Wanjei Cho

Stress-Strain Responses of Block Samples of Compressible Chicago Glacial Clays

This paper presents the results and analysis of a laboratory investigation of the behavior of lightly overconsolidated compressible Chicago glacial clays over a wide strain range. Each specimen was trimmed from high quality block samples taken from an excavation in Evanston, Illinois. Specimens were instrumented with three sets of bender elements and local LVDTs. After K0 consolidation to the in situ vertical effective stress of the block, drained stress probe tests were conducted. Results of bender elements tests obtained prior to stress probing show that compressible Chicago glacial clay initially is cross anisotropic. Propagation velocities measured by bender elements in axial direction after K0 reconsolidation and drained creep agrees well with the in situ shear wave velocity measured by seismic cone penetration tests. Results of drained stress probe tests are analyzed in terms of shear, volumetric and coupled stiffness, stiffness degradation, and direction of loading. The significant variability of s...

Effects of swelling during saturation in triaxial tests in clays

Effects of swelling during saturation in triaxial tests on compressible Chicago glacial clays are assessed from results of triaxial compression tests on block samples cut from excavations, thin-wall Shelby tube samples, and reconstituted specimens. The results are presented in terms of the stress-strain response during saturation, k 0 reconsolidation and shearing. Bender element tests were also performed to investigate the effects of swelling on the shear wave velocities during k 0 reconsolidation. Results showed that the swelling during saturation lowered the shear wave velocity, thereby inferring a change in the original structure of natural clay. Responses at strains less than 0.01 % were most affected by the saturation-induced swelling. To minimize these changes, it is recommended that the measured residual stress is applied prior to saturating the soil. Based on these results, the saturation stage should be considered as much a part of a triaxial test as consolidation and shearing.

Patterns of Nonlinear Shear Stiffness Degradation of Reconstituted Clay with Different Stress Histories

This paper describes patterns of nonlinear shear stiffness degradation with respect to the stress history of clay. An experimental study using undrained triaxial compression tests was conducted on specimens cut from reconstituted clay samples of kaolinite. The nonlinear pattern of stiffness degradation was analyzed within the frameworks of both the conventional overconsolidation ratio (OCR) and the stress path rotation angle. The experimental data were subsequently interpreted based on the concept of the kinematic sub-yield surface. The pattern of stiffness degradation is more relevant to the rotation angle of the current stress path than the OCR value. The sizes of the sub-yield surfaces are variable. Results show that the kinematic movements of sub-yield surfaces within the overall bounding surface may provide an insufficient tool to fully describe the pattern of stiffness degradation.

Effect of Pre-Shear Stress Path on Nonlinear Shear Stiffness Degradation of Cohesive Soils

The nonlinear degradation of soil stiffness from very small to small strain is a key consideration for reliable prediction of ground behavior and its interactions with structures under dynamic excitation and working load conditions. Despite high sensitiveness of stiffness measurement to testing conditions, the effect of the pre-shear stress path on the stiffness degradation has not been properly discussed. Here the authors investigate the effect of pre-shear stress path on nonlinear shear stiffness degradation of cohesive soils. Reconstituted kaolinite specimens were consolidated to be the overconsolidation ratio (OCR) = 1, 2, and 4 along Ko and isotropic stress paths. The shear stiffness degradations of the specimens during undrained shear were measured using on-specimen linear variable differential transformers. Experimental results show that the pre-stress stress path has a strong influence on the degree of shear stiffness degradation at different OCRs. This influence is interpreted within the context of the rotation angle of shear stress path, which provides a good qualitative explanation of the inconsistent observations in the literature.

Gmax of Reclaimed Ground on the Western Coast of Korea Using Various Field and Laboratory Measurements

The maximum shear modulus of soil is a principal parameter for the design of earth structures under static and dynamic loads. In this study, the statistical data of the maximum shear moduli of reclaimed ground in the Songdo area on the western coast of Korea were evaluated using various field and laboratory tests, including the standard penetration test (SPT), piezocone penetration test (CPTu), self-boring pressure meter test (SBPT), down-hole seismic test (DHT), seismic piezocone penetration test (SCPTu) and resonant column test (RCT). Soils were classified variously by using a conventional unified soil classification system and classification charts for CPTu data. For the soils containing mostly sand and silt, the soil classifications using the classification charts for CPTu data show good agreement with the unified soil classification. Based on the statistical analysis on various maximum shear moduli, new site-specific empirical correlations between the shear moduli and SPT and CPTu values were proposed. Predictions of the maximum shear moduli using the proposed correlations were compared with the data obtained from the DHT, which is comparatively exact in evaluating the maximum stiffness of soils. The good agreement confirmed that the proposed correlations reasonably predicted the maximum shear moduli of soils in western coastal area of Korea.

The Variation of Physical Properties in Frozen Soils at Various Freezing Temperatures

Due to the recent climate change, the colder and longer winter is expected in Korea. Besides, the recent participation agreement on the development of the natural gas pipeline in Russia and construction of the second Korean Antarctic research station, the Jangbogo station changes the research interests from the seasonally frozen ground to the permafrost ground. The recent development of the site investigation techniques using wave propagation and electrical resistivity enabled engineers to evaluate the physical properties of the frozen soils and further correlate them to the mechanical properties. However, the physical properties of the frozen ground change when the water in the soil solidifies to ice; this change is particularly notable between 0 and −10°C. Therefore, the physical property changes due to freezing needs to be investigated in terms of wave propagation characteristics and electrical resistivity with regard to the various freezing temperatures. In this study, the characteristics of wave propagation and electrical resistivity of frozen soils are investigated under various sub-zero temperatures. The characteristics of wave propagations are analyzed in terms of compression and shear wave velocities. The electrical resistivity is measured under various sub-zero temperatures to understand the effects of ice, which will further provide the valuable information about the location of the active layer. Furthermore, the evaluated physical properties can be used as basic data for the evaluation of the mechanical properties such as strength and stiffness.Copyright

Development of Ground Freezing System for Undisturbed Sampling of Granular Soils

The ground freezing technique was first invented for the undisturbed sampling of the granular soils. With increasing necessity of liquefaction evaluation under earthquake loading, there has been more research with high-quality granular samples, with ground freezing techniques in the world. However, there has been little research on the ground freezing techniques since Korea had no records of liquefactions until the Pohang earthquake in 2017. Since more than 10 places were reported with liquefaction phenomena, it is required to assess the liquefaction potential with high-quality samples of granular soils. Therefore, in order to obtain undisturbed samples of granular soils, a new local ground freezing equipment and an operating system were developed in this study. The applied coolant was liquid nitrogen and circulated through a double tube inserted in the ground. To evaluate the performance of the system, laboratory scale tests were performed with water only and saturated fine sands. In the laboratory evaluation, a frozen soil column of 60 cm diameter was made after 20 hours and the average freezing rate was approximately 12 mm/hr in radial direction. After laboratory evaluation, the freezing system was applied in the field and the performance was evaluated with the 2D electrical resistivity tomography. In the field evaluation, the frozen region was 4 m diameter with 6.5 m depth in a cylindrical shape.

Particle Spacing Analysis of Frozen Sand Specimens with Various Fine Contents by Micro X-ray Computed Tomography Scanning

The mechanical characteristics of frozen sand greatly depend on the frozen temperature and the fine contents according to the previous study by Chae et al. (2015). There are two hypotheses to explain this experimental results; one is the unfrozen water contents greatly affected by the fine contents and frozen temperature and the other is the sand particle spacing greatly affected by the pore-ice. To evaluate the latter hypothesis, the micro X-ray CT scan was performed. The micro X-ray CT scanning, one of the actively performed interdisciplinary research area, has a high resolution with micrometer unit allows to investigate internal structure of soils. In this study, X-ray CT technique was applied to investigate the effect of the frozen temperature and fine contents on the sand particle minimum and average spacing with the developed image processing techniques. Based on the spacing analysis, the frozen temperature and fine contents have little influence on the sand particle spacing in the frozen sand specimens.

Stiffness Nonlinearities of SCP-inserted Clay Specimens with Various Replacement Ratios in Triaxial Compression Tests

The sand compaction pile (SCP) method is one of the widely used ground improvement techniques for the embankment constructed on the soft clay. The behavior of SCP-improved ground becomes very complex due to the differences in the stiffness and strength of the SCP and the original soft ground. Since the behavior of the SCP-improved ground greatly affects the performance of the embankment, it is crucial to understand the mechanical behavior of the SCP-improved ground under the field conditions. Thus, in this study, triaxial compression tests after K 0 consolidation are performed on the SCP-inserted clay specimens to characterize the stiffness nonlinearity for the SCP-improved ground. Based on the stress-strain behaviors of SCP-inserted clay specimens with different replacement ratios, the secant Youngs modulus is obtained with strain level. The initial stiffness value and stiffness degradation patterns are greatly affected by the replacement ratio showing similar trends with those of the major component of the SCP-inserted clay specimen.