Innovated Project of Geodetic Monitoring of Bočac Dam

Transcription

1 Innovated Project of Geodetic Monitoring of Bočac Dam Slavko Vasiljević 1, Branko Milovanović 2, Zagorka Gospavić 2 1 Faculty of Architecture, Civil Engineering and Geodesy, University of Banja Luka, Vojvode Stepe Stepanovića 77/3, Banja Luka, Republic of Srpska, Bosnia and Herzegovina, svasiljevic@aggfbl.org 2 Faculty of Civil Engineering, University of Belgrade, Bulevar kralja Aleksandra 73, Belgrade, Serbia, milovano@grf.bg.ac.rs, zaga@grf.bg.ac.rs Abstract. The main task of geodetic monitoring of dams over the period of exploitation is to detect the deformation on time that can cause human and material damage. The paper presents the innovated project of geodetic monitoring of the Bočac dam on the river Vrbas in the hydroelectric power plant Bočac. This innovated project was developed for the purpose of analysing the current state of geodetic control networks and to design a new 2D and 1D geodetic control network for monitoring of the dam. The design of the new 2D and 1D geodetic control network included: 1) defining the geometry, coordinate system, geodetic datum, observation plan, the accuracy of measurements, 2) the accuracy calculation and the verification of the correctness of design solutions, 3) defining the technical conditions for the performance of measurements, processing method and analysis of measurement results, adjustment of networks, the methods of determining shift of points on the object and the methods of presenting the results. Keywords: designing the geodetic control network, geodetic control network, geodetic monitoring of dams. 1. Introduction A great performance of all geodetic works is possible only with the establishment of a quality geodetic network. This network should be established according to the following principles in geodesy: from large to small, cost effectiveness resulting from accuracy, consistency, control, protection of data. This paper describes the procedure of making the renewed project of the geodetic observation of the Bočac dam. The dam was constructed on the river Vrbas as a part of the hydropower plant Bočac located 40 km from the city of Banja Luka. The existing project of geodetic observation from 1980 is inapplicable due to a number of changes. These changes are found in: the construction of new hydrotechnical objects, construction and reparation of the river dikes and access 189

2 SIG 2016 International Symposium on Engineering Geodesy, May 2016, Varaždin, Croatia roads, as well as due to the damage or destruction of the existing geodetic control network points. None of the newly constructed objects are included in the existing project of observation. Furthermore, the methods and techniques proposed in the existing project are outdated (almost never used nowadays). The goal of the renewed project is to include all the objects and lands that form a narrower complex of the hydropower plant and to optimize the geodetic observation in accordance with the accuracy demands, of objects and relief. A practical research was performed: 1) on the basis of the available technical data about the dam, the geodetic bases and the data about the existing control networks, 2) in accordance with legal requirements of the Republic of Srpska and the practical field experience from the neighbouring countries, and 3) in accordance with the project task related to the preparation of a renewed project. The calculation of the accuracy of geodetic control networks was done with the specially developed software package for designing of geodetic networks prepared by the assistant professor Branko Milovanović. 2. Analysis of the existing geodetic networks and the observation of Bočac dam The conclusions about the existing geodetic networks for the observation of the dam, its attached objects and the surrounding fields were made on the basis of the study of available data and by visiting the hydropower plant Analysis of the existing geodetic networks for the observation of the dam The geodetic 2D control network consists of a number of points of basic geodetic 2D network and a number of points on the body of the dam, the attached objects and the surrounding field, divided equally in accordance with the conditions in the field and the disposition of objects. The network consists of 30 points in total [Vasiljević 2015]. In the geodetic 2D control network, the directions of all points of the basic network were measured according to the visible network points and 2 basic lengths (microtrigonometric network base) D ST5-ST3 and D ST5-ST6. On the basis of these measurements, the coordinate of the network points were calculated in the local coordinate system. All the points of the geodetic 2D network were stabilized with geodetic poles as planned in the Main project observation. The points on the body of the dam were stabilized. The points for the observation of the rock had to be stabilized with small bolt poles on the head of the pole serving for the observation of directions. The geodetic observation of vertical movements was done by measuring the height differences with precise geometric levelling in four independent traverses set at four height levels [Vasiljević 2015]. The network consists of 60 benchmarks. All benchmarks were stabilized according to the instructions from the Main project deep benchmarks were 190

3 installed vertically, and benchmarks on the objects were mostly installed horizontally. The level lines begin on one and end on the other coast on so-called deep benchmarks (benchmarks with the mark DR). Deep benchmarks are considered to be the points of basic geodetic 1D network. It was planned to work on the Main project of technical observation of the dam using geodetic methods, and the project of observation of suburban area should connect the level lines in one unique area network by measuring the height differences using the trigonometric levelling with the poles of geodetic 2D network base. However, we can conclude from the available documents that this was practically not done. The level lines formed without the interconnection with the measurement values cannot be considered as a unique 1D geodetic control network. Even the deep benchmarks do not make a basic 1D network because they are not interconnected with the measured height values Analysis of the previous epochs of dam observation The observation of the dam has been done according to the previously described methodology since it was constructed until today, that is, according to the Main project. A number of epochs of measurement in geodetic networks for the observation of the dam was done during the first (test) loading and discharge of accumulation. The observations were done later twice a year at the most. The observation was significantly disrupted in the period from 1992 to The results of the observation require calculated coordinate values and point heights done by cutting the lines forward and the heights in the level lines, the observation of their numerical values and the calculation of vector of movement. The vector of movement implies the length of movement and orientation calculated on the basis of the coordinates of the same points on objects in simultaneous periods of observation. On the basis of previous analysis of the given geometrical network, it can be concluded that the following criteria of quality in geodetic control network can be accomplished for the plan of observation and the accuracy of measured values: the minimum value of standard deviation of the position of 2D network points is 2 mm and the maximum is 9 mm, the minimum value of the unit of movement of points in 2D network that can be discovered is 7 mm and the maximum is 37 mm, the relation between the small and the big axis of the error ellipse is from 1.5:1 to 4:1, the minimum local measurement value of inner reliability is 0.1 and the maximum is 0.80, and the value of marginal gross errors is from 2 to 18 These criteria can be accomplished by performing an adjustment of the whole network by means of the Least Squares Method and by applying the conventional methods of deformation analysis, which was not the case in the realized periods. 191

4 SIG 2016 International Symposium on Engineering Geodesy, May 2016, Varaždin, Croatia As previously stated, the micro levelling network for the observation of the vertical movements of the dam was not established as a unique geodetic 1D control network, so the specific quality analysis of such network could not be made. 3. The innovated project of geodetic control networks The previously presented documents about the Bočac dam did not provide the conclusion about the value of the movement of the dam, its attached objects and the surrounding field, which should be revealed by geodetic observation. Taking into consideration that it is a concrete dam, the value of the movement of the points of geodetic 2D control network that should be determined is 15 mm; on the other hand, the value of the benchmark movement of geodetic 1D control network that is to be revealed is 2 mm. The geodetic 2D and 1D networks are intended for geodetic observation of the Bočac dam, its nearest objects and the surrounding field (river dikes, access roads). The networks consist of basic geodetic networks and the networks of points on objects. A separate combining of geodetic control networks was performed due to the accuracy demands, related to the field configuration, form and size of objects Innovated project of geodetic 2D control network The geodetic 2D control network implies the basic 2D geodetic network of the dam and the network of points on the dam, its attached objects and the surrounding field. Considering the fact that the geodetic 2D control network for the dam exists, this project: 1) defined the values of movement that are to be determined by the measurements in the network, 2) proposed the stabilization of new points in the network (replacement of the damaged points), 3) proposed a new plan of observation in the network, 4) defined the accuracy of the measured values in the network, 5) performed a calculation of network accuracy, 6) defined technical requirements for measuring. The value of the point movement in geodetic 2D control network which should be determined in the network is 15 mm. Taking the value of the movement that should be determined into consideration, the geodetic 2D control network should be established with the precision of the point position being σ pol/gk2dm = 3 mm. The standard deviation σ pol is defined on the basis of the movement size value. In order to determine the size d, the standard deviation of the position of points should be (1) The network consists of 62 points in total and their disposition is given in the figure 3.1. Apart from the existing points in the geodetic 2D control network, it is proposed to stabilize 2 new points of basic geodetic network and the point for the follow-up of the supporting wall on the right coast next to the dam on the 192

5 downstream side, new points for tracking the right river dike downstream from the dam, new points for the observation of overflow and ski leap of the left circulatory tunnel as an object of monitoring, and the point on the roof of the main machine building HE for its observation. The method of stabilization was defined in accordance with the objects and the requirements defined in the project for all points of the geodetic 2D control network. Figure 3.1 The disposition of points of the new geodetic 2D control network The coordinate system for geodetic 2D control network is local and defined by the previously accepted rectangular coordinates of points in the existing network. The coordinates of their points have been adopted as approximate. The datum of the geodetic 2D control network was defined by coordinates of all points (minimum trace on all points). A plan of observation and the required accuracy of measured values were defined for the constructed geometrical shape of the network. The measurement 193

6 SIG 2016 International Symposium on Engineering Geodesy, May 2016, Varaždin, Croatia of horizontal directions and lengths, that is, 262 directions and 99 lengths, were planned in the network. The accuracy of the measured values was determined with the calculation providing the estimated accuracy of the coordinate of points to be consistent with the given positional accuracy [Milovanović et al. 2014]. The review of the required measuring accuracy for the horizontal directions in 2 gyrus depending on the length perspective and for the accepted / = 3 mm is given in the table 3.1, where σ α is a standard deviation of arithmetic mean for the measured directions. The required accuracy for the length measurement is the same for all lengths = 3 mm [Milovanović et al. 2014]. Table 3.1 Accuracy of the measured directions depending on the length perspective D [m] σ α ["] D [m] σ α ["] A calculation of the accuracy of the geodetic 2D control network was performed for the designed geometrical shape of the network, for the plan of observation and the defined accuracy of the measured sizes, and the following was adopted: weight of measured directions = =, (2) where =1, = and weight of measured lengths = =, (3) where =1, =,. According to the report on the accuracy estimation for the geodetic 2D control network, it can be concluded that the following criteria of the network quality were acquired: the minimum size of movement that can be determined is 3.6 mm, and the maximum is 13.6 mm, the minimum value of standard deviation for the position of points is 1 mm, and the maximum is 3 mm, the relation between the big and the small axis of the ellipse error for all points is less than 2:1, the minimum value for local measurement of internal reliability is 0.2 and the maximum is 0.9 and the value of marginal gross errors for all planned measurements is between 5 and 7. The innovated project of the geodetic 2D control network specified the technical requirements for the measurement of directions and lengths: 1) a declared accuracy of instruments for the measurement of directions and lengths, 194

7 2) the requirements during the measurement, 3) required geodetic measurement equipment and tools, 4) measurement procedure and 5) the criteria for tracking and measurement control. This project also defined the procedure for the result analysis and adjustment of unique geodetic 2D control network Innovated project of the geodetic 1D control network The geodetic 1D control network implies a basic geodetic 1D network of objects and a network of benchmarks on the dam, its attached objects and the surrounding terrain. Taking into consideration that there are level lines for the dam that are not interconnected, this project: 1) defined the size of the movement that should be determined in the measurement, 2) proposed the stabilization of new river dikes in the network (the replacement of the destroyed ones), 3) proposed a new plan for observation in the network, 4) defined the accuracy of the measured values in the network, 5) performed a calculation of network accuracy, and 6) defined technical requirements for the measurement. The scope of the movement of the geodetic 1D control network which should be determined in the measurement is 2 mm. Considering the size of the movement to be determined, the geodetic 1D control network should be established with the precision of benchmark position / =0.4 mm. The network consists of 88 benchmarks in total and their disposition is given in the figure 3.2. Apart from the existing points of the geodetic 1D control network, the following is proposed the stabilization of new benchmarks, the observation of ground subsidence and access roads downstream the dam, new benchmarks for ski leaps observation, new benchmarks for the follow-up of the right river dike downstream the dam, new benchmarks for the observation of the main hydropower plant building and new benchmarks for the observation of the overflowing construction of circulatory tunnel. For all the points of the geodetic 1D control network, the method for the stabilization was defined in accordance with the objects and requirements defined in the project. The coordinate system for the geodetic 1D control network is local and defined with the previously accepted heights of benchmarks of the existing level lines in the national coordinate system (NCS). Even though the heights are given in NCS, the geodetic 1D control network should be analysed in the local coordinate system due to the elimination of errors of given values. The heights of benchmarks are to be used as approximate. The datum of the geodetic 1D control network is defined by the heights of all benchmarks (minimum trace on all points). 195

8 SIG 2016 International Symposium on Engineering Geodesy, May 2016, Varaždin, Croatia 196 Figure 3.2 Disposition of benchmarks of the new 1D control network For the defined geometrical shape of the network, the plan of observation is defined as well as the required accuracy of the measured values in the network. A total of 143 height differences are measured in the network. The height differences between benchmarks of the geodetic 1D control network should be determined by means of the method of precise geometric levelling for the points that are on the same height level, and the method of trigonometric levelling for the height differences that the level lines are connected with on various levels. The level lines have not been connected on the levels until now, which indicates that they were processed separately. Considering that these level lines are intended for the subsidence observation of the same object, the interconnection between these level lines was proposed, as well as forming of a unique geodetic 1D control network. The accuracy of the measured values was determined by the calculation that ensures that the estimation of the accuracy of the coordinate points is consistent

9 with the given positional accuracy. The calculation indicated that the required accuracy of the arithmetic mean of the measured height differences is smaller than 0.56 mm. The calculation of the accuracy for the geodetic 1D control network was performed for the designed geometric shape of the network and it was adopted that: the weight of measured height differences is =, (4) where: =0.2 mm is the aprioristic dispersion factor and is the standard deviation of measured height difference, the accuracy of height differences of geometric levelling = (5) where n is the number of stations according to which a height difference is established, and =0.2 mm the aprioristic dispersion measuring factor, the accuracy of height differences of trigonometric levelling [Mihailović & Aleksić 2008] σ 2 2 Z Dr 2 ΔH = (cos Z) D + D ( sin Z) k i l k σ σ σ σ σ ρ 2R ( ) ( ) " =, (6) where is slope distance, standard deviation of measured zenith deviation, standard deviation of instrument height measuring, standard deviation of signal height measuring, central Earth radius and standard refraction deviation. According to the calculation report for the geodetic 1D control network, it can be concluded that the acquired quality criteria of the network are the following: the minimum movement size that can be determined is 1 mm and the maximum is 2 mm, the minimum value of standard point position deviation is 0.35 mm, and the maximum is 0.45 mm, the minimum value of local measurement for internal accuracy is 0.2, and the maximum is 0.8 and the value of marginal gross errors of all planned measurements is between 5 and 7. The innovated project for the geodetic 1D control network specified the technical requirements for the measurement of height differences: 1) declared accuracy of instruments for the measurement of height differences using geometric and trigonometric levelling, 2) the requirements during the measurement, 3) the required geodetic measuring equipment and tools, 4) measurement procedure, 5) criteria for tracking and measuring control. This project also defined the procedure for the result analysis and the adjustment of the unique geodetic 1D control network. 197

10 SIG 2016 International Symposium on Engineering Geodesy, May 2016, Varaždin, Croatia 4. The procedure of establishing the geodetic 1D and 2D control network stability The establishment of the stability of points in geodetic 1D and 2D should be done with Pelcer method by doing the following [Mihailović & Aleksić 1994]: adjustment of the null and i epoch of measurement in the network with the minimum trace on all points, according to this method, we question the stability of points, the adjustment of the null and i epoch of measurement in geodetic control network with the minimum trace on stable benchmarks/points of the basic network, according to Pelcer method, we question the stability of benchmarks/points on objects. After performing the Pelcer method of ascertaining the stability of points in the network, it is necessary to determine and present (in tables and graphically) the sizes of movement values for which it was ascertained that they were moved. 5. Conclusion It has been proved on the practical example that even the high demands for accuracy can be met with the application of presented methodology of designing in the procedure of determining the movement of points on the dam with optimal material and human resources. Considering the social significance and economical value of objects in the hydropower sector, the financial investments in application and advancement of methodology of geodetic observation were neglected. References Mihailović, K.; Aleksić I. (2008). Koncepti mreža u geodetskom premeru (The Concepts of Networks in Surveying), Geokarta, Beograd. Mihailović, K.; Aleksić I. (1994). Deformaciona analiza geodetskih mreža (Deformation Analysis of Geodetic Networks), Monograph 2, Faculty of Civil Engineering, University in Belgrade, Belgrade. Milovanović, B.; Gospavić, Z.; Pejović, M.; Vasiljević S. (2014). Projekat osnovne mreže (Fundamental Network Project), Beograd, The Proceedings of the National and Scientific Gathering GEO2014, Kopaonik, pp Vasiljević, S., (2015). Inovirani projekat geodetskog omatranja brane Bočac (Innovated Project of the Geodetic Observation of the Bočac Dam), Master Thesis, Faculty of Civil Engineering, University in Belgrade, Belgrade. 198

11 Inovativni projekt geodetskog brane Bočac monitoringa Sažetak. Osnovni zadatak geodetskog monitoringa brana pri eksploataciji je pravovremeno otkrivanje deformacija koje mogu uzrokovati ljudske i materijalne štete. U radu je prikazan inovativni projekat geodetskog monitoringa brane Bočac na rijeci Vrbas, pri hidroelektrani Bočac. Izrada ovog inovativnog projekta podrazumijevala je analizu postojećeg stanja geodetskih kontrolnih mreža i projektiranje nove geodetske kontrolne 2D i 1D mreže za monitoring brane. Projektiranje nove geodetske kontrolne 2D i 1D mreže obuhvatilo je: 1) definiranje geometrije, koordinatnog sustava, geodetskog datuma, plana opažanja, točnosti mjernih veličina, 2) proračun točnosti i provjeru ispravnosti projektnog rješenja, 3) definiranje tehničkih uvjeta za realizaciju mjerenja, načina obrade i analize rezultata mjerenja, postupka izjednačenja, načina određivanja pomaka tačaka i načina prezentacije dobivenih rezultata. Ključne riječi: geodetska kontrolna mreža, geodetski monitoring brana, projektiranje geodetskih mreža. *scientific paper 199