Mar 2010, Volume 4 Issue 1

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  • Research articles
    Eduardo E. ALONSO, Rafaela CARDOSO,
    The basis of the design of earth and rockfill dams is focused on ensuring the stability of the structure under a set of conditions expected to occur during its life. Combined mechanical and hydraulic conditions must be considered since pore pressures develop during construction, after impoundment and in drawdown. Other instability phenomena caused by transient flow and internal erosion must be considered. The prediction of the hydro-mechanical behavior of traditional and non-traditional materials used in the construction of dams is therefore fundamental. The materials used for dam’s construction cover a wide range from clayey materials to rockfill. In a broad sense they are compacted materials and therefore unsaturated materials. A summary of the current level of knowledge on the behavior of traditional materials used in the construction of dams is presented in the paper. Regular compacted materials (with a significant clay fraction), rockfill and compacted soft rocks are studied with more detail. The latter are non-traditional materials. They are analysed because their use, as well as the use of mixtures of soil and rock, is becoming more necessary for sustainability reasons.
  • Research articles
    Jinsheng JIA,
    Along with economic, social quick development and urbanization, dams and reservoirs are of strategic importance for flood control, water supply, electricity production, irrigation, etc., both for developed countries and for developing countries. Climate change is a new challenging issue to be considered which will speed up the development of hydropower in developing countries. More and more attention will be paid on the long-term better behavior of dams to guarantee the safety of the people involved and the better development of the world. There are about 50000 old dams in the world and a lot of them have been completed and operated for more than 50 years. However, how do we evaluate the dams’ safety? How do we make the decision to do rehabilitation work or to rebuild a new dam based on evaluation results? The life span and the real safety status of old dams becomes a challenging task for the dam society, especially for China because it has more than 6000 dams to be evaluated and rehabilitated within the next few years. Based on the investigation of the Fengman gravity dam, which is 91.7 m high, operated since 1943 and suffered uplift pressure, freeze and thaw problems, etc., discussions on the life span evaluation of old concrete gravity dams have been made. The reasonable coefficient of dam safety has been discussed. The social decision for the final choice after comprehensive studies has been introduced.
  • Research articles
    Lei WANG, Cho Thanda NYUNT, Toshio KOIKE, Oliver SAAVEDRA, Lan Chau NGUYEN, Tran van SAP,
    Reservoir is an efficient way for flood control and improving all sectors related to water resources in the integrated water resources management. Moreover, multi-objective reservoir plays a significant role in the development of a country’s economy especially in developing countries. All multi-objective reservoirs have conflicts and disputes in flood control and water use, which makes the operator a great challenge in the decision of reservoir operation. For improved multi-objective reservoir operation, an integrated modeling system has been developed by incorporating a global optimization system (SCE-UA) into a distributed biosphere hydrological model (WEB-DHM) coupled with the reservoir routing module. The new integrated modeling system has been tested in the Da River subbasin of the Red River and showed the capability of reproducing observed reservoir inflows and optimizing the multi-objective reservoir operation. First, the WEB-DHM was calibrated for the inflows to the Hoa Binh Reservoir in the Da River. Second, the WEB-DHM coupled with the reservoir routing module was tested by simulating the reservoir water level, when using the observed dam outflows as the reservoir release. Third, the new integrated modeling system was evaluated by optimizing the operation rule of the Hoa Binh Reservoir from 1 June to 28 July 2006, which covered the annual largest flood peak. By using the optimal rule for the reservoir operation, the annual largest flood peak at downstream control point (Ben Ngoc station) was successfully reduced (by about 2.4m for water level and 2500m3·s−1 for discharge); while after the simulation periods, the reservoir water level was increased by about 20m that could supply future water use.
  • Research articles
    Martin WIELAND,
    Earthquakes can affect large dam projects in many different ways. Usually, design engineers are focussing on ground shaking and neglect the other aspects. The May 12, 2008 Wenchuan earthquake has damaged 1803 dams and reservoirs, and 403 hydropower plants with an installed capacity of 3.3GW. Among these dams were the 132-m-high Shapai RCC arch dam and the 156-m-high Zipingpu concrete face rockfill dam. These recently completed dams are dam types which, up to now, have not experienced strong ground shaking. The widespread mass movements have caused substantial damage to dams and surface powerhouses in Sichuan. The different features of earthquake hazard are presented, i.e., ground shaking, faulting and mass movements. It is proposed to prepare project-specific safety plans for all dams, which consist of a matrix where the possible hazards and the corresponding countermeasures are listed. The earthquake behaviors of the Sefid Rud, Zipingpu and Shapai dams, which, in the past, have experienced strong ground shaking from nearby earthquakes, are discussed. Finally, the need for strong motion instrumentation of large dams is discussed. It is proposed that major dams with large damage potential, dams located in areas of high seismicity, and dams showing signs of abnormal behavior be equipped with strong motion instruments.
  • Research articles
    Xiangyu XU, Dawen YANG,
    Physically-based hydrological models are used to predict catchment water balance through detailed simulation of hydrological processes at small temporal and spatial scales. However, annual catchment water balance can also be easily and simply predicted using lumped conceptual model. Comparison between physically-based hydrological models and lumped conceptual models can help us understand the dominant factors on catchment water balance at different scales. In this paper, a distributed physically-based hydrological model (i.e., bottom-up approach) and a simple water-energy balance model (i.e., top-down approach) are used to predict actual evapotranspiration in nine sub-catchments, and the whole basin of the Luan River in northern China. Both simulations give very close values of annual evapotranspiration and show the same complementary relationship between actual and potential evapotranspiration at annual time scale. From the analysis at different time scales through comparison of the top-down and the bottom-up methods, it is shown that the annual catchment evapotranspiration is controlled mainly by annual precipitation and potential evapotranspiration, and the variability of soil water and vegetation becomes more important at a smaller time scale in the study areas. It is also known that the relationship between potential and actual evapotranspiration shows a highly nonlinear relationship at the annual and catchment scale but can be simplified to a linear relationship at hourly temporal and hillslope scales, which is commonly used in the physically-based hydrological models.
  • Research articles
    Erich BAUER, Zhongzhi FU, Sihong LIU,
    The wetting deformation of weathered rockfill materials has been attracting growing attention from both engineers and scientists. The importance of realistic predictions of wetting deformations for high earth and rockfill dams is a strong motivation to establish a suitable constitutive model. Recently, the hypoplastic constitutive model by Gudehus and Bauer was extended by introducing solid hardness depending on the state of weathering. The extended model takes into account the influence of the current density, the effective stress state, the rate of deformation, and the time dependent process of degradation of the solid hardness. In the present paper, the performance of this model is evaluated by comparing numerical simulations with experiments obtained from a water sensitive rockfill material. In particular, triaxial compression paths and creep deformation under deviatoric stress states are considered. Finally, the constitutive model proposed is used to study the influence of a degradation of the solid hardness on the long term behavior of a hypothetical fill dam.
  • Research articles
    Yuji KOHGO, Akira TAKAHASHI, Tomokazu SUZUKI,
    In this paper we analyzed a real fill-type dam, named the Minami-Aiki Dam, to verify a numerical consolidation method with an elastoplastic model for unsaturated soils. The validities were conducted by comparing the analyzed and measured values during construction and impounding. The settlements calculated showed good agreement to those observed. Though the changing patterns of the measured and calculated pore water pressures were similar, the amounts of pore water pressures induced by the self-weights of embankment and the dissipation speeds were different. The earth pressures measured at the bottom of the core zone showed good agreement with those calculated. This method is valid for estimating behavior of fill-type dams during construction and impounding.
  • Research articles
    Predicting “regular” dam deformations for prevailing conditions and comparing them with observed deformations is an essential part of dam surveillance. In many cases prediction and comparison are carried out automatically and an alarm is triggered in the case of larger deviations.
    The main contributors to the deformations of arch dams are water loading and the fluctuation of concrete temperature. In general, the deformations exhibit an instantaneous elastic part, a time dependent reversible part and an irreversible part. Prediction of dam deformations can be based on deterministic models, purely statistical models or so-called hybrid models, which are a combination of the first two. Deterministic models rely on static analyses, statistical models on the statistical analysis of previous deformation data.
    For annual reservoirs, both water loading and temperature vary in cycles over a period of one year and it is extremely difficult to identify the various factors contributing to an observed deformation. This is particularly true for time-dependent reversible displacements due to the water loading and the portion caused by temperature variation.
    The present article deals with the analysis of arch dam deformations based on hybrid models. Particular attention is given to the analysis of the deformations due to temperature fluctuation and to the long-term deformations. The proposed procedure is employed for the analysis of the deformations of the 186 m high Zillergruendl arch dam.
  • Research articles
    Jianguo LIU, Qicheng SUN, Feng JIN,
    Dense granular matter is a conglomeration of discrete solid and closely packed particles. As subjected to external loadings, the stress is largely transmitted by heavily stressed chains of particles forming a sparse network of larger contact forces. To understand the structure and evolution of force chains, a photoelastic technique was improved for determining stresses and strains in the assemblies of photoelastic granular disks in this paper. A two-dimensional vertical slab was designed. It contains 7200 polydispersed photoelastic disks and is subjected to a localized probe penetrating at the top of the slab to mimic the cone penetration test. The interparticle contact force distribution was found a peak around the mean value, a roughly exponential tail for greater force and a dip toward zero for smaller force. The force chain network around the probe tip was depicted, and the contact angle distribution of particles in force chains was found to be well aligned in the directions of major principal stress.
  • Research articles
    Xuexing CAO, Yunlong HE, Kun XIONG,
    Studies show that the dynamic properties of rockfill are strongly dependent on the confining pressure. Therefore, confining pressure effect has become a very important factor in the seismic analysis of high rockfill dam. The relationships of dynamic shear modulus versus dynamic shear strain and damping ratio versus dynamic shear strain had been improved to a certain degree on the basic of widely used Hardin-Drnevich constitutive model in this paper. Then a new model that could consider confining pressure effect has been established. Regression analysis was carried out of the dynamic triaxial experimental data of the damming materials in the Changheba hydropower station of Sichun Province, China. The results show that, the new model can fit the test data well under various confining pressures. A corresponding computational procedure was compiled and applied in the dynamic response analysis of the Changheba Dam. Comparing the calculation results between the new constitutive model and the ordinary Hardin-Drnevich model, it can be seen that the result is conservative to some extent without considering confining pressure effect.
  • Research articles
    Two-stage (pre-placed aggregate) concrete (TSC) consists of coarse aggregate, which is first deposited into the formwork, and grout, which is later pumped into the formwork from the bottom up to bind the stone together into a monolith. During the course of this study the formation of grout for two stage concrete was investigated using readily available 5 mm sand mixed in a slow speed paddle mixer.  Silica fume and a superplasticizer were used in the mix to see if the necessary stability and fluidity could be achieved without the use of a specialized plant, essentially to see if TSC could be produced on a small scale using native Irish aggregates. A much higher degree of sedimentation was recorded during testing than was expected. As a result, the use of 5 mm sand and a slow speed mixer in combination were found to be not conducive to the successful production of TSC grout, without further research and testing.