• Journal of the Korean Geosynthetics Society
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• v.17 no.3
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• pp.9-18
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• 2018

The soil parameters important for the design of the soft ground are the compression index ($C_c$), the consolidation settlement and consolidation speed at the field. Compression index is obtained by laboratory consolidation test. In the laboratory consolidation test, sample disturbance always occurs. In order to correct the disturbance phenomena, the method of calculating the compression index proposed by Schmertmann (1955) is generally used. However, recent developments in sampling technology and Korean soil conditions are different from those proposed by Schmertmann. So it needs to be verified. In this study, each consolidation curve's cross void ratio is evaluated by doing consolidation test varying disturbance on high-plastic clay (CH), low-plastic clay (CL) and low-plastic silt (ML). The test results were $0.521e_0$ for low-plastic silt, $0.404e_0$ for low-plastic clay, and $0.458e_0$ for the high-plastic clay. This results were different from those of Schmertmann's suggested value of $0.42e_0$. Therefor we proposed a correction formula using the plastic index according to soil type. However, since the results of this study are limited test results, further studies on various korean soil are needed to suggest the compression index correction method according to the degree of plasticity index of soil.

• Journal of the Korean Geotechnical Society
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• v.18 no.3
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• pp.73-85
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• 2002

This paper is focused on studying the undrained cyclic triaxial behavior of saturated Nak-dong River sand, using anisotropically consolidated specimens. A test of isotropically consolidated specimens was performed to compare the results of the anisotropically consolidated specimens. The cyclic shear stre3ngth of the sand under various combinations of initial static shear stress and relative density was considered. Failure was defined as a 5% double amplitude cyclic strain and a 5% residual axial strain for both reversal stress and no reversal stress conditions. Using this definition, the cyclic strength of the anisotropically consolidated specimens was affected by the initial static shear stress. For anisotropically consolidated Nak-dong River dense sand, the cyclic strength is greater than that of Toyolura silica sand but is smaller than that of Dogs Bay carbonate sand. By comparing the experimental and predictecl results, it was possible to predict the residual pore pressure of Nak-dong River sand using Hyodo's model with initial static shear stress subjected cyclic loading.

• Journal of the Korean Geotechnical Society
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• v.15 no.4
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• pp.85-97
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• 1999

For a deltaic clay in the mouth of the Nakdong river, OCR was investigated through methods using the results of field measurement, laboratory and field soil tests. As a result, OCRs were obtained around the range of 0.95 to 1.20 by analysis of field measurements, although they were estimated around the values of 0.4 to 0.7 by the results of conventional consolidation tests for the clay. From the dissipation test it was found out that the excess pore pressures scarcely existed in the clay deposit and then the soil was not in the underconsolidated condition. And the OCRs obtained through methods of Mayne(1991) and Cao et al(1996) using the piezocone test and of Mayne & Kemper(1988) using the cone penetration test were in good agreement with those of field measurement.

• Geotechnical Engineering
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• v.5 no.4
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• pp.47-60
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• 1989

Soft offshore sediments quite frequently contain undissolved gas, probably methane pro- duced biogenically. The presence of gas bubbles can have a significant effect on the engineering behaviours of the seabed. One of the main difference between saturated and gassy soils is that the undrained response is not incompressible, and without volume change, may be assumed for a fully saturated soil. This paper describes the basic experimental work to further understanding of a gassy soil. The test has been performed for a gassy soil under undrained and drained conditions. It was confirmed that the gas inclusions deformed due to changes in the total stress on the sample and also the pore gas pressure response to change in total stress. but not directly to those in pore water pressure. And the test which applied the repeated load under undrained state also showed the similar behaviour as the simple load.

• Journal of the Korean Geotechnical Society
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• v.24 no.1
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• pp.51-59
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• 2008

In this study, cyclic shear characterics of sand-clay mixtures were analyzed. In order to perform cyclic triaxial tests on sand clay mixtures, natural clays with activity and silica sand were mixed variously to reproduce soils with wide range of grain size compositions. Test specimens with various fines contents were prepared by the moisture compaction and pre-consolidation methods, while paying attention to the void ratio expressed in terms of the sand structure and clay structures, and undrained cyclic shear tests were performed. In the test results, cyclic shear strength decreased with increasing of sand granular void ratio below 20% of fine contents. When the granular void ratio of the test specimen exceeded the maximum void ratio of the silica sand, the clay matrix dominated the soil structure, and soil structures were not influenced by compaction energy. It was observed that, the matrix structure of the coarse particles has great effect on the undrained cyclic shear strength characteristics for sand-clay mixtures, and therefore, it is more appropriate to pay more attention to the density of the sand structure, rather than to the fines content.

• Journal of the Korean Geotechnical Society
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• v.18 no.6
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• pp.73-81
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• 2002

Various CPT-based prediction models far profiling stress history of marine clay at the southern part of the Korean peninsula were investigated by using both statistical analysis and case history study. Preconsolidation pressures($\sigma'$p) and overconsolidation ratio(OCR) estimated by empirical correlations and cone penetration tests were compared with those of laboratory odometer test results. Stress history of marine clay determined by odometer test results was in general overconsolidated at below 10m depth from the mudline, whereas marine clay at below l0m depth from the mudline which has an around 0.3 overconsolidation ratio showed variable stresses and unstable states. Preconsolidation pressures were computed by both empirical methods of the Chen and Mayne(1996) and theoretical method of Konrad and Law(1987). It is estimated that Chen and Mayne(1996)'s prediction method based on pore water pressure is more reliable than any other prediction methods, and their method proved to be the most reliable for overconsolidation ratio estimation. However, it is recommended that Mayne & Holtz(1988) and Mayne & Bachus(1988) methods are more suitable than any other methods for predicting the overconsolidation ratio at an underconsolidated (OCR<1) clay. For these reasons, rather than making use of existing prediction models, development of site specific empirical correlations which considers local characteristics and site conditions may be required due to different local stress history and variable soil properties.

• Geotechnical Engineering
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• v.12 no.5
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• pp.17-26
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• 1996

The undrained behavior of Asan marine soil was investigated by using an automated triaxial testing device. The stress-strain behavior at the preand postfailure state of marine soil under undrained compression and eatension conditions was compared with the behavior of pure silt, pure clay and the overall behavior of Asan marine soil was predicted with the modified Camflay model and the bounding surface model. The marine soil sampled in Asan bay area was clayey silts with 70oA silt-30% clay content and the testing samples were prepared in both undisturbed and remolded conditions. All samples are normally consolidated with 400 kPa of effective mean confining pressure and each sample is unloaded to 200, 100, 67 kPa, respectively. And then the shear test was performed with different confining pressure. According to experimental results, there exists an unique failure line whose slope is lower than silt's and higher than clay's. It is identified that the undrained shear strength of normally consolidated samples increases after crossing the phase transformation line because of volume dilation tendency which is not seen in clay. Overconsolidated samples show different soil behavior compared with pure silt due to its tendency of change in volume. It is also found that the overall behavior of Asan marine soil cannot be predicted precisely with the modified Cam-clay model and the bounding surface model.

• Journal of the Korean Geotechnical Society
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• v.24 no.5
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• pp.107-115
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• 2008

This paper is performed to examine the effect of creep during the primary consolidation and the applicability of the Yin's EVP (Elasto-Visco-Plastic) model. In ordinary consolidation theories using the elastic model, the primary consolidation process can be expressed but the secondary consolidation process cannot. It is due to the viscosity, which can express the secondary consolidation, and is sometimes related to the scale effect (difference of the thickness of clay layer between laboratory sample and field condition) such as hypotheses Type A and Type B shown by Ladd et al. (1977). Usually, the existence of the creep during the primary consolidation has been conformed and the Type B is well acceped. On the other hand, from the large-scaled consolidation tests the intermediate characteristic between Type A and Type B was proposed as Type C by Aboshi (1973). In this study, to clarify the effect of creep on the settlement-time relation during the primary consolidation in detail, Type B consolidation tests were performed using the separate-type consolidation test apparatus for a peat and clay. Then the test results were analyzed by using Yin's EVP Model (Yin and Graham, 1994). In conclusion, followings were obtained. At the end of primary consolidation, the compression for the subspecimens should not be the same because of the difference of the excess pore water pressure dissipation rate. And the average settlement measured by the separate-type consolidometer coincides with the analyzed one using the Yin's EVP model. As for the dissipation of the excess pore water pressure, however, the measured excess pore water pressure dissipates faster compared with the Yin's model.

• Journal of the Korean Geotechnical Society
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• v.18 no.4
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• pp.127-135
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• 2002

The accurate evaluation of settlement is important to every steps in the constructions involving soft soils. Relating with those constructions, especially, it has been emphasized recently that the influnce of secondary settlement is important. The ratio of $C_a/C_c$ and the surcharging tests can be applied collaboratively to predicting and reducing secondary consolidation. A series of incremental loading consolidation test and surcharging tests for undisturbed samples of Kunsan clay were performed in this study. As a result of the tests, the ratio of $C_a/C_c$ for the clay was found to be 0.0329. Also, the relationship between void ratio and $t/t_p$ was shown to be linear. Accordingly, the secondary compression index, $C_a for a long term loading had a constant value regardless of time. When the total surcharge ratio was 0.4 and the dissipation ratio of excessive porewater pressure was in the range from 80% to 100%, secondary settlement was effectively reduced for Kunsan clay.

• Journal of the Korean GEO-environmental Society
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• v.9 no.1
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• pp.27-31
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• 2008

In this study, evaluation of samples' reflection of in-situ condition was performed for those by 225 mm KICT large sampler and NX size piston sampler. Disturbance analysis was conducted for the clay sampled by two kinds of sampler throughout consolidation tests, uniaxial tests and triaxial test, under the same condition. From the analysis it can be seen that not only the initial void ratio and preconsolidation pressure from KICT large sampler but strength parameters from both uniaxial and triaxial compression tests are also about 10% higher than those of NX size piston sampler's.