Analytical estimation of natural frequency in earth dams with respecting to the foundation effects (original) (raw)
Related papers
Natural frequency of earthen dams at different induced strain levels
Engineering Geology, 2019
A method is proposed to estimate the degradation of the first natural frequency of vibration of earthen dams with increase in strain levels induced due to seismic events. A synthesised wave, referred to as the 'sum of sines', generated by the superposition of sinusoidal waves with frequencies ranging from 0.01 Hz to 25 Hz, is scaled to different peak accelerations and used to simulate seismic excitations at the base of the dam. The natural frequency is then determined by studying the response of the structure in the frequency domain. For this study, four dams with varying complexities of geometry, constituent material properties, and known natural frequencies were at first selected from previously published literature to establish the validity of the proposed method for determining natural frequencies of the dams at small strain levels. Plane strain models of these structures were constructed and analysed, using a commercially available finite element method-based software that is capable of performing time-history analyses. Results from the analyses indicate a good agreement between the natural frequencies predicted using the proposed method and the frequency values reported in the literature for the corresponding structures at small strain levels. The method was later used to determine the strain-dependent natural frequency of a hydraulic-fill dam in North Texas. Twenty-one different earthquake conditions, with different peak ground accelerations, frequency contents, and mean periods, were used to thoroughly validate the applicability of the developed methodology. Numerical analyses indicate that the strain-dependent variation of the first natural frequency follows a similar trend as that obtained using the 'sum of sines' excitation when the dam is subjected to widely different earthquake conditions.
Current Science, 2017
This article presents the permanent deformation of an earthen dam located in the vicinity of a safety related structure for M w = 6.5 design basis earthquake. A nonlinear 2D dynamic analysis using a real earthquake motion compatible with the design spectrum was performed to check the earthquake-induced deformations of the dam. Deformations of the dam were also estimated by semi-empirical and empirical methods such as Seed and Makdisi's method, Newmark's double integration method, Jansen's method and Swaisgood's method. Results from different methods are compared to obtain a range for the value of permanent deformations of the dam. It is observed that the lateral deformation obtained by Seed and Makdisi's method is the highest while Jansen's method predicts the highest crest settlement. The crest settlement of the dam is found to vary between 11.8 mm and 17.8 mm, which is within the safety limits according to IITK-GSDMA guidelines.
Review Article: Numerical analysis of the seismic behaviour of earth dam
Natural Hazards and Earth System Science, 2009
The present study concerns analysis of the seismic response of earth dams. The behaviour of both the shell and core of the dam is described using the simple and popular non associated Mohr-Coulomb criterion. The use of this constitutive model is justified by the difficulty to obtain constitutive parameters for more advanced constitutive relations including isotropic and kinematic hardening. Analyses with real earthquake records show that the seismic loading induces plasticity in a large part of the shell and in the lower part of the core. Analysis shows that plasticity should be considered in the analysis of the seismic response of the dam, because it leads to a decrease in the natural frequencies of the dam together to energy dissipation, which could significantly affect the seismic response of the dam. Plastic analysis constitutes also a good tool for the verification of the stability of the dam under seismic loading.
Behaviour of Earth Dam under Seismic Load Considering Nonlinearity of the Soil
Open Journal of Civil Engineering, 2016
The stability and safety are very important issues for the dam structure which are built in seismic regions. The dam body consists of soil materials that behave nonlinearly modelled with finite elements. The numerical investigation employs a fully nonlinear finite element analysis considering linear and elastic-plastic constitutive model to describe the material properties of the soil. In this paper, seismic analysis of an earthen dam is carried out using Geo-Studio software based on finite element method. Initially, the in-situ stress state analysis has been done before the earthquake established, and then its results are used in the seismic analysis as a parent analysis. A complete parametric study is carried out to identify the effects of input motion characteristics, soil behaviour and strength of the shell and core materials on the dynamic response of earthen dams. The real earthquake record is used as input motions. The analysis gives the overall pattern of the dam behaviour in terms of contours of displacements and stresses.
MODELLING OF DAM SECTION CONSIDERING DAM-FOUNDATION INTERACTION BASED ON SEISMIC LOADING CONDITION
Concrete gravity dams are the structures which are constructed to retain large quantities of water in a reservoir. The safety of the dam must be verified by the appropriate modeling considering fluid-structure interaction and soilstructure interaction. Although there lies a complex analysis in the interaction effects, there is a need for an effective methodology that can account for water compressibility, foundation stability and absorptive reservoir bottom. In this study an effective dam-foundation model is designed using ADINA 8.8, a finite element analysis tool. In this paper a case study of the Pare dam is included with and without the dam-foundation interaction which shows a variation of frequency of the dam suggesting an effective modeling of dam foundation rather than considering it as a rigid base. Also the dynamic analysis using average acceleration spectra of IS 1893:1984 is carried out and the stresses are being analyzed for the various modes of vibration. It can be comprehended from the results that there is a significant increase in the period of the dam along with the foundation as compared to the dam alone and also there is a significant decrease of stress with the higher modes of vibration.
Parametric study of the earth dam's behaviour subjected to earthquake
Studia Geotechnica et Mechanica, 2022
Static stability of an earth dam can be established by estimating the static safety factor equal to the ratio of the shear strength to the shear stress along a critical sliding area. In contrast, it is more complicated to evaluate the dynamic stability during an earthquake. The water filling the interstices of the earth dams cannot drain during the short duration of an earthquake. An excess pore water pressure DU develops, and its role is predominant in the destabilisation of the dam. The pore water increase causes a decrease in the soil shear strength. It is, therefore, crucial to evaluate and take into consideration DU in the dam dynamic stability analysis. This research is a contribution to reach this objective. A parametric study was conducted by varying the physical and mechanical soil characteristics constituting the dam, as well as its geometrical values, in order to evaluate their effects on the dynamic safety factor. The dynamic safety factor is calculated using the pseudo-static method, taking into account the excess pore water pressure that develops during cyclic loading into the granular soil of the earth dam upstream face. The results of the parametrical analytical study were also compared to the results of numerical simulations of the dam seismic stability trough pseudostatic method. The numerical simulations were done with three different software: PLAXIS and ABAQUS (based on the finite element method) and GEOSTAB (deals with the problem at the limit equilibrium using the simplified Bishop method). At the end, on one hand, we were able to describe how and at what level of the dam upstream face the sliding occurs, and on the other hand, we were able to underline the adequate combination between the dam geometric parameters and the mechanical soil characteristics which may ensure seismic stability.
Seismic response analysis of a hydraulic fill dam
Acta Geotechnica, 2020
The seismic response of a highly heterogeneous hydraulic fill dam was evaluated by studying the natural frequencies of the first and second modes of vibration and analyzing the crest accelerations of different two-dimensional or 2D sections of the dam when subjected to two different earthquake excitations. The existing methods for determination of the natural frequency of earthen embankment structures can only be used to analyze the structural response at small strain levels. However, during seismic events, the natural frequency of an earthen dam is significantly affected by the nonlinear material behavior exhibited by the geomaterials at high strain levels. Hence, a novel method was devised to evaluate the strain-dependent natural frequency for plane strain 2D dam sections, using a synthesized multi-sine base excitation. The degradation of first and second natural frequencies of transverse vibration for all the 2D sections followed a linear trend when plotted against the respective crest's root mean square strain on a logarithmic scale. The slope of the degradation curve was found to depend on the constituent material properties prevalent in the individual sections. The observed variations in natural frequencies and crest accelerations of the 2D dam sections were also used to assess the suitability of using two-dimensional plane strain analyses for studying the response of a long earthen dam having variability in material properties. Results indicate that there is a considerable chance of erroneous estimation of the seismic response of such highly heterogeneous earthen dams that are conventionally analyzed using plane strain models. A 2D analysis was found to merely capture the seismic response of the individual sections of the dam as independent entities while ignoring the stiffening or weakening effect of the adjacent neighboring segments that may have different material properties.
Seismic Analysis as a Tool in the Design of Two Earth Dams
1981
SYNOPSIS : Two dynamic analysis studies of embankment dams are described. One dam is 43m high on alluvium, the other is 140m high on a rock foundation in a highly seismic area. The main emphasis is on the practical nature of the analytical methods and their value as design tools. The earthquake design features of both projects are described and the closing paragraphs attempt to draw attention to the main points to be considered when running an earthquake analysis of earth dams.
The seismic performance of a earth dam by different displacementbasedmethods
2011
The performance-based design of earth dams and the rehabilitation of existing ones require the evaluation of seismic performance based on permanent displacements caused by expected the earthquake. The paper reports a comparison between different methods with increasing complexity for estimating seismic displacements: simplified rigid block method, based on empirical relationships (Bray and Rathje, 1998; Tropeano et al., 2009); simplified uncoupled method, again based on the sliding block analysis, but accounting for soil deformability; coupled „stick-slip‟ approach, based on a 1D lumped mass model to calculate together dynamic response of the site and movement of sliding block (Tropeano et al., 2011); 2D finite differences analyses by the FLAC code, reproducing the heterogeneity of soil and topographic effects. The methods were applied to the case of the dam of Marello mountain across the Angitola river (Southern Italy). The parameters for static and dynamic geotechnical characteriz...