Pei-Hsun Tsai

A Numerical Study of the Screening Effectiveness of Open Trenches for High-Speed Train-Induced Vibration

This study used the 2D boundary element method in time domain to examine the screening effectiveness of open trenches on reducing vibration generated by a high-speed train. The parameters included configurations of the trench, train speed, the distance between the source and the trench, and the Poisson’s ratio of the soil. A reducing displacement level (in dB scale) was defined and used to evaluate the screening effectiveness of a wave barrier. The maximal reducing displacement level reached 25 dB when an open trench was used as a wave barrier. The depth of an open trench is a main influential parameter of screening effectiveness. The cutoff frequency of the displacement spectrum increases with decreasing trench depth. The maximal screening effectiveness occurs when the depth is 0.3-0.4 Rayleigh wavelength. Using an open trench as a wave barrier can reduce 10–25 dB of vibration amplitude at frequencies between 30 and 70 Hz. A considerable increase in screening effectiveness of the open trench was observed from 30 to 70 Hz, which matches the main frequencies of vibration induced by Taiwan High Speed Rail. The influence of trench width on screening effectiveness is nonsignificant except for frequencies from 30 to 40 Hz. Poisson’s ratio has various effects on the reduction of vibration at frequencies higher than 30 Hz.

Effects of Core on Dynamic Responses of Earth Dam

This paper investigates the dynamic response of the Pao-Shan II Dam subjected to the Chi-Chi earthquake (ML=7.3) in Taiwan by using FLAC3D. The elastic modulus of the dam is considered to vary with mean stress in this study. Staged construction, seepage, static equilibrium and dynamic response are sequentially analyzed. Fourier power spectra are analyzed as the earth dams subjected to a sweep frequency dynamic loading. Influences of core dimensions on the dynamic responses of the earth dam are investigated. The influence of the core width-height ratio and length-height ratio of the dam on the first natural frequency is studied in this study. The results show that 3D effect could be neglected for η > 4 cases. The first natural frequency decreases with the increase of core width-height ratio or length-height ratio of an earth dam. The first natural frequency increases slightly after the seepage phase. The stiffness of the dam decreases at the end of an earthquake which causes the first natural frequency to decrease.

Simulating the Loading Behavior of Reinforced Strip Footings with a Double-Yield Soil Model

AbstractGeosynthetics such as geogrids or geotextiles are often used for the reinforcement of weak soils. Much research has been performed to investigate the bearing capacity of reinforced shallow foundations using the limit equilibrium method with Mohr–Coulomb failure criterion. In this paper, the loading behavior of reinforced strip footings was studied numerically using a commercial finite difference program with a double-yield soil model. First, the load–settlement curve of an unreinforced strip footing was simulated using both Mohr–Coulomb failure criterion and the double-yield model. The simulated bearing capacities were then compared with Terzaghi’s solution. The effects of various parameters, such as burial depth and length of the reinforcement, as well as arrangement of two reinforcement layers, were also studied. Results of this study indicate that the loading behavior of strip footings simulated with the double-yield model is more realistic, and the optimum burial depth and length of reinforcem...

Stability Analysis of Landslide Dam under Rainfall

Failure of a landslide dam might occur by river discharge or rainfall. A rise of the upstream water level of a landslide dam and rain infiltration into the dam body increase pore water pressure and the weight of the dam. In this study, transient seepage analysis of rainfall infiltration and dam stability analysis are performed. A two-phase flow simulation using the FLAC finite difference code is adopted to analyze unsaturated seepage flow in transient fluid-mechanical calculations. The safety factor of dam stability is evaluated using the shear strength reduction technique. The parameters discussed in this study include the rising speed of the water level, rain infiltration, and the hydraulic conductivity of soil. The results show that the time to slope failure of the dam is approximately 247 min. when only the effect of the rising upstream water level of the dam is considered. The failure time decreases to 189 min. when the rain infiltration and rising upstream water level of the dam are considered. The results also indicate that the hydraulic conductivity of the dam affects dam failure time.

Effects of Open Trench Dimension on Screening Effectiveness for High Speed Train Induced Vibration

2D boundary element method in time domain is used to investigate the screening effectiveness of open trenches on reducing vibration generated by a high speed train in this study. The parameter used in the study is the configurations of the trench. In order to evaluate the screening effectiveness of a wave barrier, a reduction of dB value is defined and used in this study. The maximum reduction of dB values is about 25 dB when an open trench is used as a wave barrier. The depth of open trench is a main influential parameter of screening effectiveness. The deeper the open trench, the better vibration reduction. The screening effectiveness increases slightly due to the increase of trench width. Current study facilitates the basics for the optimum configuration of the open trench as the wave barrier for reducing vibration.

A study on dynamic properties of cement-stabilized soils

In this paper the dynamic property (shear modulus and damping ratio) of cement-stabilized soil is studied with using the resonant column test. The amount of cement admixed, the magnitude of confining pressure, and shearing strain amplitude are the parameters considered. Test results show that the maximum shear modulus of cement-stabilized soil increases with increasing confining pressure, the minimum damping ratio decreases with increasing confining pressure. The shear modulus of cement-stabilized soil decreases with increasing shearing strain while the damping ratio increases with increasing shearing strain. In the paper the relationship of shear modulus versus shearing strain is fitted into the Ramberg-Osgood equations using regression analysis.

Application of MASW Method for Evaluating Dynamic Properties of Lu-Liao-His Earth Dam

In the paper the shear wave velocity and Poisson’s ratio profile are studied using the MASW test. Slant stacking was adopted in experimental dispersion curve constructing. Theoretical dispersion curve can be constructed by thin layer stiffness matrix method. A real-parameter genetic algorithm is required to minimize the error between the theoretical and experimental dispersion curves. Test results show that spectrum using slant stacking shows the fundamental mode of Rayleigh wave in the frequency range from 15 Hz to 50Hz. To reduce the error of experimental and theoretical dispersion curve using real-parameter genetic algorithm is feasible. The results also show that the strata of Lu-Liao-His Earth Dam can be modeled as 3 soil layers with an underlying half space.

Numerical Modeling on the Stress-Strain Behavior of Sand via Disturbed State Concept Model

The disturbed state concept (DSC), proposed by Desai, is a powerful constitutive modeling for the simulation of the material behaviors of strain hardening or softening of soil. These material parameters can be decided by the results of the triaxial compression test of soil. Some triaxial drained test were performed to obtain the deviator stress vs. axial strain behaviors of Ottawa sand with relative densities Dr ＝20% and 90% under three different confining pressures. The result shows that the experimental data and the results of the DSC model showed good correspondence. In study for disturbance parameters to effect on the curve of axial strain vs. deviator stress, it can be found that the deviator stress after peak markedly decreases when the values of disturbance parameters are reduced. The effect of disturbance parameter A on the trend for deviator stress decrease after peak is higher than that of parameter B.

Using Continuous Wavelet Transform to Construct the Dispersion Image for Soil Layers

This study used a time-frequency domain analysis for estimating the dispersion curve of a Rayleigh wave by using two receivers. The signals were first transformed using continuous wavelet transform. A similar slant stack procedure was used to analyze the wavelet transform signals and extract a dispersion image. This method is advantageous because it requires no empirical judgment in phase unwrapping and few receivers. To examine the applicability of the method for evaluating the dispersion curve for soil layers with lateral heterogeneity, three synthetic examples and an experience example were investigated. In these examples, numerical simulations of the surface wave seismic test were performed using the finite difference FLAC code. The results revealed that the estimates of the surface wave dispersion curve, obtained using the method, coincide with those of the theoretical values. A high-resolution dispersion image is generated by increasing the spacing of receivers. The method is applicable for extracting a dispersion image for lateral heterogeneous soil layers.

Estimation of 2D Shear Wave Velocity Profile of Soil Layers Using Surface Wave Seismic Tests

The 2D shear wave velocity profile of strata is estimated using the active and passive surface wave seismic tests. The experimental dispersion curves were obtained after the recorded signals were transformed by the slant stack procedure. The phase velocity in the relatively high frequency range can be obtained using the dispersion curves deduced from the active tests. On the other side, dispersion curves obtained from the passive tests can be used to estimate the phase velocity in the relatively low frequency range. From the higher frequency portion of the dispersion curves that stand for the fundamental mode, we obtained the phase velocities about 190 m/s for the sandy surface fill. Theoretical dispersion curves can be constructed by the thin-layer-stiffness-matrix method. For theoretical dispersion curves, the soil layers of the test site were modeled as the sandy surface fill overlying a half space soil layer. A real-parameter genetic algorithm was programmed to minimize the difference between the theoretical and experimental dispersion curves. We prove that the real-parameter genetic algorithm is capable to reduce the error between experimental and theoretical dispersion curves. The estimated 2D geometry of the sandy surface fill using the active and passive surface wave seismic tests was verified with the borehole data.