Gye-Chun Cho

Long-wavelength P-wave and S-wave propagation in jointed rock masses

Field data suggest that stress level and joint condition affect shear-wave propagation in jointed rock masses. However,thestudyoflong-wavelengthpropagationinajointedrock mass is challenging in the laboratory, and limited data are available under controlled test conditions. Long-wavelength P-wave and S-wave propagation normal to joints, using an axially loaded jointed column device, reproduces a range of jointconditions.Theeffectsofthenormalstress,loadinghistory, joint spacing, matched surface topography i.e., joint roughness, joint cementation e.g., after grouting, joint opening, and plasticity of the joint filling on the P-wave and S-wavevelocitiesandonS-waveattenuationarenotable.The ratio VP / VS in jointed rock masses differs from that found in homogeneous continua. The concept of Poisson’s ratio as a functionof VP / VSisunwarranted,and VP / VScanbeinterpreted in terms of jointed characteristics. Analytic models that consider stress-dependent stiffness and frictional loss in joints as well as stress-independent properties of intact rocks can model experimental observations properly and extract joint properties from rock-mass test data. Thus, joint propertiesandnormalstresshaveaprevalentroleinpropagationvelocityandattenuationinjointedrockmasses.

Shear Strength Estimation of Sandy Soils Using Shear Wave Velocity

Typically, shear strength is associated with large strain phenomena, while shear wave propagation is associated with small strain phenomena. Yet, the effective stress and void ratio, both key determinants of sandy soil shear strength, are also the primary factors affecting shear wave velocity. This study presents a shear wave velocity-void ratio-shear strength correlation through experimental tests. Natural sands taken from various reclaimed or recently deposited sandy fields are used for reconstituting specimens at different void ratios in an oedometer cell. Shear wave velocities are measured while changing the state of the stress in the cell for each specimen prepared at a specific void ratio. The relationship between shear wave velocity and vertical effective stress is found at extreme values of void ratios (emin and emax). Direct shear tests are also performed on specimens with various void ratios. Experimental results show that the internal friction angle of each sand type increases with decreasing void ratio, rendering a unique relationship between friction angle and void ratio. Finally, a procedure is suggested to evaluate the in-situ shear strengths of a sandy soil based on in-situ shear wave velocities. Results show that the suggested method effectively estimates in-situ shear strength.

Small-Strain Stiffness, Shear-Wave Velocity, and Soil Compressibility

AbstractThe small-strain shear modulus depends on stress in uncemented soils. In effect, the shear-wave velocity, which is often used to calculate shear stiffness, follows a power equation with the mean effective stress in the polarization plane Vs=α(σm′/1 kPa)β, where the α factor is the velocity at 1 kPa, and the β exponent captures the velocity sensitivity to the state of stress. The small-strain shear stiffness, or velocity, is a constant-fabric measurement at a given state of stress. However, parameters α and β are determined by fitting the power equation to velocity measurements conducted at different effective stress levels, so changes in both contact stiffness and soil fabric are inherently involved. Therefore, the α and β parameters should be linked to soil compressibility CC. Compiled experimental results show that the α factor decreases and the β exponent increases as soil compressibility CC increases, and there is a robust inverse relationship between α and β for all sediments: β≈0.73−0.27⁡log...

Site characterization and geotechnical aspects on geological storage of CO2 in Korea

The long-term storage of carbon dioxide (CO2) in deep geological formations, known as geological CO2 storage (GCS), has the potential to reduce CO2 emissions by 20%, a figure considered necessary to stabilize atmospheric CO2 levels over the next century. The purpose of this paper is to present the current state and future direction of geological CO2 sequestration in Korea. This study reviewed current storage technologies and strategies related to GCS worldwide, and the most suitable basins for GCS in Korea were selected from current available geophysical and geological research results. Finally, scientific questions and technical challenges were discussed in relation to the injection, storage, and monitoring processes from geotechnical engineering perspectives.

A New Alternative for Estimation of Geotechnical Engineering Parameters in Reclaimed Clays by Using Shear Wave Velocity

The consolidation behavior of reclaimed clay can be categorized as large strain deformation. Findings from previous studies indicate that the effective stress and the void ratio are important geotechnical engineering parameters for the characterization of large strain consolidation behavior. However, existing in situ consolidation characterization methods of reclaimed clay cannot adequately estimate changes of the effective stress and void ratio during a consolidation process. This paper suggests an alternative method for estimating the geotechnical engineering parameters of reclaimed clays using a shear wave. An in situ self-weight consolidation process of reclaimed clay is simulated in laboratory while shear wave velocity is continuously measured. Experimental results show that there are single trends in relationships among the shear wave velocity, effective stress, void ratio, and geotechnical engineering parameters for a normally consolidated clay (e.g., reclaimed clay). As a practical application, the in situ parameters and the expected settlement are predicted by incorporating the obtained relationships with the in situ shear wave velocity. The predicted values are in good accordance with the values measured in field. Therefore, the proposed method can be used effectively for geotechnical engineering parameter estimations of reclaimed clay during/after self-weight consolidation with the aid of in situ seismic exploration techniques.

PROPERTIES OF LOW-PH CEMENT GROUT AS A SEALING MATERIAL FOR THE GEOLOGICAL DISPOSAL OF RADIOACTIVE WASTE

The current solution to the problem of using cementitious material for sealing purposes in a final radioactive waste repository is to develop a low-pH cement grout. In this study, the material properties of a low-pH cement grout based on a recipe used at ONKALO are investigated by considering such factors as pH variation, compressive strength, dynamic modulus, and hydraulic conductivity by using silica fume and micro-cement. From the pH measurements of the hardened cement grout, the required pH (＜ pH 11) is obtained after 130 days of curing. Although the engineering properties of the low-pH cement grout used in this study are inferior to those of conventional high-pH cement grout, the utilization of silica fume and micro-cement effectively meets the long-term environmental and durability requirements for cement grout in a radioactive waste repository.

Effect of Soil Mineralogy and Pore-Water Chemistry on the Electrical Resistivity of Saturated Soils

AbstractThe electrical characteristics of soil-water mixtures reflect the soil type, ionic concentration, surface conduction, fluid saturation, porosity, and pore connectivity of the mixtures. Archie’s law commonly is used to analyze the electrical resistivity measurement results of soil-water mixtures. This paper explores the pore-fluid effect on Archie’s law. Experimental tests were performed on sand and clay specimens to measure the variation in their electrical resistivity at different porosities and for different electrical resistivities of the pore fluid. The results demonstrate that the sand specimens (i.e., low specific surface soil) have a unique Archie’s law curve; however, the clay specimens (i.e., high specific surface soil) have an inconsistent Archie’s law curve owing to the surface conduction induced by the double layer of clay particles. In particular, Archie’s law should be applied and analyzed cautiously when high specific surface soils are subjected to pore fluids with high electrical r...

Rock Cutting Depth Model Based on Kinetic Energy of Abrasive Waterjet

Abrasive waterjets are widely used in the fields of civil and mechanical engineering for cutting a great variety of hard materials including rocks, metals, and other materials. Cutting depth is an important index to estimate operating time and cost, but it is very difficult to predict because there are a number of influential variables (e.g., energy, geometry, material, and nozzle system parameters). In this study, the cutting depth is correlated to the maximum kinetic energy expressed in terms of energy (i.e., water pressure, water flow rate, abrasive feed rate, and traverse speed), geometry (i.e., standoff distance), material (i.e., α and β), and nozzle system parameters (i.e., nozzle size, shape, and jet diffusion level). The maximum kinetic energy cutting depth model is verified with experimental test data that are obtained using one type of hard granite specimen for various parameters. The results show a unique curve for a specific rock type in a power function between cutting depth and maximum kinetic energy. The cutting depth model developed here can be very useful for estimating the process time when cutting rock using an abrasive waterjet.

Application of Microbial Biopolymers as an Alternative Construction Binder for Earth Buildings in Underdeveloped Countries

Earth buildings are still a common type of residence for one-third of the world’s population. However, these buildings are not durable or resistant against earthquakes and floods, and this amplifies their potential harm to humans. Earthen construction without soil binders (e.g., cement) is known to result in poor strength and durability performance of earth buildings. Failure to use construction binders is related to the imbalance in binder prices in different countries. In particular, the price of cement in Africa, Middle East, and Southwest Asia countries is extremely high relative to the global trend of consumer goods and accounts for the limited usage of cement in those regions. Moreover, environmental concerns regarding cement usage have recently risen due to high CO2 emissions. Meanwhile, biopolymers have been introduced as an alternative binder for soil strengthening. Previous studies and feasibility attempts in this area show that the mechanical properties (i.e., compressive strength) of biopolymer mixed soil blocks (i.e, both 1% xanthan gum and 1% gellan gum) satisfied the international criteria for binders used in earthen structures. Economic and market analyses have demonstrated that the biopolymer binder has high potential as a self-sufficient local construction binder for earth buildings where the usage of ordinary cement is restricted.

Evaluation of Particulate Materials using Wave-Based Techniques

Elastic and electromagnetic waves provide important information about particulate materials. In order to facilitate the application of wave-based techniques to soil characterization, fundamental soil properties are first reviewed, and then experiments are performed for both elastic and electromagnetic waves. The first application is related to characterization of particulate materials using shear wave, concentrated on the change of effective stress in consolidation process, multi-phase phenomenon with relation to capillarity, and microscale characteristics of grain particles. The second application is relevant to the electromagnetic wave, focused on stratigraphy detection in layered soils, estimation of void ratio and its spatial distribution, and conduction in unsaturated soils. Experimental results suggest that the shear wave measurements allow studying the evolution of effective stress in unsaturated soils as well as in saturated soils while the electromagnetic wave measurements give an insight into conduction process and allow estimating void ratio and its spatial distribution.