Jiunnren Lai

Development of a Transparent Material to Model the Geotechnical Properties of Soils

Measurement of three-dimensional deformation patterns and flow characteristics within a soil continuum are usually limited by the fact that soil sensors do not provide a continuous image of the measured continuum. Additionally, soil sensors exhibit static and dynamic characteristics that are different from those of the surrounding soils and therefore can change the response of the measured continuum. Tests conducted with a transparent material which has properties that closely model the geotechnical properties of natural soils can potentially circumvent these experimental problems if the response of a model transparent continuum can be measured using nonintrusive optical visualization techniques. This paper demonstrates the feasibility of producing transparent materials which exhibit macroscopic properties representative of the geotechnical properties of natural soils. The transparent “soils” discussed in this paper were made by consolidating suspensions of amorphous silica in liquids with matching optical refractive indices. The measured shear strengths and permeabilities of the transparent soils are characteristic of natural clays and silts. The stress-strain response of transparent soils is characterized by large strains during consolidation and shear.

Hazard Assessment of Debris Flows by Statistical Analysis and GIS in Central Taiwan

This paper aims to build an analytical process of assessing debris-flow hazards using multivariate analysis and geographic information system (GIS) techniques. The watershed of the Chen-Yu-Lan River is investigated in this study. Factors that are believed to be critical in the occurrence of debris flow are identified and considered in the assessment of debris-flow hazards. These factors used for assessing the debris-flow hazard are: (1) rock formation, (2) fault length, (3) naked-land area, (4) slope angle, (5) slope aspect, (6) stream slope, (7) watershed area, (8) form factor, and (9) cover and management factor. Using the spatial analysis feature of GIS, the indexes of these factors are calculated. By using principal component analysis (PCA) and discriminant analysis (DA) of all indexes according to each factor, the discriminant function of overall debris-flow hazard at any particular creek in the Chen-Yu-Lan River may be assessed. The applicability of the proposed approach for hazard assessment of debris-flow in the watershed of the Chen-Yu-Lan River has been confirmed with other researches and field observations in recent debris-flow events.

Laboratory Testing of Enhancing the Bearing Capacity of Strip Footing with Woven Geotextiles

Laboratory sand box tests were performed in this study to investigate the improvement in bearing capacity of a strip footing reinforced with woven geotextile in dry sand. The test cell is 0.9m wide, 0.9m long and 1.0m high. A hydraulic loading system was used to apply the normal force to a 0.85m × 0.1m × 0.05m (length × width × height) strip footing. The unreinforced bearing capacity was obtained and compared with value calculated using Terzaghi’s equation. Various parameters such as: burial depth, length of reinforcement, number of layers and distance between layers were varied to investigate their effects on the bearing capacity. Results of these tests indicate that the optimum burial depth of reinforcement is about 0.4 times the width of footing, with a bearing capacity ratio (BCR) of about 1.67. The optimum reinforcement length is about 3~4 times the width of footing with a BCR value of 1.81. The improvements in bearing capacity obtained from laboratory testing are in accord with previous numerical simulation. However, the loading behaviors are quite different due to the failure mechanism assumed in the numerical simulation.

Numerical Study on Enhancing the Bearing Capacity of Shallow Foundation Using Geosynthetics

Geosynthetic materials such as geogrid or geotextile are often used to compose a reinforced earth to improve the engineering properties of weak soils. These materials provide extra tensile resistance by interaction with surrounding soils. The geosynthetic materials have long been used to reinforce earth slopes, but relatively little study have been performed on their usage for the reinforcement of shallow foundation. In order to provide practice engineers with parameters to optimize their design, the effectiveness of reinforced shallow foundation was studied numerically by using a commercial finite difference program – Fast Lagrangian Analysis of Continua (FLAC). The bearing capacity of an unreinforced strip foundation was simulated and compared with Terzaghi’s theoretical solution to verify the correctness of the numerical model. The effects of various parameters, such as the burial depth, length of the reinforcement material, number of reinforcement layers, arrangement of reinforcement layers, and the strength of soil on the bearing capacity were then studied. Results of this study verify that the reinforced shallow foundation can provide higher bearing capacity, especially for weak soils. It was found that a buried depth of 0.4 times the width of foundation yields the optimum bearing capacity increase. The most economical reinforcement length is about 2 to 3 times the width of the foundation. Adding an additional layer of reinforcement was found to further increase the bearing capacity. 64 Innovative and Sustainable Use of Geomaterials and Geosystems

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.

Application Of Closed-Form Solution For Normal Surface Displacements On Impacted Half Space : Quantification Of Impact-Echo Signals

In order to investigate the feasibility of deducing a simulated transfer function based on the Rayleigh wave form in an Impact-Echo signal, the analytical solution for the normal sur- face displacement due to a heaviside force at the half-space was reviewed and used to compute the surface displacement responses resulted from various types of impulse forces. Based on a series of numerical studies on the characteristics of Rayleigh wave form in the surface displace- ment responses, this paper presents the idea of using an equivalent impact force to derive an in- tentionally scaled transfer function. The pseudo force can be obtained from using Rayleigh wave form as a pseudo force or by generating an equivalent half-sine impact force accordingly. The effect of using such pseudo and equivalent force functions was discussed in details. In the pro- posed method, the force amplitude was first estimated from an amplitude curve established from numerical simulations using half-sine force functions. The recovery of a simulated transfer func- tion was next achieved via the use of an estimated force amplitude and a selected force function. The proposed procedure also results in steady thickness amplitudes when measurements on two concrete plates were taken for various impacts associated with different steel balls and different impact locations. The success in recovering constant thickness amplitudes for plate-like struc- tural members proved that the derivation of simulated transfer function is a useful tool in ex- tending the Impact-Echo test. The quantitative evaluation of the interfacial property of the sub- strate layer will also benefit from this simulated transfer function.

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...

ASSESSMENT OF LOCAL STIFFNESS FOR SLENDER CONCRETE MEMBERS USING IMPULSE RESPONSE TEST

In a typical dynamic test of structural members, signals within low frequency range can normally reflect the overall stiffness of the member, while they seem not useful in assessing stiffness of a specific region. In this study, a simple back-calculation procedure was developed to potentially assess quasi-static stiffness for a specific portion of a linear member. The purpose of this study is to briefly introduce the theoretical background of such methodology and to verify the feasibility of such procedures with experimental data obtained from tests on concrete specimens. Based on the numerical and experimental data, it was found that the proposed method gives reasonable estimates of stiffness for the test portion of a symmetric reinforced concrete beam. Certain modifications are also required in extending such procedures to members with various boundary conditions.

Boundary Effects of Pile Cap on the Integrity Testing of Group Piles

Low-strain surface reflection pile integrity testing (PIT) methods have been successfully used for the quality assurance of newly-built individual piles. However, for existing piles with pile cap, previous studies have shown that the stress-wave reflected from the boundaries of pile cap may cause difficulties in determining their integrity. In this paper, the boundary effects of pile cap on the signal of PITs were studied by numerical simulation and tests on model piles. Feasibility of minimizing the boundary effects by embedded accelerometers was also investigated. Results from this study indicate that signals obtained from numerical simulation are similar to those obtained from tests performed on model piles. It also shows that boundary effects of pile cap on low strain pile integrity tests can be reduced by using embedded accelerometers, thus making it easier to assess the integrity of piles with a pile cap.

Foundation stiffness in the linear modeling of wind turbines

Effects of foundation stiffness on the linear vibrations of wind turbine systems are of concerns for both planning and construction of wind turbine systems. Current study performed numerical modeling for such a problem using linear spectral finite elements. The effects of foundation stiffness were investigated for various combinations of shear wave velocity of soil, size of tower base plate, and pile length. Multiple piles are also included in the models such that the foundation stiffness can be analyzed more realistically. The results indicate that the shear wave velocity of soil and the size of tower base plate have notable effects on the dominant frequency of the turbine-tower system. The larger the lateral dimension, the stiffer the foundation. Large pile cap and multiple spaced piles result in higher stiffness than small pile cap and a mono-pile. The lateral stiffness of a mono-pile mainly depends on the shear wave velocity of soil with the exception for a very short pile that the end constraints may affect the lateral vibration of the superstructure. Effective pile length may be determined by comparing the simulation results of the frictional pile to those of the end-bearing pile.