Geodesy - New


Auscultation by GPS of rockfill dam

Apr 2018 | No Comment
The paper aims to analyze the behavior of rockfill dams using the ANSYS computer code; an application was made on Altinkaya Dam in Turkey

BOUHOUCHE Nadia

Centre of Space Techniques, Department of Geodesy, Oran, Algeria

GOURINE Bachir

Centre of Space Techniques, Department of Geodesy, Oran, Algeria

Dams are one of the most important engineering structures used for water supply, flood protection and agricultural activities. Besides, it is the most natural and cheapest way of energy production for a country. Dams constructed with high cost expenditures are subjected to deformation due to various loading factors such as water level, air, water temperature and rock deformability. Controlling these dams has become compulsory in order to prevent disasters [2].

There are generally two methods used to carry out the deformation surveys, namely geodetic and geotechnic.

Geodetic methods use total station, precise leveling instruments and geotechnic rely on instrumentations such as inclinometer and extensometer, which can effectively monitor one or two- dimensional modes of motion to sub-millimeter level.

On the other hand, the space-based geodetic method for instance, the Global Positioning System (GPS) offers a reliable and efficient method for three-dimensional survey [3].

This technology has been used for various surveying and mapping activities, including cadastral, engineering, hydrographic survey applications. Its ease of use and capability of very high accuracy make GPS an applicable system in dam stability monitoring of dams.

The objective assigned to this paper is to establish an analysis of geometric deformation of the Altınkaya dam in Turkey whose sealing is ensured by a central clay core.

Safety of Rockfill Dams

The most common causes of failure of the embankment dams are internal erosion of finegrained soils from the embankments, erosion under the foundation or abutment, stability problems resulting from the high pore pressures, hydraulic gradients, and overtopping of the dam or spillway.

Deformations of rockfill dam start occurring during the construction of the dam. These deformations are caused by the increase of effective stresses during the construction by the consecutive layers of earth material and also by effects of creep of material. Deformations are also influenced by the deformations of the foundation, the transfer of stresses between the various zones of the dam and the other factors [4].

Role of Monitoring

Deformation monitoring is generally a working procedure to ensure the safety of engineering structures and also to validate the engineering structures’ designs [3].

All engineering structures are subject to deform and displace due to several factors such as environmental stress, structural overload, tectonic movements etc. Two methods used to carry out the deformation surveys, namely geotechnic and geodetic.

Geotechnical Surveillance: rely on instrumentations such as inclinometer and extensometer, which can effectively monitor one or two- dimensional modes of motion to sub-millimeter level. However, spatial distribution of geotechnical instrumentation is usually limited to the locations that the instruments can be installed during dam construction.

Geodetic surveillance or structural monitoring: use terrestrial instruments as precise leveling instruments, total stations, etc. It has become very important as more and more new engineering structures had been erected over the years. Comparatively, it is a slow process to GPS method.

Geometric survey of a rockfill dam by GPS

Since its inception in the early 1970s, GPS has become a widely used surveying tool. Today, accuracies at the centimeter level or better are routinely achieved using a variety of relative positioning techniques. These techniques range from nearinstantaneous positioning over relatively short reference receiver to unknown receiver distances, to solutions requiring many hours of data and advanced modelling for distances between receivers of up to several thousand kilometers. Removal of the line-of-sight dependency for survey observation has radically altered the practices of the survey community, allowing larger areas and more points to be measured [5].

Monitoring using GPS, observation is done in triangulation style. Two or more GPS receiver are seated on the control points while some other GPS receiver will be place on the monitoring stations. In most situations, due to the limited numbers of available GPS receivers, the whole monitoring campaign is to be carry out in separate session. There must be at least one common station between two sessions.

Figure 1 shows the sample of a dam control network. The stability of the control network can be determined using conventional triangulation observation method (total station) or GPS observation (static observation). The blue triangles represent the stable reference points scattered around the dam. The red dots represent the monitoring (object) points situated on the dam surface.

After the observation, the raw data have to go through data processing in order to obtain the desired results.

With geodetic techniques it is possible to measure the displacement of pillars with respect to a reference network made up of stations that are supposed fixed (stable). This technique has the advantage of determining absolute displacements. To achieve this goal, we must follow these steps [6]:

1) Establishment of the reference network or guard network: Choose points out of the deformation zone (geologically stable points) and materialize them by concrete terminals. The altitude of the reference stations should be close to that of the study area. This network, which is supposed to be stable, serves as a backbone for all subsequent work carried out as part of the study.

2) Implementation of the Auscultation Network: Choose points that are well distributed and delimit the area of influence of the reservoir (upstream and downstream sides and banks of the reservoir) and which are materialized by concrete terminals or sealed markers (stainless steel) in concrete sites. They will be used to study the behavior of the vicinity of the dam and as points of support for the target network.

3) Target network implementation: Choose homogeneously distributed targets, according to the form and type of dam, by an appropriate mesh. The targets (concrete markers and sealed markers in the concrete sites) must be materialized on the structure (the axis of the ridge, downstream / upstream facing, some ancillary works) and on its immediate vicinity (downstream foot of the book), in a sustainable way. This network will be used to monitor the behavior of the dam.

Application

Definition of Work Area

Altınkaya dam is 35 km south west of the Bafra, Samsun. This dam is structured by one of the Turkish Government establishments that is called State Hydraulic Works.

Altinkaya Dam is 22nd among rock fill large dams in Turkey and also it is 32nd dams in the world. The dam is built on the Kizlirmak River as rock fill with clay having seeds. Height of the dam (from river bed) is 195.0 m. and crest length is 634.0 m. Reservoir area at normal water surface elevation is 118.31 km2. Volume of the dam is 16 x 106 m³. The dam is convex towards the water [7].

Building of monuments of reference and object points

Monitoring network consists of six reference stations (1001, 1002, 1003, 1004, 1005 and 1006); they were built as pillars on the stable ground which surrounds the dam.

In order to monitor and measure possible displacements at the crest, 10 object points were established at the crest when the dam was built (Figure 2). Since that time only one object point, numbered 0023, was added to the deformation network.

This point was built because of the physical changes that had been observed in the surrounding area. The deformation measurements related to the reference and object network were made with 3 Ashtech Z surveyor GPS receivers and 700700B_Mar.III_L1/L2 GPS antennas [8].

Deformation measurements of Altinkaya dam

The deformation measurements of the dam involved four measurement campaigns and two separate measurements were made at the Altinkaya dam: one between reference points and the other within the object points.

For the measurement plan on the reference network, the observation period was selected 45 minutes with a sampling rate of 10 seconds. The satellite elevation mask was selected at 15°in order to avoid multi-path effect and cycle slip error.

Before commencing deformation measurements, all the equipment was calibrated. In order to avoid or diminish any equipment errors, the same GPS receivers and antennas were used at the same points in all periods [8].

The measurements were processed with GeoGenius 2000 software. The deformation network showed a maximum value of 0, 9 mm horizontally and 1, 7 mm vertically for the 4 observational periods. The adjusted coordinates and their covariances were obtained from a free network adjustment. The accuracy of the measurements of the horizantal deformations and the vertical displacements.

On remarque que le barrage ainsi que la zone environnante ont subi des déplacements horizontaux et verticaux, durant les différentes périodes d’observations.

Analysis of the movements of the GPS monitoring network

The differences of the local coordinates (E, N, U) of the surveillance network points, between the different periods, are illustrated as vectors form of horizontal and vertical displacements, according to the figures (6 and 7) we notes that the dam and the surrounding area have undergone horizontal and vertical displacements during the different observation periods.

Indeed, the average modulus of horizontal displacements at the reference points is of the order of 10 mm, 7 mm and 8 mm, according to the periods 1 & 2, 2 & 3 and 3 & 4, respectively. For the target points of the dam crest, the amplitude of these displacements is of the order of 2 to 10 mm.

Vertical displacements were observed at reference points:

With an uprising of the order of 33 mm, between periods 1 & 2;

▪ With a subsidence of the order of 19 mm, between periods 2 & 3;

▪ With a subsidence of about 20 mm, between periods 3 & 4.

▪ For the target points, these movements are of the order of a few millimeters to a few centimeters.

According to a study conducted by [8] which consisted in the analysis of horizontal deformation of the GPS dam monitoring network, by the determination of unstable points, the results obtained are as follows:

▪ The points 0003, 0007, 0017, 0011, 0013 and 0019 on the crest dam and also the points 1002 and 1006 of the reference network; are unstable between 1st and 2nd periods. During these periods, the reservoir level decreased from 170.34 m to 167.53 m.

▪ The points 1001, 0015 and 0017 showed significant movements between the 1st and 3rd periods, during which the water level decreased from 170.34 m to 164.20 m.

▪ The points 0003, 0007, 0009, 0011, 0013, 0015 and 0019 showed significant movements between the 1st and 4th periods.

▪ The most of the crest dam points during these periods moved and the level of the reservoir increased from 170.34 m to 177.23 m.

▪ The direction of these displacements, according to the figure 8(a) is characterized, generally, by an upstream – downstream direction. However, the middle of the dam crest (targets 0007, 0009, 0011, and 0015) has undergone an upstream movement between the 2nd and 3rd periods figure 8 (b).

All these results lead us to the following conclusions:

▪ The horizontal movement of the dam crest was mainly affected by the hydrostatic charge of the reservoir at different water levels.

▪ The most significant movements were recorded at target points 0003, 0007, 0013 and 0019, in the middle and surrounding areas of the dam crest.

▪ Reference points such as 1001, 1002 and 1006 were considered unstable. Since most of the reference points are on the upstream side of the dam, it is probable that this area is affected by the hydrostatic charge at different water levels.

According to Figure 9, the vertical movement of the crest dam was characterized by a settlement of the medium (the targets 0009, 0011, 0013 and 0015) and a uprising of their surroundings (the targets 003, 005, 0007, 0017, 0019, 0021) of the order of 3 to 16 mm, during the period 1 & 2 (Fig. 8a).

On the other hand, during periods 2 & 3 (Fig. 9b) and 3 & 4 (Fig. 9c), the targets of the dam crest underwent a settling phenomenon of the order of 10 to 40 mm and 3 to 12 mm. , respectively.

Generally, this settlement phenomenon is the consequence of the effect of the self weight of the dam conjugated with the hydrostatic charge.

Conclusion

This analysis has verified the stability of the Altinkaya Dam using GPS survey measurements over a period of 2 years at four epochs (September 2000 to May 2002).

It showed that the crest of the dam has undergone horizontal and vertical movements characterized by a dominant direction towards the downstream and by a settling phenomenon.

The filling of the reservoir dam is modeled by the application of a hydraulic pressure on the upstream facing. The Altınkaya Dam showed maximum deformation at its crest due to its own weight and hydrostatic charge; its act is very important on the evolution of deformations and displacements. The whole solid rockfill dam has a high compactness. It was found a general low settlement of the structure due to the actions of the self weight and the pressure of the water. Based on the results of the analysis carried out, the structure can be considered to be quite satisfactory during the GPS observation period (09/2000 to 05/2002); the dam has all the guarantees of good durability.

References

[1] Szostak-chrzanowski, A. et al. 2008. Study of a longterm behavior of large earth dam combining monitoring and finite element analysis results. 13th FIG International Symposium on Deformation Measurements and Analysis, 4th IAG Symposium on Geodesy for Geotechnical and Structural Engineering, Lisbon, Portugal.

[2] Berkant .k, Gökalp.E. 2017. Deformation Measurements and Analysis with Robust methods: A Case Study, Deriner Dam.

[3] Wan, A et al. Research On The Application of GPS Technique in Dam Stability Monitoring.

[4] Szostak-chrzanowski, A. et al. 2006. Kinematic Analysis of Behavior of Large Earth Dams, TS 68 – Deformation Measurements of Dams. Munich, Germany.

[5] Stewart.M, Tsakiri.M. 2001. The Application of GPS To Dam Surface Monitoring, Journal of Geospatial Engineering, Vol. 3, No. 1, pp.45-57.

[6] Kahlouche, S. 2012.Projet de mise en place d’un réseau d’auscultation géométrique du barrage de Sikkak (Tlemcen) à partir d’observations spatiales .Atelier technique de l’Agence Spatiale Algérienne sur l’Utilisation des technologies spatiales au service des ressources en eau.

[7] Levent.T, 2010. Analysis of dam deformation measurements with the robust and non-robust methods, Scientific Research and Essays Vol. 5(14), pp. 1770-1779.

[8] Gökalp, E & Taşçı, L. Deformation monitoring by GPS at embankment dams and deformation analysis. Survey Review, 41, 311 pp.86-102 (2009).

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