| As the development of water resources in South- and North-Western areas of China where the topographical condition and engineering geological as well as environmental conditions are often rather complex and the seismic activity is sometime high, large-scale major infrastructures such as high dams have been built or to be constructed or under planning or design. As a potential type of dam structures, earth and rock-fill dams (ERFD) or concrete-faced rockfill dams (CFRD) have been most frequently employed in engineering practice. However, the evaluation of earthquake-resistant behavior and seismic safety of these high dams during earthquake is one of the key technical issues in the design and construction of earth or rockfill dams and concrete-faced rockfill dams. It has been not well solved at present. At the same time, embankments are widely used in the river banks and coastal banks as a protective precaution. It has been recognized that the conventional pseudo-static method for evaluating seismic stability of dams or embankments has many shortcomings while the nonlinear dynamic finite element method for analyzing seismic response of dams and embankments subjected to earthquake shaking is somewhat complicated for practicing engineers. At the same time the seismic safety cannot be authentically evaluated by the single factor of safety against sliding during transient overloading. The sliding movement or overall permanent deformation was proposed by Newmark in 1965 to evaluate the overall dynamic stability of dams and embankments. In order to get good understanding and then to offer rational evaluation of earthquake-resistant characteristics and seismic stability of dams or embankments, the numerical methods for evaluating seismic dynamic response and for assessing the earthquake-induced residual deformations or displacements are to be developed in the thesis. The investigation mainly consists of the following portions.1. As a simplified analytical procedure, the shear-beam or shear-slice model is widely employed for evaluating dynamic response of earth dams or embankments. In this thesis, the modification on the equivalent linear technique for analysis of seismic response of dams and embankments which is based on one-dimensional shear-slice model is made. In the proposed method, the discrete shear-slice model is developed to consider the layered distribution of shear moduli and damping ratios of soils along depth. The differential equations for governing the vibration behavior of dam or embankment are solved mathematically under the assumption that the soils of dam display visco-linear-elastic behavior and the closed-form solutions for the natural characteristics and seismic response of the dam are obtained. The resulting linear solutions are incorporated with equivalent linearization technique for seismic response of dam or soil strata. A series of trial-and-error iterations are performed in order to make the moduli and damping ratios of all individual layers be compatible with their strain amplitudes. Then a linear system which is overall equivalent to nonlinear dam system is achieved and the seis-mic response for this equivalent linearization system can be taken as a good approximation of nonlinear seismic response of dam. Finally, example studies are performed for both a homogeneous dam and a clay-core dam and the numerical results are compared with the solutions obtained by the finite element method to illustrate the rationality and applicability of the proposed method.2. A numerical procedure by using elasto-plastic finite element method based on shear strength reduction technique is presented for pseudo-static seismic stability analysis of dams and embankments. In the proposed procedure, two alternatives are employed to determine the seismically-induced inertia loads as following: (1) The inertia forces are computed by using the routine employed in the pseudo-static limit equilibrium slices method proposed in earthquake-resistant design code of earth and rockfill dams; (2) Based on the dynamic response computed by the nonlinear dynamic finite element analysis of dams or embankments, the inertia forces corresponding to the computed nodal acceleration are defined by a given empirical manner. In addition to the seismically-induced inertia loads which is distributed non-uniformly along dam height, the seepage forces or dynamic pore water pressure caused by steady seepage or normal drop of reservoir are taken into account. Under such coupling circumstances, the elasto-plastic finite element analysis is performed to assess the pseudo-static safety factors of the dam by using the shear strength reduction technique. Finally, numerical computations for example dams are conducted to illustrate the rationality and reliability of the proposed analysis method, and the results obtained from two considerations of inertia loads induced by earthquake are compared. It is shown that the finite element procedure for seismic stability analysis of dams or embankments is proposed to prefer to be used in practice.3. In order to evaluate the earthquake-iriduced sliding displacements of a potential sliding mass in dam slope, the average seismic response of the potential sliding mass assessed on the basis of seismic response of dam or embankment subjected to earthquake shaking computed by equivalent dynamic analysis are compared with the critical safety factor to define the condition and state of the transient overloading. Then the rigid-plastic sliding-block model for evaluating sliding movement proposed by Newmark is used estimate the sliding displacement caused by such an overloading. All the sliding displacement caused by all individual overloading are accumulated to obtain the overall movement induced by earthquake. In the proposed method, the Newmark model is modified in order to take account the degradation of strength of soils due to the build-up of pore water pressure caused by earthquake-induced cyclic or transient loading. At the same time, the effect of vertical acceleration of potential sliding mass induced by earthquake shaking on seismic stability of dam which is previously overlooked are taken into account. Two types of dynamic loading including sine-type excitation and earthquake-induced irregular excitation are considered. Finally, numerical analyses for typical cases are made and a number of findings are given through comparison.4. As an alternative mechanism of earthquake effect on dam or embankment, a finiteelement procedure is proposed for evaluating seismically-induced permanent deformation of dam or embankment. In the proposed procedure, both concepts of equivalent nodal force and step-by-step gradually softening modulus are integrated together. The earthquake duration is divided into a certain number of time increments and for each time increments the residual strain and dynamic pore water pressure which is likely induced during previous time increments under undrained condition are estimated on the basis of the stress condition obtained by the finite element dynamic analysis and the empirical patterns of both residual strain and pore water pressure achieved experimentally. Then, the computed accumulative dynamic pore-water pressure at the end of each time increment is used directly to modify the static hyperbolic relationship of stress and strain which is to be used for the next time period, and at the same time, the equivalent nodal forces equivalent to incremental residual strain potential are defined. By using the modified stress-strain relationship, the incremental deformations are computed when the nodal forces equivalent to earthquake effect on dam defined as above are imposed on the dam or embankment. The computed incremental displacements of the dam or embankment for each time increments are accumulated and the accumulative displacements can be regarded as approximation of the residual deformation which is to be initiated by earthquake shaking. In fact, the proposed numerical procedure has taken both the inertia effect and the softening effect of earthquake-induced loading on the permanent deformation of the dam or embankment into consideration.
|
PDF Full Text Request