Dynamic properties of fill materials and aseismatic analysis for high earth and rock fill dams.
Earth and rock fill dam is a essential dam type in hydraulic and hydropower engineering, since it has following advantages such as, easy to be constructed, low geology requirements, comparable low cost, and abundance materials in site. This is a long construction history of earth and rock fill dam engineering. Statistically, there are about 86000 earth and rock fill dams above 15m have been constructed in China since 1950s. With the rapid economic development, the construction of hydraulic and hydropower engineering advanced rapidly too. In recent years, a number of clay core seepage prevention earth and rock fill dams are at the height of 100m or even 200m. The Xiaolangdi dam is inclined core earth and rock fill dam with the height of 167m and constructed on a cover layer with the thickness of 70m. It is the most typical project in clay core earth and rock fill dam construction. The Nuzhadu dam in Lancangjiang River will be clay core rock fill dam with the height of 265m, which will be the highest dam in China.
Since 1980s, concrete faced rock fill dams have been under construction and developed very fast in China. Most of the dams, such as Xibeikou, Gouhou, Chengping (phase I) and Zhushuqiao Dam are all in the height of 70m and some of them above 100m. Recently, there are a lot of concrete faced rock fill dams above 100m and parts of them are close or above 200m. For examples, Jiangshanxi Dam in Zhejiang province is constructed with excavated rock fill materials at the height of 132.5m, Qiezishan dam in Yunnan province is constructed with the granite at the height of 107m. Gongboxi aconcrete faced rock fill dam on the Yellow River is at the height 133m in seismic intensity VIII area. Tianshengqiao (phase I) dam is at the height of 178m. Shuibuya dam on Qingjiang River in Hubei province is under construction and its height will be 233m.
A number of high earth and rock fill dams have been under construction as a result of the recent Western Development Plan in China. Most of these dams are located in the high seismic activity area of the southwestern China. The dynamic properties of rock fill materials and dynamic response analysis of the high earth and rock fill dam are the essential research topics for situation under strong seismic motion. This paper presents state of arts on the dynamic properties of fill materials and aseismatic analysis for high earth and rock fill dams
LABORATORY DYNAMIC TESTS AND DYNAMIC PROPERTIES OF FILL MATERIALS
Laboratory dynamic tests
The development of dynamic properties of coarse granular soil mostly relies on the improvement of testing technique. There are three kinds of approaches on studying dynamic properties of coarse granular soils in laboratory: (1) test wave velocity and attenuation properties based on wave theory (ultrasonic and pulse technique); (2) test soil coefficients of stiffness and damping based on vibrate theory (resonant column triaxial test); (3) obtain the relationship of stress-strain directly based on cyclic loading tests such as cyclic triaxial test, hollow torsional test, simple shear test, torsinal simple shear test etc. The advantages of cyclic loading test are prone to controlling initial stress condition, dynamic stress condition, drainage condition. Therefore, it is widely used and developed in the past few years and there are a few new progresses were achieved in this field in recent years.
The field stress condition can be reasonably reflected through stress path laboratory test on dynamic deformation properties of coarse granular soil. The direction of main stress in conventional cyclic triaxial test is fixed and the deformation condition is axial symmetrical which is greatly different from the field stress condition. But the cyclic torsional simple shear laboratory test can simulates many conditions such as the torsion of main stress axis, cyclic simple shear deformation, the uniformity distribution of vertical shear strain etc. It can also reflect the field loading condition. So it is used recently. Attempt experiment study on the shear strength through true triaxial apparatus was performed by Guo Qingguo which will be a base and trend in coarse granular soil mechanics. It is obvious that dynamic properties experiment study under complex loading conditions is still under development.
Each test method has different strain level and test index, so it is difficult to generally compare the test results. The dynamic properties can be measured in relatively wide strain range, especially under microcosmic strain condition by introducing high precise microcosmic displacement transducer. Then the continuous measure method of dynamic properties can be put into a substantiality stage for the rock fill materials specimen. It not only predigests the testing method of characteristic parameters but also degreases the measure cost and enhances the testing precision.
Dynamic deformation properties of rock fill materials
Under all kinds of load effect, the dynamic deformation properties is mostly affected by the factors of strain level, stress condition, testing measures, grain properties and drainage condition. The studies of coarse granular soil at present is mainly based on testing the rule of shear modulus ,which is calculated in equivalent and linear expression ,and damping ration non-linear changing with strain value. In the following it will be concluded from the experience description and influence factors of dynamic shear modulus and dynamic damping ratio.
Dynamic shear modulus of rock fill materials
The void ratio and relative density are also important influence factors on dynamic shear modulus of rock fill materials besides the shear strain value, secondly the grain size (especially maximum grain size), lithology, sediment history and cycle of cyclic stress etc. According to strain magnitude it can be divided into dynamic shear modulus under microcosmic strain condition and dynamic shear modulus under middling strain condition.
(1) Dynamic shear modulus under microcosmic strain condition and its influence factors
The dynamic shear modulus under microcosmic strain condition which is also called initial shear modulus ([G.sub.max]) is an important parameter to description dynamic properties of rock fill materials. It keeps constant under microcosmic strain condition ([less than or equal to] 10-4%). Normally it is obtained by resonant column triaxial test in laboratory or field wave velocity test. In recently using cyclic loading test to test initial shear modulus becomes true because of the use of microcosmic strain transducer test apparatus.
Confined pressure and the density of specimen are the main influence factors on shear modulus of rock fill materials under microcosmic strain condition. Many laboratory test results indicate that the modulus present the increasing trend with the increase of confined pressure, while the modulus gradually degrease with the increase of void ratio. Considering the main factors, all kinds of experimental relationships of initial shear modulus are proposed. The experience expressions combined the field testing results by Seed et al. (1970) and combined the lab test results by Hardin et al. were two typical examples.
The effects of grain properties (lithology, grain size etc), sampling method and test measure were also studied besides two main factors reflected by experience expressions. It was indicated from the test on rock fill materials with the same density but different coarse content that the initial modulus increases obviously with the increases of coarse content. The effect of the grain diameter of rock fill materials on initial modulus was studied by Kong Xianjing et al. (1988) from another view of point through large model dam in laboratory vibrating table. It was pointed out that when shear strain held the line the modulus increased with the increase of average grain diameter but the rise rate tend to a smooth way when the grain diameter increased to a certain value. Liang Yongxia (1990) indicated that initial shear modulus decreased with the decrease of grain diameter of testing material with the same density and also suggested that it should times a correct coefficient between 1.0-1.5 when the test results of rock fill materials was applied into practice. The test results from high precise and large hydraulic pressure serve triaxial apparatus by Jia Gexu (1998) indicated that elastic modulus of rock fill materials was very close to each other by all kinds of test measures under microcosmic strain condition (less than 10-30%).
(2) The rule of shear modulus should be considered among a relatively wide strain value for the soil dynamic problem of dynamic shear modulus and its effects under moderate strain conditions. In order to describe the attenuation of dynamic shear modulus with the increase of strain value the shear modulus G in a certain strain value was commonly normalized the maximum shear modulus [G.sub.max]. The shear modulus ration is denoted as [sup.delta] = G/[G.sub.max]. It is difficult to collect and manage the parameter in the model because the test data obtained from dynamic deformation soil test are scattered. However based on the before-mentioned initial experience expression of shear modulus it can be expressed in a wide strain range by the variety of modulus parameter K and index m. The experience expression was proposed by Wu Xingzheng et al. considering the comparability between the curve changing trend of modulus ration and shear modulus in semi-logarithm coordinate and properties curve of non-saturated soil water proposed by Fredlund et al.
It is indicated that there aren't obvious difference of the degressive curve of modulus between saturated sample and dry sample of rock fill materials. The reason is probably that the change of pore water pressure had little effect on sample made up of large grain diameter. So the field shear modulus G could be obtained by the laboratory anisotropic consolidation test only the magnitude of average main stress to be corrected.
As for the shear modulus normalized curves from cyclic triaxial test and torsional simple shear test, it indicates that the two curves are basically the same. While Iida et al. (1984) concluded that the shear modulus normalized curve from trosional simple test was lower than the curves from dynamic triaxial test, which should be verified by more test studies.
Furthermore, specimen lithology and sediment history also affected the shear modulus of rock fill materials.
The accumulated residual deformation properties of rock fill material are also experimental studied.
Dynamic damping properties of rock fill materials
The soil damping is mainly hysteresis damping. It is described by the corresponding equivalent viscosity damping ratio [xi] that ratio of the soil losing energy in a cycle and the maximum elastic shear strain energy. The test data is much scatter as the friction of test apparatus is sometimes great and difficult to obtain precisely. It becomes more and more deeper for the properties of dynamic damping of rock fill materials with the development of some high precise and microcosmic displacement test apparatus
There are commonly two experience relationships to describe the dynamic damping properties of coarse granular soil. The first one is to directly set up the relationship between the dynamic modulus and damping ratio. The second one is to directly set up the experience relationship considering all kinds of effect factors of dynamic damping ratio.
The main effects on rock material damping ratio are shear strain, void ratio or relative density and confined pressure.
Dynamic strength properties of rock fill materials
Hatanaka et al. (1988) firstly conducted the cyclic undrained test and studied the dynamic strength. Yasuda et al. (1997) also studied the dynamic strength properties of rock fill materials systematically, and used the large scale torsional single shear apparatus first time.
Characteristics of dynamic strength properties of rock fill materials
The results of cyclic undrained loading tests indicate that compare with sand, the dynamic strength of coarse granular soil are different in the following two aspects, (1) the increasing pattern of pore water pressure is different from dense sand remarkably. In early stage of cyclic test, with the times of cyclic loading increasing, the pore water pressure of the compression direction increases up to almost 100% of confining pressure, the effective stress reduces to zero, the maximum ratio of pore water pressure reaches up to 0.9 or more. (2) As rock fill materials considered, with the times of cyclic loading increasing, the strain increases gradually. Even the cyclic time reaches 100, sudden increment of strain will not happen in the liquefaction process of loose sand. It has been studied that the rock fill materials have higher cyclic deformation properties, but the accumulative deformation increases with the cyclic times increasing, which can not be ignored as for the aseismatic stability of rock fill materials structures which is dominated by the deformation control.
The effect and reasonable description of rapid increasing pore water pressure of saturated rock fill materials on the dynamic deformation and strength properties should be taken into full consideration.
Description methods of dynamic strength properties of rock fill materials
For strength properties of coarse granular soil, there is no uniform description method so far. Usually one of ratio of cyclic shear stress, peak value of dynamic strain and vibration times is considered as criterion, different description patterns will be achieved with the other two considered. For example, (1) The relationship of cyclic shear stress ratio and corresponding vibration times at a certain strain peak value. (2) The cyclic shear stress ratio at certain vibration time at certain strain peak value. (3) The vibration time at certain cyclic shear stress ratio to the appointed strain peak value etc.
Influence factors of dynamic strength of rock fill materials
The influence factors of dynamic strength mainly include relative density, confining pressure, initial shear stress, test method, sampling pattern and lithology etc.
The results of undrained cyclic test of different relative density of rock fill materials indicate that the relative density increases with the cyclic shear stress ratio. The cyclic shear stress depends on confining pressure at relative higher density. With the average stress decreasing, the cyclic shear stress ratio increases, which is consistent with the conclusion that cyclic shear stress depends on confining pressure. While the cyclic shear stress ratio of rock fill materials doesn't depend on initial confining pressure at relative lower density. The cyclic shear stress ratio increases with the initial shear stress increasing at a relative higher density. The result of cyclic shear stress of torsional single shear test is lower than that of triaxial compression test, while the mean principal stress is dependent of initial shear stress. With the same relative density, the cyclic shear stress ratio of disturbed sample is 50% of undisturbed sample.
The results of vibrating table tests with different contents of gravel indicates that the sandy gravel have aseismatic properties when the content of gravel reaches up to a certain limit. It is also indicates that grained material can not form framework when the gravel content is lower than limit content. The permeability of sandy gravel depends on sand and liquefaction performance is also close to sand.
In addition, the dynamic strength of coarse granular soil is also remarkably influenced by the sediment history and lithology of samples.
The key technique study on high earth and rock fill dam of the 8th 5-year plan special topic is combined with 5 earth and rock fill dams at the height of 200m such as Pobogou, Xiaolangdi, Jilintai, Zipingpu, Daliushu. The relationship of dynamic stress and strain is conducted with 3 different kinds of fill materials, 1 kind of cushion material, 2 kinds of transition materials and dam foundation sandy gravel. The main results on cyclic loading dynamic triaxial apparatus and testing technique and the dynamic properties of coarse grained soil are as follows:
(1) China Institute of Water Resources and Hydropower Research (IWHR) developed the wave velocity testing apparatus and testing demarcates technique of laboratory large-scale triaxial apparatus, which improved the testing precision up to 10-4s to 10-6s, and also improved and perfected the testing method for dynamic deformation properties of coarse granular soil from a range of small strain to large strain under max confining pressure, 1Mpa with the wave velocity testing apparatus of large-scale dynamic triaxial test combined with testing large-scale samples, proposed in the 7th 5-year plan period,
(2) In Dalian University of Technology, Dynamic deformation properties of sand from small strains to large strains are tested on a medium and a small dynamic triaxial apparatus with testing set up for local axial strain at the stress level of max confining pressure from 0.2MPa to 0.4MPa, which is proved feasible.
(3) The equivalent visco-elastic model parameters of former several kinds of dam material are achieved from tests, which can be used for the dynamic response analysis in the mentioned projects. When testing the relationship of stress and small strain, the wave velocity and local axial microcosmic deformation testing technique are applied.
(4) With the mentioned test apparatus, consolidate drained cyclic loading tests on air-dry materials and saturated materials are conducted in China Institute of Water Resources and Hydropower Research and the relationships of dynamic stress and residual shear strain of air-dry saturated material are presented. It is also indicated the static strength is approximately equal to the dynamic strength for the kind of saturated sand. The relationships of dynamic shear stress ratios and residual shear strains of rock fill materials from Guanmenshan and Pubugou are obtained in the consolidated drained or air exhausted cyclic loading tests in Dalian university of technology, which can be used for calculating the permanent deformation of earth and rock fill dam under seismic action.
(5) In order to reduce the seismic deformation of dam, it is better to study out reliable engineering measures in the aspects of high quality fill materials, enhancing fill density, increasing the confining pressure, and seepage control and reducing saturated water zoon.
ASEISMATIC ANALYSIS OF EARTH AND ROCK FILL DAM
According to the constitutive model, the methods of seismic response analysis can be divide into two kinds, one kind is equivalent-linear method based on the equivalent visco-elastic model, and another was real nonlinear analysis based on elastic-plastic model. The first model is convenient for application and abundant test data and project experience, which can be accepted for engineering field, had been accumulated in the aspect of parameter determination and model application. While the second model, which had more reasonable theory, could approach well actual response of soil, and could directly calculate the residual deformation of dam.
According to the whether considering pore water pressure in the earthquake process, the methods of seismic response analysis may be divide into total stress method and effective stress method. The effective stress method also has two kinds; one does not consider the dissipation and diffusion of pore water pressure, and another considered it. The closed relevant research, such as dynamic pore water characteristics, dynamic strength and liquefaction properties, had achieved important development. The liquefaction mechanism theory has been established, and the relationship and difference of soil liquefaction, failure and limiting equilibrium state also had obtained thoroughly knowledge. All these established the base for the earth and rock fill dam earthquake disaster mechanism research.
From present national and international research situation, the dynamic response analysis method of earth and rock fill dam gradually developed from, equivalent linear, total stress method to three-dimensional, real non-linear, the dissipation and diffusion of pore water pressure considering, effective stress method. And thorough research work should be performed in the aspects, such as reservoir water-dam-foundation couple nonlinear analysis, nonlinear constitutive model under complicated stress condition, pore pressure calculating mode, earthquake residual deformation calculating method, interface modeling and boundary condition processing, seismic input, high-precision numerical simulation, nonlinear calculating method and so on. The equivalent static method is used to carry on anti-slide stability analysis and safety evaluation of earth and rock fill dam in project. But this traditional method could not well consider the internal stress-strain relationship and real work state of soil, which is related closely with seismic characteristics, and the obtained safety factor was only so-called factor along the supposed potential slip surface. It is unable to obtain actual internal force and determinate the deformation, and forecast the occurrence and the developing process of failure. Moreover, it could not consider the effect of partial deformation on the stability of the dam. Therefore, dynamic response analysis method of earth and rock fill dam and ground are gradually developed.
Based on the nonlinear earthquake responds analysis, combining some research methods, such as laboratory test, in-situ test and so on, studies of hazard mechanism, seismic security and preventing measures, maybe the essential questions of the high earth and rock fill dam during the earthquake. For the research of failure mechanism under the earthquake, the nonlinear problems should be thoroughly studied, including nonlinear material properties, nonlinear failure parameters, nonlinear seismic respond characteristics, nonlinear solution theory and laboratory test methods. And the research of these aspects at present is very imperfectly, the innovative research work is required. According to the structure failure characteristic and the project destruction mechanism, the reasonable seismic design methods and preventing measures of high earth and rock fill dam should be proposed, and the function mechanism of preventing measures should be studied. The decrease of worry about the earthquake is the key point and the goal of seismic research work. It's important to value the pertinence, the reliability, the usability and the efficiency of the measures in the research.
Aseismatic key technique research on high earth and rock fill dam of special topic of the 8th 5-year plan achieved main results in the aspect of the seismic respond analysis method and the computational technique are as follows,
(1) The structural nonlinear visco-elastic-plastic analysis method or named incremental linear visco-elastic dynamic respond analysis method was proposed by Beijing Hydraulic Research Institute. The main characteristic is using incremental method and superposition method in turn, which can control the error accumulation of incremental method and make the analysis result to be more reasonable.
(2) Adopting the cyclic shear testing results of the interface between the cushion material and the concrete, which was conducted by Hohai University using the vibrated drag plate installation and refitting dynamic simple shear apparatus, the dynamic model of the interface between the face slab and the cushion is used to compare the effect of element shape on the calculating results by Dalian University of Technology. The results of comparison show that it is feasible to use slab interface element in the face slab dam analysis. Comparing Goodman element with thin element, it indicates that the maximum relative error between the two calculating results does not exceed 1%, when the thickness of thin element is about 10cm. Therefore, it is suggested to use anisotropic thin element with certain thickness to substitute for the Goodman element for simplified calculation.
(3) The development of permanent deformation of earth and rock fill dam. The structure nonlinear visco-elastic-plastic analysis method, which proposed by Beijing Hydraulic Research Institute, regards the soil as visco-elastic-plastic material, and considers the mathematical pattern of soil elastic-plastic deformation rule under dynamic loading as the combination of initial load curve, skeleton curve and hysteretic loop. The tangent shear modulus is calculated by established nonlinear elastic-plastic shear strain model, and the secant shear modulus and (secant or tangent) damping ratio is calculated by nonlinear elastic model. The secant Young's Modulus is calculated by supposing the damping ratio equals const. Then, the permanent deformation of dam can be computed by the structure nonlinear visco-elastic-plastic analysis method. During the research, the semi-empirical methods of permanent deformation, such as the whole deformation analytic method (mainly is nodal force method) and rigid body slide method have been improved significantly.
(4) Development of the computational method of dynamic hydraulic pressure. The interaction of dynamic hydraulic pressure and the concrete faced rock fill dam is studied by Hohai University. When calculating dynamic hydraulic pressure 3 conditions (water is not compressible, water is compressible, and calculating with Westergaard formula) are considered. The analyze results show that it is reasonable to use the numerical model with uncompressible water to perform dynamic couple analysis. For the low dam, it is wise to use Westergaard formula which calculating dynamic hydraulic pressure is considered as additional mass to the couple dynamic analysis. The workload is too much for the model with compressible water, which is not recommended.
(5) Influence of traveling wave to the earth and rock fill dam. The studies by Sichuan United University showed that the influence mainly displayed in the earthquake direction and the occurrence of the strengthen area. The traveling wave velocity and direction had the significant influence to seismic respond: when the traveling wave input along the axes of the dam, a greater vertical acceleration, displacement difference, and horizontal stress occurred. The respond of traveling wave had enhancement under the most situations for ditch earth and rock fill dam, and the enhancement would be about 30%.
(6) Development of computer programme. Nanjing Hydraulic Research Institute developed the total stress 3D static and dynamic analyze general programme (TOSSD3), and effective stress 3D static, dynamic analyze programme (EFFESD3). Hohai University developed the dynamic finite element programme (WWCC2D) and (WWCC3D). Sichuan United University developed the 3D nonlinear dynamic analyze programme with traveling wave input function.
EARTHQUAKE INDUCED PERMANENT DEFORMATION OF EARTH AND ROCK FILL DAMS
At present, it is regulated in national specifications of seismic actions on hydraulic structures that surface sliding method is applied in aseismatic stability analysis of rock fill dams. However, harzards and investigations indicate that safety factor against sliding can not correctly reflect whether rock fill dams are failure after earthquake. Rock fill dams may be instantaneous failure or instantaneous limit equilibrium state under strong seismic actions. However, the instantaneous instability of dams may not induce engineering failure. Deformation of dams induced by instantaneous instability is permitted if it has no great influence on engineering function. Thus, it is obvious that the safety factor of dams under seismic actions is not the direct parameter. Therefore, researchers started to look for more reasonable method to evaluate dam stability. For concrete faced rock fill dam, safety standard and design basis mostly depend on the deformation of the dam body, so it is widely accepted that permanent deformation is applied to assess the dam safety. For many years, the researchers have studied in this field and several simplified analysis methods are proposed through the results of dynamic response analysis based on numerical analysis. The method for earthquake induced permanent deformation includes two respects of determinacy and randomness. The so-called determinacy analysis of earthquake induced permanent deformation is a method that permanent deformation is calculated through dynamic stress and dynamic displacement which is obtained by numerical integrating one or several seismic wave spectrums. It can also be classified to sliding block deformation and whole deformation analysis.
Sliding block deformation analysis
It is firstly proposed by Newmark (1965) based on limit equilibrium theory and assuming that permanent deformation is the sliding displacement of sliding body along the most possible sliding surface during instantaneous failure under seismic actions. Sliding surface will start to move when the acceleration of some point exceeds the critical value. The permanent deformation can be calculated by integrating the value of the ground acceleration minus the critical value twice. Based on Newmark method, lots of other researchers (Markdisi and Seed, Chen Shengshui and Shen Zhujiang et al.) also developed the method.
Whole deformation analysis
The basic assumption of whole deformation analysis is regarding soil deformation as continuous medium. The method is developed with applying visco-elastic model is in constitutive law and using FEM for calculation combined with experimental study. According to the mechanism of permanent deformation, it includes two kinds of methods, (1) Softening modulus method. It is based on the assumption that permanent deformation is due to a depressing of static shear modulus by seismic stress. The permanent deformation is calculated by twice finite element based on the difference of static strain induced by depressing shear modulus and before earthquake. It is firstly proposed by Lee and several methods, such as initial approximate estimations, linear modified modulus and nonlinear modified modulus, were proposed by Serff et al. (2) Equivalent node force method. It is suggested that the influence of seismic induced deformation is replaced by a batch of static nodal force (equivalent node force) acting on element nodes. The permanent deformation is obtained by using FEM calculating superimposed deformation under equivalent node force according to the relationship between residual deformations and dynamic stress received from experiment. Taniguchi and Whitman proposed some typical method and equivalent nodal method were proposed by Zhang Kexu and Liu Hanlong et al.
The stochastic analysis of permanent deformation also includes sliding block deformation analysis and whole deformation analysis. (1) Stochastic analysis of Sliding block deformation. Lin (1984) firstly proposed the method of permanent deformation of earth and rock fill dams. Later on Lin (1986) introduced particularly the process through which the permanent deformation of perfect sliding block was calculated. Wu Zaiguang et al. (1991) carried out further studies based on Lin's research. (2) Stochastic analysis of Continuous deformation. At present, less investigation is carried on in this aspect and only Wu Zaiguang et al. deals with some preliminary study. Wu introduces the accumulative damage model of liquefaction analysis into the softening modulus approach under stochastic seismic excitation. Liu Hanlong established a stochastic analytical method of permanent deformation based on the equivalent nodal force scheme and seismic randomicity.
RESEARCH ON ASEISMIATIC MEASURES OF THE HIGH EARTH AND ROCK FILL DAM
In order to meet the aseismiatic requirements of earth and rock fill dam some measures have to be taken. When there are fine sand crystals in the shallow layer of the foundation, vibro replacement stone column is commonly used. In general, the slope of the dam is slow. For example, the grade ratio of a concrete panel sandy gravel dam in Jilingtai, Xinjiang province is 1:1.7~1:1.9, the design earthquake intensity of which is 8 degree. For the same example, when the rock fill of which the antiskid ability is better is used in the outer dam slope, the grade ratio could be 1:1.4 or little slower. The aseismiatic performances of most rock fill dams with face slab are good, which are related to the attributes of the rock fill and earth. Furthermore, in order to reduce the reclamation materials and meet the aseismiatic requirements, concrete sashes or bar meshes are installed in the dam.
(1) The cracking at the top of the impermeable barrier in the core wall rock fill dam is one of the common phenomena of earthquake damage. For the sake of preventing the cracking caused by earthquake, it's important to select better reclamation materials, take notice of the reclamation quality, and optimize the fracture surface design. In addition, the designs of the inverted filter and transition layer of the impermeable barrier at the upriver and downriver surfaces are much more important, especially at the downriver surface .
(2) The earthquake damages for rock fill dam with face slab are mostly at the top of dam, such as the cracking of the face slab, the deformation of the dam body, the land slip of the downriver slope, and so on. In addition, the fractional damages of the peripheral joint and the watertight seal parts always appear. Based on the height of dam and the earthquake response, different measures can be taken, such as: slowing the top slope, the addition of the packways at the proper altitude, the setting of the face slabs at the top of the dam and at the upriver and downriver surfaces, the setting of tension braces and geotextile at the top of the dam, the adoption of reclamation with roller compacted concrete, and so on. When it's necessary, the horizontal seams can be added in upriver face slab at the proper altitude, and the antiseep ability of the cushion should be increased at easily-cracked parts.
RESEARCH ON EXPERT SYSTEM OF ASEISMIATIC SAFETY EVALUATION AND DISASTER PREVENTION MEASURES OF HIGH EARTH AND ROCK FILL DAM
Regarding the earthquake damage data and the data of reclamation materials dynamic behavior as knowledge base, considering the constitutive models applied in the earthquake response analysis of earth rock and fill dam and earthquake response analytic methods as model base, taken the aseismiatic safety evaluation criteria and method for evaluation standard base, regarding the research system of aseismiatic reinforcement measures as countermeasure base, and based on the artificial intelligence theories and expert system tools, the aseismiatic safety evaluation and disaster prevention countermeasure expert system of high earth and rock fill dam can be founded, as shown in Fig 1. If the proper initial conditions have been given, the aseismiatic safety evaluation conclusion of dam could be achieved with the expert system, no matter the dam is being constructed, have been built or will be built. Furthermore, the corresponding aseismiatic measures and the further work could also be suggested. At the same time, the use of the artificial intelligence will make the system learn the new knowledge, so that the system will self-perfect and develop continuously during the application of the system in the process of decision, scientific research, design and construction of the earth and rock fill dam projects.
[FIGURE 1 OMITTED]
In order to meet the requirements of the economic and water resources development, it is necessary to construct high dams in strong seismic area. As for the construction of high earth and rock fill dam in strong seismic area, the aseismiatic problems are always the controlling factors. Once high earth and rock fill dams were failure, the result will be an awful disaster. On the other hand, so far, there aren't any cases and field data of earthquake failure of high earth and rock fill dam. Therefore, seismic actions on high earth and rock fill dams should be studied. Further researches are suggested as follows,
(1) Research on the mechanism of the earthquake failure of high earth and rock fill dam. In order to prevent and mitigate disaster effectively, the mechanism of the earthquake failure should be clearly understood.
(2) Research on the dynamic properties of fill materials and the acquisition and management of earthquake failure data of high earth and rock fill dam. The reliable dynamic properties of fill materials and earthquake failure data are the bases of the research on mechanism of the earthquake failure.
(3) The dynamic hydraulic pressure and dynamic residual displacement can be calculated directly by elasto-plastic method, so 3-D elasto-plastic seismic response analyze method should be developed.
(4) Research on nonlinear problems of high earth and rock fill dam, such as nonlinear material propertiesr (constitutive laws), nonlinear failure parameters, nonlinear seismic response characteristic, nonlinear solve theory and experiment method, etc.
(5) The evaluation theory and method of the aseismiatic safety of high earth and rock fill dam should be established, according to the structure failure characteristic and failure mechanism.
(6) Research on the engineering measures of resisting earthquake damage of high earth and rock fill dam and the mechanism of the measures.
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H. L. LIU
Geotechnical Research Institute (GeoHohai), Hohai University, 1 Xikang Road Nanjing, 210098, China
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|Publication:||Geotechnical Engineering for Disaster Mitigation and Rehabilitation|
|Article Type:||Conference news|
|Date:||Jan 1, 2005|
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