Papers by Author: Gye Chun Cho

Abstract: Sedimentation is one of the most basic processes in the formation of a soil structure in nature. Many studies have been performed to describe the characteristics of clay sedimentation, based on settlement and water content measurement. In addition, there have been some attempts in numerical modeling to describe soil structure formation as a whole. However, these effects still fall short in explaining the overall process of soil structure formation because some relevant properties are measured after a self-weight consolidation is completed. Furthermore some measurement techniques significantly alter soil structure. Thus, a non-destructive evaluation is necessary for the effective description of soil characteristics during the sedimentation process. In this study, a testing device is designed that continuously monitors the self-weight consolidation process of sedimentation with shear waves. Piezoelectric bender elements are installed into a testing cell to generate and receive shear waves in a small strain regime. Slurries are prepared with kaolinite-type clay and placed in the cell. Shear wave velocities are continuously measured as a function of time during the whole process of the self weight consolidation. The experimental results suggest that as clay sediment is subjected to a certain loading, the shear wave velocity increases as time increases, showing an abrupt change in log time. This abrupt change is relevant to the formation of a stable soil skeleton. It is concluded that the time-dependent variations in shear wave velocity reflect sedimentation and self weight consolidation behavior and the evolution of the effective stress increment.

Abstract: Geophysical prospecting using electrical resistivity is one of the principal methods for subsurface exploration. However, the majority of such application methods are restricted to coarse descriptions of underground conditions. The Q-system is commonly used as a representative rock mass classification system in modern rock engineering. In this paper, electrical resistivity is related to the Q-system through theoretical analyses. The analyses are based on Coulomb's law and Gauss' law considering electrical characteristics of constituent parameters for rock mass classification such as joint thickness, joint condition, joint spacing, intact rock strength, and RQD. The results show that there is a strong correlation between electrical resistivity and rock mass classification.

Abstract: This study proposes a tomography-based method for evaluating grouting performance after injection. Tomography is a convenient approach for solving the boundary measurement inverse problem of capturing discrete pixels and synthesizing these pixels into a unified image. Four arrays of eight electrodes are installed into large triaxial cell specimens to simulate in-situ crosshole resistivity testing. Sand is used as a base material and a wet cement mixture is grouted into the specimens. Electromagnetic waves are used as a means of accumulating the physical properties of the specimens. Each measured electrical resistivity is considered as a discrete signal. The electrical resistivity distribution is calculated and optimized through an iterative modified least-squares inversion based on a forward solution of Coulomb and Gauss’s law equations. Results show that the electrical properties of an injected grout material and its location and size can be effectively estimated from a series of resistance measurements.

Abstract: The purposes of this study are to analyze post liquefaction shear strength and to explore the potential use of wave-based techniques to monitor liquefaction and post liquefaction response. The first part presents a detailed analysis of triaxial test results to identify robust strength criteria. The second part documents experimental data on the characterization of liquefaction events with P-wave reflection imaging and S-wave trans-illumination techniques. The relevance of multiple coexisting temporal and spatial scales is highlighted. The following results are obtained: 1) the post liquefaction shear strength can be estimated within the framework of critical state soil mechanic; 2) the P-wave reflection images obtained before and after liquefaction represent the depression of the soil-water interface; 3) excess pore pressure migration from liquefied deep layers may cause zero-effective stress in dilative shallow layers. P-wave reflection is a valuable tool to monitor the evolution of subsurface structures and S-wave trans-illumination technique can be used to yield a comprehensive picture of the spatial evolution of liquefaction.

Abstract: An electrical needle-size probe is developed to effectively assess one-dimensional spatial variability of laboratory soil specimens in high resolution. A calibration procedure is also presented to determine resistance from the measured complex impedance. The capability of the developed electrical needle probes to resolve interfaces and spatial variability is explored using sand specimens prepared by various conditions. The complex impedance is measured 0.2~0.5 mm for every specimen. Results show that the coefficient of variation increases as the size of the probe reaches the size of the particle while a very large ratio of probe size to grain size would decrease the detectability of local soil variations due to averaging effects and smoothening. The attainable spatial resolution depends on the needle diameter: submillimetric resolution is typically achieved in laboratory applications and it can be scaled for field applications. The local electrical parameters permit one to infer the soil porosity and the electrolyte conductivity.

Abstract: The behavior of a jointed rock is different from that of an intact rock, and the characteristics of elastic wave propagation in a jointed rock are different from those of an intact rock. In this study, a rock resonant column testing device is designed to measure the longitudinal and flexural wave velocities of jointed rocks under different states of stress. A column of more than 12 rock discs is stacked on a steel base, which acts as a free-fixed system. This configuration ensures that waves propagate under an equivalent continuum condition, thereby rendering a constant and unique velocity. The effect of joint conditions on the wave velocities is investigated through rock resonant column testings. The results show that velocities are sensitive to the state of stress and increase nonlinearly with stress. The velocities are also affected by joint conditions such as roughness, spacing, and filling. The results are useful for rock mass classification based on near-surface geophysical characterization.