FIRST EXPERIENCES WITH THE DEFORMATION ANALYSIS OF A LARGE DAM COMBINING LASERSCANNING AND HIGH-ACCURACY SURVEYING

FIRST EXPERIENCES WITH THE DEFORMATION ANALYSIS OF A LARGE DAM COMBINING LASERSCANNING AND HIGH-ACCURACY SURVEYING Diego González Aguilea a*, Javie Gómez Lahoz a and José Antonio Sánchez Seano b a Land and Catogaphy Engineeing Depatment, Univesity of Salamanca, Spain b ECONTOP S.L. Avila, Spain. * daguilea@usal.es KEY WORDS: Civil Engineeing, Lasescanning, Suveying, Defomation Analysis. ABSTRACT: Monitoing the static and dynamic behaviou of lage dams has always been a topic of geat impotance, due to the impact these stuctues have on the whole landscape whee they ae built. Many instuments and suveying methods have been applied so fa in ode to continuously access to the safety of this kind of huge stuctues. Howeve, the common appoach in a lot of developments has been focused towads the possibility of measuing, with high accuacy and eliability, displacements of a singula numbe of points. This pape pesents peliminay ideas and fist esults of a poject in pogess fo monitoing displacements of big stuctues such as lage dams using new technology, teestial lase scanne. Paticulaly, this pape poposes the development and testing of a methodology that allows us to pefom the ecoding and stuctual monitoing of lage concete dams combining lasescanning and high-accuacy suveying. Fo this pupose, a test field has been established on the dam of Las Cogotas (Avila, Spain). Fist esults and analyses have been focused on two main aspects: the netwok design; and the netwok adjustment that incopoates statistic tests in ode to povide moe accuacy and eliability. Finally, some futue poposals fo the defomation analysis of the lage dam exploiting the powe of lasescanning technology ae outlined. 1. INTRODUCTION The theoy about defomation analysis has been well-known since the eighties (Caspay, 1987). The main easons that contibute to its development wee its own necessity, as well as its technological advance, especially suppoted by accuate instuments and poweful computes. Nowadays, the defomation analysis is applied to a wide spectum of fields: contol of movements and defomations applied to engineeing woks, glide sufaces, cotical movements, glacies evolution and last but not least as phenomenons of subsidence. The basic goal is focused on analyzing the geometic vaiation of an object o suface with the passage of time. In this sense, thee ae seveal elements that take pat duing the pocess. i) Geometic evolution of the theoetical model with the passage of time. ii) Chaacteization about the geometic vaiation based on elative position of contol points. iii) Necessity of measuing the geometic vaiation though two basic methods: diect measuements (inclinomete, pendulum o extensometes) o indiect measuement based on the classical suveying methods and obsevables (angles, distances, height vaiations, etc.). Both appoaches need to be contasted though diffeent epochs. Paticulaly, egading stuctual monitoing of lage dams by high-accuacy suveying, seveal appoaches can be founded. Stating with the use of pseudolites which can be taced back to the late 1970s (Haington and Dolloff, 1976), whee the pseudolites in fact wee used to validate the GPS concept befoe launch of the fist GPS satellites. The concept of inveted pseudolite positioning was fist intoduced by (Raquet et. al., 1995). In thei expeiment, a gound-based test was conducted to investigate the feasibility of using the mobile pseudolites fo pecise positioning of militay aicaft; (O Keefe et. al., 1999) also discussed the pseudolite-based inveted GPS concept fo local aea positioning and pesented thei expeimental esults. In this system, moe flexibility was obtained and cost was educed because all the hadwae equipment and softwae wee configued on the gound, whee the powe, size and computational load constaints could be easily esolved. Regading defomation analysis based on GPS technology, a lot of appoaches have been developed. Paticulaly, in the context of lage dams, (Jeffey and Kenneth, 1999) pesented a monitoing stuctual defomation applied to a lage dam using GPS technology. Nealy thee yeas of data wee analyzed fom this sub-aay to povide quate-daily estimates of station-tostation baseline lengths. Quantitative methods wee assessed to allow the identification of a numbe of outlying data points that could be misintepeted as GPS station motion in a eal-time system. The expeiment successfully demonstated the applicability of continuous GPS to the field of stuctual monitoing. Moe ecently, with the emegence of lase scanne technology new appoaches and compute methods have been applied fo defomation monitoing of lage stuctues. (Stathas et. al., 003) pesented a new monitoing technique combining total station and gound-based lase scanne though plane fitting. Nevetheless, thei appoach to test lase scannes ability in detemining defomations has not been tested on moe complex

sufaces; (Godon et. al., 003) outlined an investigation designed to exploe the sensitivity of teestial lase scanne systems fo stuctual defomation monitoing tasks. Two expeiments ove wooden and concete beams wee pefomed involving contolled load testing of stuctues. A negative and citical aspect was focused on the selection of the most appopiate suface modelling technique. They used a gid fo modeling beams, but no intepolation was consideed in ode to getting an optimal and adaptative gid spacing; (Schäfe et. al., 004) intoduced fist opeational expeiences in the usage of Leica Geosystems DS1 500 lase scanning system fo the detemination of suface defomations of seveal centimetes which can be obseved at the lock gates of a hydopowe station duing the filling and emptying pocess; moe ecently, (Tsakii et. al., 006) pesented and developed new algoithms and methods that assessed and exploited the metic accuacy of a commecial lase scanne, especially accuate algoithms fo the detection of the cente of etoeflective tagets. Seveal expeiments wee tested in ode to assess the eliability and accuacy of the algoithms. Howeve, the expeiments wee only focused on small objects, so thee was nothing that involved expeiments of lage scales and applications such as lage dams; (Guaniei et. al., 006) showed a multi-senso appoach fo stuctual analysis applications in Cultual Heitage. To this aim a medieval wall was suveyed with combined suveying techniques: eflectoless total station, GPS fo establishing a local netwok and teestial lase scanne fo the geneation of a 3D model of the walls. Seveal cosssections and pofiles wee extacted and compaed with the passage of time in ode to pefom the stuctual analysis. Paticulaly, the stuctual analysis was based on finite element methods which wee moe suitable fo complex stuctues. The study is cuent unde development and it is aimed to detect the aeas of the wall equiing most impotant consolidation woks. To sum up, stuctual monitoing appoaches based on the lase scanne and emaked upon above, outline that lase scannes constitute a eal and effective solution. Lase scannes ae capable of acquiing a vey lage numbe of points, so that the stuctual contol could be extended to the whole stuctue instead of being limited to a few points. Howeve, although long ange lase scannes allow us to measue 3D points also at a distance of 800-1000 m, geate anges ae not consideed because the data quality shaply deceases and in stuctual monitoing accuacies aound a few millimetes ae always equied. The pape pesents the following stuctue and oganization: afte this intoduction, Section develops the netwok design. Section 3 explains in detail the data acquisition pocess. Section 4 descibes an accuate and eliable appoach fo the netwok adjustment. A final section is devoted to outline some futue poposals fo the defomation analysis of the lage dam exploiting the powe of lasescanning technology.. NETWORK DESIGN optimal netwok configuation and an optimal set of obsevations that satisfy the following goals: Reach a detemined global accuacy. Establish a eal stochastic model that can be tested. Design an obsevation campaign suitable unde pactical and economical consideations. The fist goal is conditioned diectly by the obsevables accuacy, the netwok configuation, the obsevation campaign, as well as the efeence system definition also known as the datum poblem. Unde no cicumstances the efeence system should be constained, due to lack of accuacy. The second goal is diectly elated with the eliability of the model. Finally, to fulfil the thid goal, it is vey useful to dispose of digital catogaphy about the emplacement which allows us to get a fist appoximation to obsevation campaign. In the past, it was vey difficult, if not impossible, to solve all aspects of the netwok optimization in a single mathematical step. Instead, the poblem of netwok design was divided into subpoblems in each of which some pogess could be made. The accepted classification poposed by (Gafaend, 1974) was: Zeo-Ode Design (ZOD): the datum poblem (efeence coodinate system) Fist-Ode Design (FOD): the configuation poblem. Second-Ode Design (SOD): the weight poblem. Thid-Ode Design (TOD): the densification poblem. Nowadays, the development of compute methods enables us to design, compute and contol the complex netwok design poblem with moe accuacy and eliability. Moeove, if the equiements of netwok design have not been ovetaken, an optimization of the netwok can be pefomed modifying o intoducing new netwok s paametes. Paticulaly in ou case, a high-accuacy netwok was designed composed by two diffeent blocks (Figue 1): A efeence block which was situated out of the dam stuctue, consisting of 4 main vetices mateialized by special tagets fixed in concete elements on the gound. The efeence block povided an extenal and fixed stuctual suppot, as well as the efeence system definition. An object block which was composed by 44 natual contol points placed on the dam font and 6 atificial contol points situated at the cest. Natual contol points wee pefectly measued thanks to some cicula maks pesent in the concete stuctue. Atificial tagets wee pefectly measued as well, because a special suppot was designed to place these tagets in the same position in each epoch. A fundamental step in the stuctual monitoing of lage dams is the netwok design. Conceptually, the pupose of the netwok design is to plan an optimal netwok configuation and an optimal obsevation plan that satisfy the peset quality with minimum effot. In othe wods, afte the definition of the netwok quality equiements (accuacy and eliability) the technique of netwok optimization allows us fo finding such an

3. Range data acquisition Regading ange measuements, a time of flight lase scanne, a medium ange Timble GS00 (URL1), was used in each epoch. This instument is motoized and allows angula and distance accuacies about of.5 mgon and 1.5 mm at distances below 100 m espectively. In ode to avoid alignment eos, lase scanne was always stationed in a fixed position (Figue 3) at mean distance of 100 m fom the dam suface. The dam was scanned with a diffeent esolution. The cental potion of dam, which is the pat subject to majo defomations, was scanned with mm esolution; while the est of dam was scanned with cm. Moeove, contol points belonging to the efeence and object block and epesented by special tagets wee scanned automatically with the highest esolution. Figue 1. Netwok design. 3. DATA ACQUISITION Data acquisition was accomplished by combining two diffeent sensos, lase scanne and total station. In May 006 (sping), when the dam was completely full, the fist campaign was pefomed. 3.1 The test field The dam of Las Cogotas (Figue ) was built in 1994 on the Adaja ive, geneating a basin of about 58.6 Hm 3 of wate. The dam pesents an ac stuctue featuing 66 m of height and 300 of length at the cest with a maximum wate level height of 1050.50 m. The consevation of the stuctue is good, thanks to the monitoing which is caied out by taditional sensos (stain gauges, inclinometes, etc.) and by peiodical geodetic measuements (levelling and geodetic contol netwoks, optical collimatos). Moeove, the availability of othe defomation measuements is vey impotant to access the esults obtained fom lase scanning. Figue 3. Lase scanne. This pocess was put in pactise in each epoch maintain the same citeions stictly. 3.3 Suveying data acquisition Regading high-accuacy suveying, the designed netwok (Figue 4) was obseved in each epoch with a total station, Leica TCA 003 (URL). This instument is motoized and allows angula and distance accuacies of 0.15 mgon and 0.5 mm at distances below 10 m espectively. Paticulaly, efeence and object blocks wee measued fom the 4 fixed vetices using angula measuements based on multiple intesections techniques. Moeove, contol points belonging to the efeence block and those placed at the dam cest wee measued using distances as well. Figue. Downsteam face of the dam of Las Cogotas. Figue 4. Total station.

4. NETWORK ADJUSTMENT In the stuctual monitoing context, the netwok adjustment poblem is undestood as the estimation of seveal paametes that descibe the stuctual and geometic suppot beginning with the obsevation of diffeent measuements. In this way, two aspects have to be taken into account: on one hand the elationships between obsevables and paametes, functional model, have to be established, and on the othe hand the stochastic pefomance of andom vaiables, statistic model, has to be descibed. Both models constitute the mathematical model. Paticulaly, the mathematical model consideed in this netwok adjustment was the Gauss Makov Model. The functional model was expessed as (1): L = F(X ) (1) The equations system was obtained fom the lineaization though Taylo s method () esticting its development to the fist ode: L F F = 0 X ( X ) + x The statistic model was expessed as (3): Ax = b + whee the b and vectos ae andom magnitudes which follow statistic popeties. Theefoe, the Gauss Makov Model established by the functional and statistic models emaked above was expessed as (4): GMM ( b, Ax, σ 0Q ) ll In ou case, two diffeent adjustments wee pefomed sequentially. At fist, the efeence block was adjusted based on least squaes adjustment and a fee netwok solution. As a esult, a efeence coodinate system (datum) was mateialized though the efeence block. Secondly, both blocks (efeence and object) wee adjusted consideing the efeence block fixed and the object block fee. Finally, once the adjustment was solved in its fist iteation, both blocks wee checked fo possible outlies using seveal statistic tests. Paticulaly, Baada (Baada, 1968) and Pope (Pope, 1976) tests wee applied iteatively, allowing us to detect wong obsevations coesponding to the object block. The next table (Table 1) shows some of the esults elated with the standad deviations of contol points belonging to the efeence block. The final aveage standad deviation in the efeence block was mm in XY and 3 mm in Z. X 0 Ax = b + N(0, σ 0Qll ) () (3) (4) Epoch May 006 Refeence Block (4 th iteation) Contol Standad Deviation (mm) Points σ X σ Y σ Z I7 1.3 1.4.8 I1 0.7 0.5 1. I4.4 1.8 3.1 I3.6 1.5.4 Table 1. Refeence block: standad deviations. Regading the object block, the final aveage standad deviation was 4 mm in XY and 5 mm in Z. The next table (Table ) shows some of the esults elated with the standad deviations of contol points belonging to the object block. Epoch May 006 Object Block (6 th iteation) Contol Standad Deviation (mm) Points σ X σ Y σ Z PM1 4.5 3.4 5.1 PM15 3. 4.5 6.1 PM1 4.1 3.1 3.0 PM37 4.3 3.7 5.8 Table. Object block: standad deviations. 5. TOWARDS A DEFORMATION ANALYSIS Once the netwok adjustment has been computed and checked fo possible outlies, a stuctual monitoing could be pefomed based on the analysis between epochs. A unification of each epoch should be pefomed taking the same efeence system, efeence block, fo each epoch. Cuently, this stage is wokin-poject and only the guidelines established ae outlined hee. In ou case, the idea will be to apply a stuctual monitoing following a twofold citeion. Fistly, in tems of the classical defomation analysis applied in Geodesy (Caspay, 1987) extacting the defomation vectos and the vaiance-covaiance matix. Secondly, applying a moe igoous analysis exploiting lase scanne dataset and making a compaison between epochs by consideing mathematical sufaces fitted to measued points. Regading classical defomation analysis, fom the combined adjustment of two epochs we will aange of defomation vectos and its vaiance-covaiance matix of obsevables. The defomation model will be established by the equation (5) whee, = oi oj Q = Q + Q σ B p 0 Bp = + ε E T [ ε ] = 0; E[ εε ] = Σ = σ 0Q ε N(0, σ 0Q ) oi oi oj oj Q oi oj Q oj oi (5) is the defomation vecto is its cofacto matix is the posteioi vaiance is the design matix is the paametevecto The analysis fom two obsevation epochs will be fundamental, because it will constitute the best way to analyze the movement poduced in two diffeent peiods. The main objectives of this analysis will be:

Confim the stability of the points belonging to the efeence block and detect singula movements. Contast a movement model fom the defomation vectos. Detect citical defomations not pedicted and which could have seious consequences. Regading lasescanning stuctual monitoing, the idea will be to apply fo defomation analysis an aea-based method (Schneide, 006), i.e. to make a compaison between data acquied fom diffeent epochs by consideing mathematical sufaces fitted to measued points. Since diffeent point clouds wee aleady geoefeenced into the same efeence system, the compaison between intepolated sufaces will allow us to evaluate displacements applied to the whole dam stuctue. The next figue (Figue 5) illustates a global lase scan model obtained fom dam ecoding. Refeences fom Jounals: Baada W. A testing pocedue fo use in geodetic netwoks. Nethelands Geodetic Com-mision, Publ. on Geodesy, vol., nº 5, Delf, 1968. Pope AJ. The statistics of esiduals and the detection of outlies. NOAA Technical Repot NOS 65 NGS 1, U.S. Depatament of Commece, National Geodetic Suvey, Rockville, Md, 1976. Refeences fom Books: Caspay W.F., Concepts of Netwok and Defomation Analysis. Monogafía 11, School of Suveying, The Univesity of New South Wales, Kensington, N.S.W., Austalia. Mazo de 1987. Refeences fom Othe Liteatue: Bouke P. Efficient Tiangulation Algoithm Suitable fo Teain Modelling. Pan Pacific Compute Confeence, Beijing, China. 1989 Godon S., Lichti D., Stewat M. and Fanke J., 003. Stuctual Defomation Measuement using Teestial Lase Scannes. Poceedings of 11th Intenational FIG Symposium on Defomation Measuements, Santoini Island, Geece Gafaend, E.: Optimization of Geodetic Netwoks. Bollentino di Geodesia a Acience Affini, Vol. 33, No. 4, pp. 351-406, 1974. Guaniei A., Piotti F., Pontin M., Vettoe A. Combined 3d suveying techniques fo stuctual analysis applications. Wokshop Meste 3Dach. 005. Venice (Italy). Figue 5. Global lase scan model applied to the whole dam stuctue. Paticulaly, in ode to get a stuctual monitoing of the dam suppoted by a compaison of sufaces, a double-step appoach will be pefomed. At the fist step, we will pass fom point cloud to suface (Figue 6) based on incemental Delaunay tiangulation algoithm (Bouke, 1989). In the second step, we will pefom a defomation analysis based on othogonal sections applied ove both sufaces belonging to diffeent epochs. The esulting sections will allow us to extact automatically displacements vectos not only applied to a limited numbe of points, but also to the whole dam stuctue, especially in the fontal pat of the dam. Haington, R.L. & J.T. Dolloff (1976). The inveted ange: GPS use test facility, IEEE PLANS 76, San Diego, Califonia, 1-3 Nov., 04-11. Jeffey and Kenneth. Monitoing Stuctual Defomation at Pacoima Dam, Califonia Using Continuous GPS. FIG Intenational Symposium on Defomation Measuements, 1999. O Keefe, K., J. Shama, M.E. Cannon & G. Lachapelle (1999). Pseudolite-based inveted GPS concept fo local aea positioning, 1th Int. Tech. Meeting of the Satellite Division of the U.S. Inst. of Navigation GPS ION-99, Nashvile, Tennessee, 14-17 Sept., 153-1530. Raquet, J., G. Lachapelle, W. Qui, C. Pelletie, A. Nash, P. Fenton & T. Holden (1995). Development and testing of a mobile pseudolite concept fo pecise positioning, 8th Int. Tech. Meeting of the Satellite Division of the U.S. Inst. of Navigation GPS ION-95, Palm Spings,Califonia. Schäfe T., Defomation Measuement using Teestial Lase Scanning at the Hydopowe Station of Gabčíkovo. INGEO 004 and FIG Regional Cental and Easten Euopean Confeence on Engineeing Suveying Batislava, 004. Shewchuk J.: Delaunay Refinement Algoithms fo Tiangula Mesh Geneation. Depatment of Electical Engineeing and Compute Science. Univesity of Califonia at Bekeley, 001. Schneide, D. Teestial Lase Scanning fo Aea Based Defomation Analysis of Towes and Wate Dams. In: Poc. of 3dIAG/1thFIG Symp., Baden, Austia, 006. Figue 6. Tiangula suface model of the dam.

Stathas, Aabatzi, Dogouis, Piniotis, Tsiniand and Tsinis. New monitoing techniques on the detemination of stuctue defomations. FIG Symposium on Defomation Measuements, Santoini, Geece, 003. Tsakii and Valani. THALES Poject No. 65/1318. 006. Refeences fom websites: URL1: http://www.timble.com URL: http://www.leica.com