MONITORING OF LONG-TERM DEFORMATIONS OF ATATURK ROCKFILL DAM BY CALIBRATED FINITE ELEMENT MODEL

Sujan Malla 1, Martin Wieland 2 and Ruedi Straubhaar3

ABSTRACT

The 170 m high Ataturk rockfill dam was completed in 1990 and dam safety has been monitored continuously since then. The monitoring system comprises visual inspections, comprehensive geodetic surveys, and various types of sensors and instruments. For the safety monitoring, a detailed finite element (FE) model of a representative section of the dam was created, in which slip movements were allowed at the core-filter and filter-rockfill interfaces. The elasto-plastic model was assumed for the main dam materials. The material properties were determined based on laboratory tests, published data, and engineering judgment. Two basic load cases were considered, i.e. (i) dead load and (ii) water load. The geodetically measured dam displacements could be fitted satisfactorily with a linear combination, with time-dependent coefficients, of the computed displacements due to the gravity and water loads obtained from the FE analysis. It was further shown that the history of the time-dependent coefficients, which represented the creep-type post-construction deformations of the dam, could be fitted with asymptotic exponential trends. This model was calibrated based on displacements measured until the year 2000. The calibrated model was used to predict the dam displacements for a period of ten years. The dam displacements measured since 2001 show good agreement with the predicted values. This comparison of measured and predicted displacements is most beneficial for the safety monitoring of the Ataturk dam. The calibrated dam model allows an insight into the processes in the dam body that lead to the observed deformations of the dam surface. The possible effect of the increase of the reservoir water level on the dam deformations could also be estimated using the calibrated dam model.

INTRODUCTION

The ongoing deformations of the Ataturk Dam are being regularly and systematically monitored. The dam is instrumented with different types of sensors, which provide information about the internal state of the dam. The most reliable data of the dam deformations are, however, from geodetic measurements carried out twice a year at several sections of the dam. In this paper, a semi-empirical method for the interpretation of the geodetic deformation measurements of one particular dam section is presented and discussed. The main objectives are (i) to analyse the deformations that have occurred within the dam, and (ii) to predict the future displacements for monitoring the dam behaviour in the coming years. These objectives have been attained with the help of a two-dimensional finite-element model of the dam-foundation system.

The prediction of the future displacements of individual observation points could also be accomplished by means of a statistical or neural network model. However, such a non-physical model would not provide any physical insight into the deformation processes occurring in the dam. In contrast, a finite element model has the advantage that the

mechanism of the deformation of the whole dam can be simulated based on measurements made at observation points located on its exterior surface.

It may be argued that it is not possible to know what has occurred inside a dam by measuring deformations at reference points located only on the accessible surface. The answer is simple: all geophysical methods that are used to describe the global structure and properties of the earth, and geophysical exploration methods are based on measurements on the earths surface. These methods have been most powerful in conjunction with models of the underground and are universally accepted. In the case of a dam, we are in a much better position because the interior structure and the basic properties of the different zones are known. Thus, it is possible not only to set up a model with geometrically correct material zones but also to establish the best estimates of the material properties by fitting the calculated deformations with the measurements made at the dam surface, taking into account also the values determined by laboratory and field tests. For the Ataturk Dam, a relatively simple finite element model could be created and calibrated to achieve an excellent fit with the measured deformations.

The concept described above differs from a blind prediction of dam behaviour made on the basis of the material properties from laboratory tests, although this cannot be avoided in analyses performed at the design stage. The quality of such a prediction can vary widely, especially in the case of embankment dams. For instance, in the ICOLD Benchmark Workshops, predictions made by different analysts of a benchmark problem tend to show a large scatter, even when they use identical basic geometry data, material properties and applied actions. The inverse analysis discussed in this paper is a rational way to overcome the problem of a pure design model of a dam and its foundation. The material properties obtained from an inverse analysis must, however, be checked and compared with laboratory data, which exist for most large dams. Processes that cannot be explained by data available from standard laboratory tests may have to be investigated further with the objective of understanding the underlying physical processes.

MAIN FEATURES OF ATATURK DAM

The Ataturk Dam is a zoned rockfill dam with a central core and is located on the Euphrates River. The dam has a height of 170 m, the crest length is 1670 m and the dam volume is 84 106 m3. The reservoir volume amounts to 48 km3 and the installed capacity at the power plant is 2400 MW. The maximum and minimum operation reservoir levels are 542 m and 526 m a.s.l. respectively.

The construction of the cofferdam lasted from 1985 to 1987. The fill work for the main dam began in 1987 and was completed in 1990.

OBSERVATIONS ON DAM DEFORMATIONS The main observations on the dam deformations during and after the construction are as follows:

1. It is estimated that the central portion of the dam crest has settled by around 7 m since the end of the construction in 1990. Since the start of the detailed geodetic monitoring of the dam crest in autumn 1992, crest settlements of up to 4.3 m have been measured. The settlements are, however, very small near the downstream edge of the crest, roughly beyond the interface between the core and the downstream filter.

2. The total post-construction settlements of the downstream slope and the downstream edge of the dam crest have been relatively small (max. 1.5 m). The largest settlement of the downstream slope has occurred at about three-quarters of the dam height. In particular, the post-construction settlements at the downstream edge of the dam crest are very small.

3. The horizontal (radial) displacement increases steadily from the bottom to the top of the downstream slope; the maximum horizontal displacement since the end of construction is about 2.9 m.

4. The dam crest developed visible irregularities because of the large deformations. 5. During the crest reinstatement carried out from 1997 to 1999, clear evidence of the

downward movement of the core relative to the downstream filter was found.

6. The highest deformations have occurred in the central part of the dam. The deformations become smaller towards the abutments and towards the downstream toe of the dam.

7. The available information shows that the deformation of the foundation rock is not a significant factor in the deformation observed on the dam surface.

8. Settlements and horizontal displacements are still taking place under more or less constant loading conditions. However, the deformation rates are slowing down with time.

9. High pore pressures developed in the clay core during the construction of the dam, especially towards the end of the construction. The dissipation of the pore pressures with time has been extremely slow due to the very low permeability of the core material. In general, the piezometric heads in the clay core remain significantly higher than the reservoir level.

GEODETIC DEFORMATION MEASUREMENTS

The geodetic measurements of the dam deformations began in July 1990, just before the end of the construction. For this purpose, a number of benchmarks were established on the dam surface. Most of the benchmarks were located on 7 cross-sections distributed evenly along the dam axis. The settlements and horizontal (radial) displacements of benchmarks located in various cross-sections situated away from the abutments are of similar magnitudes. This indicates that the dam behaves essentially as a two-dimensional structure. Hence, only the geodetic measurements made approximately at the central cross-section of the dam were used as the basis of the present calibrated model. A complete set of geodetic measurement data is available for five benchmarks located on this cross-section.

BASIC CONCEPT AND METHODOLOGY

From the geodetic measurements, the settlements and the radial (horizontal) displacements of the dam surface are known accurately. The relative contributions of the gravity and water loads to the ongoing deformations are, however, not known a priori. The basic idea of the proposed model is to split the inelastic deformations that have occurred since reservoir impoundment into a term proportional to the static deformation due to the gravity load (self-weight) and a second term proportional to the static deformation due to the water load. This is not a new idea; for example, creep strains in concrete structures are usually assumed to be proportional to the elastic strains. Moreover, the concept of separating elastic displacements of a structure into several so-called shape functions is an established concept forming, e.g. the basis of the finite element method. In the present model, the dam deformations due to the

gravity load and the water load are used as shape functions as these have direct physical meaning. The new concept is that the shape functions are not obtained from a linear-elastic analysis but from an inelastic analysis and the measured deformations that have to be fitted are of inelastic nature.

The main steps of the procedure adopted for the analysis and calibration of the dam displacements are outlined below:

A two-dimensional plane-strain finite-element model that included all relevant material zones in the dam and the foundation was prepared. In this model, relative sliding displacements governed by the Coulombs friction law could occur along the core-filter and filter-rockfill interfaces on both the upstream and downstream sides of the core.

The dam displacements were calculated for the principal load cases: (i) gravity load and (ii) water load.

The measured displacements, D, on the dam surface were fitted with the following linear combination:

D = a G + b W (1)

where G: computed displacements due to gravity load (including buoyancy) W: computed displacements due to water load a, b: time-dependent coefficients.

The coefficients a and b were determined by the method of least squares. For simplicity, the displacements G and W were taken for a fixed reservoir level of 535 m a.s.l.

The material properties of the various zones and the angles of sliding friction of the material interfaces were adjusted in the computational model taking into account also the field and laboratory test results. The aim was to achieve the best fit of the geodetically measured displacements from July 1990 to June 1994 with the calculated dam displacements expressed in the form a G + b W, so that the coefficient b was equal to about 1.0. In other words, when the water load corresponding to a reservoir level of 535 m a.s.l. was applied to the calibrated dam model, the computed displacements were about the same as the estimated contribution of the water load to the measured displacements until June 1994, i.e. during the first filling of the reservoir.

The ongoing creep deformations were represented by coefficients a and b, expressed as gradually increasing functions of time. The increasing values of these coefficients could also have been expressed in terms of an equivalent reduction of the stiffness of the fill materials with time.

The trends of the time-dependent coefficients a and b were fitted by regression analysis with exponential time functions that approached asymptotic values. This type of creep response corresponds to that of a Kelvin viscoelastic model.

The expected dam displacements in the future were predicted based on the exponential trends of the time-dependent coefficients a and b.

This method has the advantage that the time-dependent inelastic deformations can be characterised by two time-dependent coefficients only. The spatial information is contained in the two deformation shapes G and W obtained by the static analysis of the dam subjected to the gravity and water loads. It is implicitly assumed that the rate of creep deformation is uniform over the whole dam body. In reality, the rates of creep deformations could be different in the various material zones in the dam.

FINITE ELEMENT ANALYSIS

The following basic assumptions were made for the finite element analysis:

1. A two-dimensional plane-strain model of the central cross-section of the dam (Fig. 1) and its foundation was used.

2. The various zones in the dam were modelled as Mohr-Coulomb materials (elasto-plastic material).

3. The Coulombs law of friction was assumed along the core-filter and filter-rockfill interfaces. The coefficient of friction of each sliding surface was treated as a constant over the whole height of the dam.

4. Effects of inelastic deformations and uplift in foundation rock were assumed to be negligible.

5. As this paper deals only with the post-construction deformations taking place under the self-weight of the whole dam, the gravity load was applied in one step to the whole structure. The buoyant unit weights were used in the case of the submerged zones.

6. The full hydrostatic pressure from the reservoir was applied on the upstream face of the core. This is supported by the information obtained from the pore pressure measurements, which indicate that the seepage process has hardly developed through the clay core due to its very low permeability.

The finite element analysis was carried out with the computer program ADINA (ADINA R& D, 1996). The finite element model was nonlinear because of the elasto-plastic material model and the contact algorithm used to model slippage along the core-filter and filter-rockfill interfaces. In the dam analysis, first, the gravity load was applied to the whole dam, and then the upstream face of core was subjected to the water load. The water load displacements were obtained by subtracting the displacements due to the gravity load from those due to the combination of the gravity and water loads.

Fig. 1: Central cross-section of Ataturk Dam with different material zones(1: clay core; 2: filter (2a: fine filter, 2b: coarse filter); 3: random fill (plicated limestone); 4: river alluvium; 5: basalt (5a: fine basalt; 5b: sound basalt; 5c: slightly weathered basalt)

DISCUSSION ON COMPUTATIONAL MODEL

The displacements computed with the help of the calibrated finite element model agree well with the observed displacement pattern based on the geodetic monitoring of the downstream slope as well as the crest region. It is clear that the dam behaviour can be simulated quite

realistically using an elasto-plastic model in which the core is allowed to move relative to the filters along the material interfaces (Fig. 2).

Fig. 2: Dam deformations from 1990 to 2000 obtained from calibrated finite element model: (a) deformed shape, (b) close-up view of deformed shape of crest region, (c) distribution of settlements, and (d) distribution of horizontal (radial) displacements

The time-dependent inelastic displacements occurring in the dam under almost constant loading conditions are nearly proportional to the displacements that occurred during the first impoundment. The results of the sensitivity studies indicate that the stiffness reduction of one particular zone alone cannot explain the observed time-dependent displacements in the whole dam. For instance, the decreasing stiffness of the core alone could not be the reason for the

increasing settlements and horizontal displacements along the downstream slope; the creep deformations in the downstream shell itself are the most likely cause of the observed displacements of the downstream slope. On the other hand, the relatively large ongoing settlement of the crest cannot be the result of the creep processes in the downstream shell.

Inelastic or creep deformation commonly occurs in a rockfill material due to particle breakage and rearrangement induced, for example, by wetting (Naylor et al., 1997; Justo and Durand, 2000). The post-construction settlement of the downstream shell could be taking place under the self-weight of the dam and the influence of the percolating rainwater; therefore, it could be a rather slow process. On the other hand, this process could have been mostly completed in the upstream shell already during the first impoundment, when it became submerged under the reservoir water.

The time-dependent deformations in the clay core could be related to the ongoing consolidation. As the permeability of the core is quite low, the dissipation of the excess pore pressures that developed during the dam construction is, however, progressing very slowly. Hence, the time-dependent inelastic deformation could go on for tens of years in the core.

PREDICTION OF FUTURE BEHAVIOUR

The displacements that have occurred in the dam since 1994, when the reservoir level first reached 535 m a.s.l., are primarily due to the creep deformations occurring under more or less constant loads. Some deviations from the constant loading condition have, however, occurred during the lowering of the reservoir by about 8 m in 1997-2000 and the crest reinstatement works carried out in 1997-1999.

The time-dependent dam displacements since 1994 were assumed to have exponential asymptotic trends. Hence, the multipliers a and b were fitted with exponential asymptotic time functions by regression analyses (Fig. 3). These functions were then utilized to predict the expected future displacements.

Fig. 3: Fitting of time-dependent coefficients a and b (for linear combination of computed displacements due to respectively gravity and water loads) with asymptotic exponential trends; dashed trend line shows possible effect of rise of reservoir level to 542 m a.s.l. on development of b

The predicted displacements along the dam surface for the period July 1990 - Dec. 2010 are shown in Fig. 4. The radial (horizontal) displacement at the top of the downstream slope would increase by about 0.5 m over the period 2000-2010, provided that the reservoir level would not exceed about 535 m a.s.l. Figures 4 and 5 also show the possible effect of a rise of

the reservoir level from 535 m to 542 m a.s.l., in which case, the additional radial displacement over the same 10-year period would be up to about 2.3 m at the top of the downstream slope. The incremental radial displacement due to the rise of the reservoir level decreases almost linearly from the top to the bottom along the downstream slope. The additional settlement of the dam crest above the core top is expected to be about 1.1 m over the period 2000-2010. The rise of the reservoir level from 535 m to 542 m a.s.l. has only a small effect on the settlement of the core top in the computational model.

Fig. 4: Expected displacements along dam surface from July 1990 to December 2010 predicted based on observed trends of a and b until 2000 Displacements for reservoir level of 535 m a.s.l.: combination 1.183 G + 1.986 W Displacements for reservoir level of 542 m a.s.l.: combination 1.183 G + 3.134 W Sign convention: Settlement (positive upwards, negative downwards); horizontal

(radial) displacement (positive towards downstream, negative towards upstream)

COMPARISON OF PREDICTED AND MEASURED DEFORMATIONS

The present model for the prediction of the dam displacements was calibrated in 2001 on the basis of the available displacement data from the geodetic surveys completed until 2000. The

dam displacements that have been geodetically measured since then are regularly checked against the values predicted in 2001. The new displacement data since 2001 up to now have shown very satisfactory agreement with the predicted values, as illustrated in Fig. 5 for benchmark 134 (position shown in Fig. 4) in the central cross-section.

Fig. 5: Comparison of predicted and measured displacements of benchmark 134 in central cross-section (Note: Prediction was made in 2001 based on geodetic measurements until 2000.)

SUMMARY AND CONCLUSIONS

The main results and the conclusions are as follows:

1. The ongoing displacements measured on the downstream slope of the dam over any given period could be satisfactorily fitted with a linear combination a G + b W of the displacements G and W computed under the gravity and water loads respectively. The time-dependent coefficients a and b were determined by the method of least squares.

2. The present model for the prediction of the dam displacements was calibrated in 2001 on the basis of the geodetic measurements made until 2000. The dam displacements that have been measured biannually since then agree very well with the values predicted in 2001 using the calibrated model. The measured displacements of the crest region and the measured settlements of the core top also show satisfactory agreement with the displacements computed using this model.

3. The good agreement between the measured and computed displacement patterns shows that the dam behaviour can be realistically simulated using an elasto-plastic model in which the core is allowed to move relative to the filters along the material interfaces.

4. The large settlement of the crest region is related to the substantial post-construction settlement of the clay core, taking place primarily under the effect of the gravity load (self-weight). The core tends to move downwards relative to the substantially stiffer filter zones on both sides, in particular, along the interface between the core and the downstream filter.

5. The horizontal displacements of the downstream slope of the dam can be predominantly attributed to the hydrostatic pressure exerted by the reservoir water on the upstream face of the clay core.

6. Rockfill materials commonly undergo creep deformations because of settlements induced primarily by wetting. Such settlements result from crushing of sharp corners at the contact points, and the rearrangement of the rockfill pieces facilitated by the lubricating effect of water as well as the stress redistribution in the upstream shell during impoundment.

7. The settlement of the downstream shell could be taking place under the self-weight of the dam and the influence of the percolating rainwater; therefore, it could be a rather slow process. On the other hand, this process could have been mostly completed in the upstream shell already during the first impoundment, when it became submerged under the reservoir water.

8. In the clay core, the time-dependent deformation may be related to the ongoing consolidation process and possibly some reduction of the Poissons ratio. As the permeability of the core is quite low, the dissipation of the excess pore pressures that developed during the dam construction is progressing very slowly. Hence, the creep deformation of the core could go on for a long time.

9. The results of the sensitivity studies clearly show that the stiffness reduction of one particular zone alone cannot explain the observed time-dependent displacements in the whole dam. For instance, the decreasing stiffness of the core alone could not be the reason for the increasing settlements and horizontal displacements along the downstream slope. The creep deformation of the downstream shell itself is the most likely cause of the observed displacements along the downstream slope. On the other hand, the relatively large ongoing settlement of the crest cannot be the result of the creep processes in the downstream shell.

10. The additional horizontal displacement at the top of the downstream slope from 2000 to 2010 is estimated to be about 0.5 m if the reservoir level would not exceed about 535 m a.s.l.

11. A rise of the reservoir level from 535 m to 542 m a.s.l. could cause the radial displacement at the top of the downstream slope to increase by up to about 2.3 m during the period 2000-2010. The incremental radial displacement due to the rise of the reservoir level decreases almost linearly from the top to the bottom along the downstream slope.

12. Based on the expected future settlements predicted using the present model, the additional settlement of the dam crest above the core top is expected to be about 1.1 m during the period 2000-2010. The rise of the reservoir level from 535 m to 542 m a.s.l. has only a small effect on the settlement of the core top in the computational model.

13. The crest re-instatement during 1997-1999 had a relatively minor effect on the dam displacements. A slight reduction of the deformation rate could be observed at that time, but similar deviations from the long-term trends have also occurred at other times. Also the lowering of the reservoir by about 10 m over the period 1996-2001 had only a small effect on the long-term trends of the dam deformations.

14. The simplified model of the present study shows that the observed dam deformation pattern can be explained satisfactorily in terms of the combined time-dependent effects of the main loads, i.e. the gravity and water loads. An exact computational model of the complex behaviour of such a fill dam would require a great deal of effort, but may still not yield much better results.

ACKNOWLEDGMENT

The authors are grateful to DSI (Ministry of Energy and Natural Resources, General Directorate of State Hydraulic Works) for permitting publication of the paper. The investigations and studies described in this paper were carried out in cooperation with DSI and Dolsar Engineering Ltd. The contributions of other experts, who have participated in this

project and are not listed explicitly, are greatly acknowledged. The opinions expressed in this paper are those of the authors and are not necessarily those of DSI. This paper is a slightly edited version of a paper, which was published in November 2006 in International Water Power and Dam Construction.

REFERENCES ADINA R & D (1999). ADINA User Interface Command Reference Manuals, Vol. I: ADINA Model Definition. Report ARD 99-2, ADINA R & D, Inc., Watertown, Massachusetts, USA.

Justo, J.L. and Durand, P. (2000). Settlement-Time Behaviour of Granular Embankments. International Journal for Numerical and Analytical Methods in Geomechanics, Vol. 24, No. 3, pp. 281-303.

Malla S., Wieland M., Straubhaar R. (2006): Monitoring Ataturk: Interpretation of measured deformations and safety monitoring of Ataturk dam by calibrated dam model, International Water Power and Dam Construction, November 2006

Naylor, D.J., Maranha, J.R., Maranha Das Neves, E. and Veiga Pinto, A.A. (1997). Back-Analysis of Beliche Dam. Geotechnique, Vol. 47, No. 2, pp. 221-233.

1 Pyry Energy Ltd., Hardturmstrasse 161, CH-8037 Zurich, Switzerland, sujan.malla@poyry.com 2 Pyry Energy Ltd., Hardturmstrasse 161, CH-8037 Zurich, Switzerland, martin.wieland@poyry.com 3 Pyry Energy Ltd., Hardturmstrasse 161, CH-8037 Zurich, Switzerland, ruedi.straubhaar@poyry.com