpoints will depend on the size of the dam.Figure 9 shows an arrangement for a large dam(780 m long, 120 m high). The asymmetriclayout of the point profiles in Fig. 9 is due tothe increasing dam height towards the East.

    Short and (Potential) Long Term NeedsThe geodetic measurements will be morefrequent during the construction of the dam andduring the first filling and emptying of thereservoir. This is to check the actualdeformations against the expected ('design')ones. After that, geodetic measurements (fullnetwork or reduced network) are less frequentunless, naturally, an abnormal behaviour isdetected (by other means). Since the geodeticnetwork provides the reference for anyemergency measurements, it is sensible(Biedermann 1996) to measure a reducednetwork at least every 5 years (during safetychecks of the dam) and the full network every15 years (when the reservoir is emptied). Thegeodetic deformation network must be designedfor the same working life as the dam and mustbe maintained and protected accordingly.

    Coordinate System, Grid Orientati