Koyna Dam Research Articles

A thermodynamics-based damage constitutive model of concrete

A thermodynamics-based damage constitutive model is developed for concrete, which describes tensile and compressive damage well and follows the second thermodynamic law. Constitutive model should meet general thermodynamics, while it describes behaviour characteristic of concrete material, which are often not able to be satisfied by existing models because of various of hypotheses. Generalised stress functions and dissipative generalised stress functions are constructed from free energy function and energy dissipation function according to the physical meaning of the thermodynamic potential and the characteristic of the constitutive model of concrete. Then, a damage constitutive model of concrete is developed and the parameters of the model are identified. The numerical method of the model is also presented. The model in the paper can satisfy the second thermodynamic law automatically and simulate the tensile and compressive behaviour of concrete well. Finally, the feasibility and practicality of the model is verified by the analysis of classical experiments and Koyna dam.

Numerical Study of Koyna Dam Failure Process Using Extended Finite Element Method

This article expound the development process and the basic principles of the extended finite element, In the large commercial software abaqus on the platform of India koyna dam failure process are numerically simulated in earthquake, and koyna dam crack initiation, through ,propagation path analysis. The results show that the earthquake began to crack initiation site often at the large dam slope fold. The study provides a theoretical foundation and scientific basis for the actual engineering improves safety.

Seismic cracking analysis of concrete gravity dams with initial cracks using the extended finite element method

The seismic crack propagation of concrete gravity dams with initial cracks at the upstream and downstream faces has rarely been studied during strong earthquakes. In this paper, a numerical scheme based on the extended finite element method (XFEM), which has been widely used for the analysis of crack growth, is presented to deal with the numerical prediction of crack propagation in concrete gravity dams. The validity of the algorithm is discussed by comparing results obtained from the proposed XFEM with those reported in the literature. For this purpose, the cracking process and final crack profile of Koyna dam during the 1967 Koyna earthquake are simulated numerically by employing the XFEM. The computed distribution of cracking damage is consistent with the actual condition and the results of model test and available methods in literature, which verifies the validity of the calculation model. Subsequently, the Koyna dam with single and multiple initial cracks is also analyzed using the proposed approach, which is investigated to evaluate the seismic crack propagation of the concrete gravity dam with initial cracks. The effects of initial cracks on the crack propagation and seismic response of the concrete gravity dam are discussed.

Damage evaluation of concrete gravity dams under mainshock–aftershock seismic sequences

A large mainshock may trigger numerous aftershocks within a short period, and large aftershocks have the potential to cause additional cumulative damage to structures. This paper investigates the effects and potential of aftershocks on the accumulated damage of concrete gravity dams. For that purpose, 30 as-recorded mainshock–aftershock seismic sequences are considered in this study, and a typical two-dimensional gravity dam model subjected to the selected as-recorded seismic sequences is modeled. A Concrete Damaged Plasticity (CDP) model including the strain hardening or softening behavior is selected for the concrete material. This model is used to evaluate the nonlinear dynamic response and the seismic damage process of Koyna dam under mainshock–aftershock seismic sequences. According to the characteristics of the cracking damage development, the local and global damage indices are both established to study the influence of strong aftershocks on the cumulative damage of concrete gravity dams. From the results of this investigation, it is found that the as-recorded sequences of ground motions have a significant effect on the accumulated damage and on the design of concrete gravity dams.

The effects of strong motion duration on the dynamic response and accumulated damage of concrete gravity dams

Strong motion duration is one of the challenging characteristics of ground motion, which affects the cumulative damage of structures significantly. Many researchers have conducted investigations related to the effects of strong motion duration on the response of building structures. However, the corresponding studies of concrete gravity dams are limited. In this paper, the effects of strong motion duration on the accumulated damage of concrete gravity dams are investigated. A Concrete Damaged Plasticity (CDP) model including the strain hardening or softening behavior is selected for the concrete material. This model is used to evaluate the nonlinear dynamic response and seismic damage process of Koyna dam during 1976 Koyna earthquake. Subsequently, the damage analyses of Koyna dam subjected to earthquake motions with different strong motion durations are performed. 20 as-recorded accelerograms, which are modified to match a 5% damped target spectrum, are considered in this study. Strong motion durations are obtained based on the definition of significant duration. According to the characteristics of the cracking damage development, both local and global damage indices are established as the response parameters. The results show that strong motion duration is positively correlated to the accumulated damage for events with similar response spectrum, and has significant influence on the cumulative damage of the dam. Longer duration will lead to greater accumulation damage to which aseismic design of the dam should be given attention.

A mesh-free approach for fracture modelling of gravity dams under earthquake

Fracture is a major cause of failure for concrete gravity dams. This can result in the large-scale loss of human lives and enormous economic consequences. Numerical modelling can play a crucial role in understanding and predicting complex fracture processes, providing useful input to fracture-resistant designs. In this paper, the use of a mesh-free particle method called smoothed particle hydrodynamics (SPH) for modelling of gravity dam failure subject to fluctuating dynamic earthquake loads is explored. The structural response of the Koyna dam is analysed with the base of the dam being subjected to high-intensity periodic ground excitations. The SPH prediction of the crack initiation location and propagation pattern is found to be consistent with existing FEM predictions and experimental results from physical models. The transient stress field and the resulting damage evolution in the dam structure were monitored. The amplitude and frequency of the ground excitation is shown to have considerable influence on the fracture pattern and the associated energy dissipation. The fluctuations in the kinetic energy of the dam wall and its fragments are found to vary with different frequencies and amplitudes as the structure undergoes progressive fracture. The dynamic responses and the fracture patterns predicted establish the strong potential of SPH for fracture modelling of dams and similar large structures.

Continuum large cracking in a rate‐dependent plastic–damage model for cyclic‐loaded concrete structures

SUMMARYFormulation and algorithmic treatment of a rate‐dependent plastic–damage model modified to capture large tensile cracking in cyclic‐loaded concrete structures are presented in detail for a three‐dimensional implementation. The plastic–damage model proposed by Lee and Fenves in 1998 was founded based on isotropic damaged elasticity in combination with isotropic multi‐hardening plasticity to simulate cracking and crushing of concrete under cyclic or dynamic loadings. In order that the model can capture large crack opening displacements, which are inevitable in plain concrete structures, the excessive increase in plastic strain causing unrealistic results in cyclic behaviors is prevented when the tensile plastic–damage variable controlling the evolution of tensile damage is larger than a critical value. In such a condition, the crack opening/closing mechanism becomes similar to discrete cracking. The consistent tangent operator required to accelerate convergence rate is also formulated for the large cracking state including viscoplasticity. The validation and performance of the modified algorithm implemented in a special finite element program is exemplified through several single‐element tests as well as three structural applications. The last example examines the model in the seismic fracture analysis of Koyna dam as a benchmark problem and the resulting crack profile is compared with the available experiment. The numerical experimentations well demonstrate that the developed model whose modification is necessary to properly simulate the cyclic behavior of plain concrete subjected to large tensile strains is robust and reasonably accurate. Copyright © 2012 John Wiley & Sons, Ltd.

Optimal Reservoir Operation for Hydropower Generation using Non-linear Programming Model

Hydropower generation is one of the vital components of reservoir operation, especially for a large multi-purpose reservoir. Deriving optimal operational rules for such a large multi-purpose reservoir serving various purposes like irrigation, hydropower and flood control are complex, because of the large dimension of the problem and the complexity is more if the hydropower production is not an incidental. Thus optimizing the operations of a reservoir serving various purposes requires a systematic study. In the present study such a large multi-purpose reservoir, namely, Koyna reservoir operations are optimized for maximizing the hydropower production subject to the condition of satisfying the irrigation demands using a non-linear programming model. The hydropower production from the reservoir is analysed for three different dependable inflow conditions, representing wet, normal and dry years. For each dependable inflow conditions, various scenarios have been analyzed based on the constraints on the releases and the results are compared. The annual power production, combined monthly power production from all the powerhouses, end of month storage levels, evaporation losses and surplus are discussed. From different scenarios, it is observed that more hydropower can be generated for various dependable inflow conditions, if the restrictions on releases are slightly relaxed. The study shows that Koyna dam is having potential to generate more hydropower.

A comparative study of the different procedures for seismic cracking analysis of concrete dams

Different procedures are compared for the three-dimensional seismic cracking analysis of gravity and arch dams during strong earthquakes. The fracture procedures include the extended finite element method with cohesive constitutive relations, crack band finite element method with plastic-damage relations, and the finite element Drucker−Prager elasto-plastic model. These procedures are used to analyze the nonlinear dynamic response of Koyna dam to the 1967 Koyna earthquake and the seismic cracking of the Dagangshan arch dam subjected to design earthquake. The cracking process and profiles of the two dams using the three different procedures are compared. The applicability and the suitability of the three procedures for seismic cracking analysis of gravity and arch dams are discussed.

A hybrid distinct element–boundary element approach for seismic analysis of cracked concrete gravity dam–reservoir systems

This paper proposes a new algorithm for modeling the nonlinear seismic behavior of fractured concrete gravity dams considering dam–reservoir interaction effects. In this algorithm, the cracked concrete gravity dam is modeled by distinct element (DE) method, which has been widely used for the analysis of blocky media. Dynamic response of the reservoir is obtained using boundary element (BE) method. Formulation and various computational aspects of the proposed staggered hybrid approach are thoroughly discussed. To the authors' knowledge, this is the first study of a hybrid DE–BE approach for seismic analysis of cracked gravity dam–reservoir systems. The validity of the algorithm is discussed by developing a two-dimensional computer code and comparing results obtained from the proposed hybrid DE–BE approach with those reported in the literature. For this purpose, a few problems of seismic excitations in frequency- and time-domains, are presented using the proposed approach. Present results agree well with the results from other numerical methods. Furthermore, the cracked Koyna Dam is analyzed, including dam–reservoir interaction effects with focus on the nonlinear behavior due to its top profile crack. Results of the present study are compared to available results in the literature in which the dam–reservoir interaction were simplified by added masses. It is shown that the nonlinear analysis that includes dam–reservoir interaction gives downstream sliding and rocking response patterns that are somehow different from that of the case when the dam–reservoir interaction is approximated employing added masses.

Transforming Results from Model to Prototype of Concrete Gravity Dams Using Neural Networks

A new method using neural networks for the transformation of results from dam models to prototypes has been proposed and validated through application to Koyna and Pine-Flat Dams, which have also been investigated by other researchers. The neural network has been called the neurotransformer. The common method for building a suitable experimental model for a dam to be tested on a shaking table is linear dimensional analysis or simply linear scaling (LS). However, because LS is theoretically applicable to linear systems, it generally provides imprecise results of transformation for extreme loading when the model or the prototype experiences noticeable nonlinearity. In this paper, it is shown through numerical simulation of the dynamic behaviour of Koyna Dam and its 1/50 model under strong earthquakes, which cause nonlinear behavior in both the dam and its model, that transformation by neural networks is considerably more precise than LS. To show the method can also be applied to other dams, the same procedure was successfully applied to Pine-Flat Dam; again, the neurotransformer outperformed the LS.

Modelling the hydromechanical response in the vicinity of the Koyna reservoir (India): results for the initial filling period

SUMMARY The seismic activity in the Koyna area is clearly related to the water impoundment of the Koyna dam in 1962, and has reached a remarkable level with the occurrence of a major event of magnitude 6.3 in 1967 December 10. We present a homogeneous poroelastic model based on analysis of the first eight years of seismicity, which aims to link the water-level fluctuations of the reservoir with the seismicity. Starting from a discretized lake, we calculate the stress field resulting from the water-level fluctuations and the pore pressure changes due to the undrained and the diffusive responses of the medium. Then, we compare the Coulomb stress variations with a set of relocated seismic events. We find that more than 80 per cent of the relocated events before the M6.3 event are well described by this poroelastic model, leading us to derive a suitable diffusivity cp = 0.2 m 2 s −1 . Then, we model the response of the system after the M6.3 event of 1967 December 10, by comparing the variation of the Coulomb stress field with the spatio-temporal characteristics of the relocated post-seismic events and the decay of aftershocks with time. We find that compared to before the main event a tenfold increase in hydraulic diffusivity is required to satisfactorily describe the aftershock decay with the appropriate Omori exponent. Although this increase in diffusivity may be physically related to the main shock we also note that events later than 9 months after the main shock are not well explained. We therefore propose an alternative hydrological model, which involves two compartments of contrasting diffusivities.

INVESTIGATION OF DYNAMIC RESPONSE CHARACTERISTICS OF GRAVITY DAMS BY SUB-STRUCTURE APPROACH

ABSTRACT In the sub-structure approach for dynamic response analysis of gravity dams the dam-foundation rock and the dam-reservoir water interaction forces are first estimated by solving the wave equation for the foundation-rock and the reservoir domains with suitable boundary conditions, and the response of the dam by treating it as a separate sub-structure with the earthquake and the interaction forces applied externally. Using this approach, the effect of various controlling parameters on the dynamic response of the tallest monolith of Koyna dam has been investigated with the accelerogram of the M6.5 earthquake of 10 December 1967 as input excitation. Nine different combinations of the Young's modulus of foundation rock, water level in the reservoir, thickness of the sedimentary deposits in the reservoir, and the Young's modulus of the rock underlying the sedimentary layer have been considered to investigate the significance of each of these parameters on the dam response. It has been established that the sub-structure approach is able to predict the response of gravity dams by accounting for the effect of various interaction forces in a quite realistic way, provided the foundation can be approximated to be a half-space.

Estimation of local magnitude of rockbursts using strong motion accelerograms in the mines of Kolar Gold Fields

Rockbursts are minor earthquakes induced due to mining operations. The seismic signals recorded using geophones in the near field are generally saturated and are not suitable for estimating the true magnitudes. The strong-motion accelerograms recorded due to rockbursts are therefore used to obtain the Wood-Anderson synthetic seismograms for getting accurate and reliable values of the local magnitudes. Using several typical strong-motion accelerograms of rockbursts in the mines of Kolar Gold Fields, the magnitudes have been computed in the present study. Correlations of peak ground acceleration and peak ground velocity with the magnitude, studied for the rockbursts vis-a-vis the natural earthquakes in the Koyna dam area, suggest the mechanical similarity between the two different types of events. The results and findings are described in this paper.

Unified plastic-damage model for concrete and its applications to dynamic nonlinear analysis of structures

In this paper, the energy-based plastic-damage model previously proposed by the authors [International Journal of Solids and Structures, 43(3-4): 583-612] is first simplified with an empirically defined evolution law for the irreversible strains, and then it is extended to its rate-dependent version to account for the strain rate effect. Regarding the energy dissipation by the motion of the structure under dynamic loadings, within the framework of continuum damage mechanics a new damping model is proposed and incorporated into the developed rate-dependent plastic-damage mode, leading to a unified constitutive model which is capable of directly considering the damping on the material scale. Pertinent computational aspects concerning the numerical implementation and the algorithmic consistent modulus for the unified model are also discussed in details, through which the dynamic nonlinear analysis of damping structures can be coped with by the same procedures as those without damping. The proposed unified plastic-damage model is verfied by the simulations of concrete specimens under different quasistatic and high rate straining loading conditions, and is then applied to the Koyna dam under earthquake motions. The numerical predictions agree fairly well with the results obtained from experimental tests and/or reported by other investigators, demonstrating its capability for reproducing most of the typical nonlinear performances of concrete under quasi-static and dynamic loading conditions.

Global positioning System in disaster monitoring of Koyna dam, Western Maharashtra

GPS measurements carried out over the last five years have been used to monitor the structural deformation of the Koyna dam, Western Maharashtra, India. A GPS network that has been established by the GPS research group of Indian Institute of Technology Bombay for the purpose of deformation measurements and analysis of the Koyna Dam has been observed over ten epochs, from December 2000 to September 2004. This paper highlights the methodology and analysis techniques used for determining deformation and the results of the research work being carried out. The analysis of the results determines the degree of correlation between the change in reservoir level and the observed structural deformation of the dam.

Global positioning System in disaster monitoring of Koyna dam, Western Maharashtra

GPS measurements carried out over the last five years have been used to monitor the structural deformation of the Koyna dam, Western Maharashtra, India. A GPS network that has been established by the GPS research group of Indian Institute of Technology Bombay for the purpose of deformation measurements and analysis of the Koyna Dam has been observed over ten epochs, from December 2000 to September 2004. This paper highlights the methodology and analysis techniques used for determining deformation and the results of the research work being carried out. The analysis of the results determines the degree of correlation between the change in reservoir level and the observed structural deformation of the dam.

Precursory changes in source parameters for the Koyna-Warna (India) earthquakes

SUMMARY Precursory changes of stress drop and corner frequency are found for five main earthquakes of M w 4.1 to 4.7 that occurred near the Koyna and Warna reservoirs during the period of our close monitoring from 1994 onwards. Earthquakes had started to occur in the area in 1962 soon after the impoundment of the Koyna reservoir and are still continuing. The epicentres are located in a 30-km-long, N‐S seismic zone extending southwards from the Koyna dam. An ew Warna reservoir, situated 25 km south of Koyna, was filled to 60 m in 1992. Most subsequent epicentres are close to the new Warna reservoir. Most M ≥ 4 earthquakes in this region have been associated with foreshocks for 15‐30 days and aftershocks for over a month. At the beginning of various foreshock sequences of M w 1.5 to 2.4, the maximum stress drop values ranged from 0.25 to 0.65 MPa; the corner frequency ranged from 7.0 to 10.0 Hz for all the sequences. The stress drops decreased by over 50 per cent of their maximum value during the precursory period of 4‐17 days prior to main shocks and remained at that low level for a few days after the main shocks. Following an initial increase, the corner frequency decreased by 8t o4 5per cent. The decrease in corner frequency implies a 9‐54 per cent increase in fault length and is inferred to be caused by increased pore pressure as a result of dilatancy.

Failure analysis of fractured dams during earthquakes by DEM

A comprehensive study on the dynamic behaviour of the fractured Koyna dam during earthquakes is performed using the distinct element method (DEM). The extensional functions of earthquake input and hydrodynamic simulation have been incorporated into the current program. The method of choosing the damping by the properties of each part of the system, mesh dependency and ground motion amplification are discussed in detail. Results show that the safety of the dam is ensured if the crack shape is horizontal or upstream-sloped; however, it is very dangerous if the crack slopes downstream. The whole failure processes of the two possible failure modes of the cracked dam, i.e. overturning or sliding of the top separated block during earthquakes under abnormal conditions, are also simulated. Some important phenomena such as the bounce of the top block and the relationship of the sliding and overturning are examined in detail.

Surface loading and triggered earthquakes in the Koyna–Warna region, western India

Earthquakes of M≥5.0 continue to occur in the vicinity of Koyna reservoir in western India, the largest known case of Reservoir Triggered Earthquakes (RTS). The region remains seismically active even after four decades of the impounding of the Koyna reservoir. The earthquakes occur repeatedly along two major fault systems in NE–SW and NW–SE directions. We performed cross-correlation analysis between time series of the Koyna reservoir levels and the strain factor (energy 1/2) calculated for earthquakes of M≥3.0 occurring in the region during 1963–1999. Four time windows of 5 years or more are selected for cross-correlation to predict the periodicities, if any, in the seismic energy release caused by the annual reservoir water level fluctuations. A similar analysis was performed for the newly-impounded Warna reservoir, 25 km south of the Koyna dam. Our results suggest that the initial seismicity in the Koyna region during 1963 was triggered after the region attained steady state pore pressure by diffusion processes, particularly occurring along vertical strike-slip faults. Subsequently, major episodes of earthquake energy release until 1999 show a periodic behavior related to the annual filling of both the Koyna and Warna reservoirs. Two stages of earthquake energy release are evident till 1996 and coincide with annual filling and draining cycles of the reservoirs. Since 1996, the seismic energy release episodes correlate mostly with the draining cycle of the reservoir levels indicating a shift in the present day earthquake activity in the region, which may be due to a combined effect of the Koyna and Warna reservoirs. To explain the causal relationship between the reservoir water level fluctuations at the surface and earthquakes at hypocentral depths in terms of diffusion processes, we modeled the pore pressure front diffusion with time, in an inhomogeneous medium. It is seen that a water level change of the order of 1 m in 5 days in the surface loading can propagate 5–15% of pore pressure front, corresponding to 0.75–2.25 bar, to the hypocentral depth of 6–8 km in the presence of a vertical conducting fault. These small stress perturbations are sufficient to trigger seismicity on pre-existing, critically stressed faults in the Koyna–Warna region.