• Journal of the Korean Geosynthetics Society
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• v.16 no.4
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• pp.93-101
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• 2017

In this study, a subjective weighting factors were awarded based on some indication of the difficulty of assessing the preconsolidation stress using traditional methods (Casagrande, Onitsuka et al., Silva, Becker et al., Janbu and Karlsrud methods) such as those proposed by Casagrande and Janbu using undisturbed sample obtained from Gwangyang dredged clay with high plasticity located in the southern area of Korean peninsular. These numbers only assess the relative ease of finding preconsolidation stress and say nothing regarding the accuracy of the value. The data were compared with measurements of undrained shear strength using strength incremental ratio, checking where or not the values are in the range of 0.25 to 0.35 (typical values of Korean marine clay) and analyzing standard deviation(degree of variability). The measurements of undrained shear strength were obtained from unconfined compression tests (UCT). When determining preconsolidation stress of Korean marine clay, at first, the work method proposed by Becker et al. and the bilogarithmic method proposed by Onitsuka et al. should be used. In addition, preconsolidation pressure should be estimated using the traditional Casagrande method as a basic of comparison.

• Journal of the Korean Geotechnical Society
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• v.19 no.6
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• pp.169-179
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• 2003

Undrained shear strength of clay deposit is one of the most important properties in the design of geotechnical structures. The use of piezocone test is rapidly growing due to its merit that can measure the in-situ undrained shear strength continuously with less error. The reliability of the shear strength from piezocone test depends upon the cone factor applied. Many researchers have suggested different ranges of values for the factors. This study performs to find out the validity of the suggested values in Korea and their charateristics related to the mechanical properties of clay. Piezocone tests were performed at the site of pilot project of ground improvement at Yangsan-Mulgeum Gyeongnam to investigate the charateristics of piezocone factors. The piezocone fators$(N_{kt}, N_{ke}, N_{\Delta u})$ based on the undrained shear strength from quick triaxial compression test are generally within the suggested range. And there appears considerable relations between undrained shear strength and $(N_{kt}, N_{ke}, N_{\Delta u})$ and between preconsolidation pressure and $(N_{kt}, N_{ke})$, while plasticity index, rigidity index and friction ratio do not show any relations with cone factors. The results also reveal that factor $(N_{\Delta u})$ shows higher reliability than factors $(N_{kt} and N_{ke})$, which show smaller standard deviation, breadth of change and scattering.

• Journal of the Korean Geotechnical Society
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• v.16 no.5
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• pp.93-105
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• 2000

일정변형률압밀사시험의 효율성 및 적용성을 구체화하기 위해 인공시료 또는 한정된 지역에서 채취한 불교란 점성토 시료가 아닌 광범위한 지역에서 채취한 불교란 점성토 시료에 대해 시험을 실시하였다. 시료채취의 대상지역은 우리나라의 서해안에서 동해안에 이르는 13개 지역이며, 29개의 불교란 자연시료를 채취하여 일정변형률압밀시험을 113회 실시하였다. 또한, 비교하기 위해 표준압밀시험도 병행하였다. 시험결과로부터 얻어진 압밀정수에 대한 공학적 특성을 고찰하였고, 회기분석을 통해 통계식을 제안하였다. 두 시험법에 의해 산출된 압밀정수를 비교 분석한 결과 수압비가 20% 이내인 범위에서는 일정변형률압밀시험의 변형률 속도가 압밀시험의 정수에 미치는 영향은 거의 없는 것으로 나타났다. 압축지수 및 압밀계수는 일정면형률시험의 결과 값이 표준압밀시험의 결과 값보다 크게 산출되었으나 일관성을 보였다. 또한, 선행압밀압력에 있어서는 두 시험법에 Cassagrande의 방법을 적용할 경우 서로 일치하는 것으로 나타났다. 본 연구로부터 일정변형률압밀시험을 사용하여 표준압밀시험에 상용하는 상호관계식 개발의 타당성을 확인할 수 있었다.

• The Journal of Engineering Geology
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• v.12 no.4
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• pp.459-469
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• 2002

We have remolded marine clay sample collected along the vertical and horizontal directions and investigated the characteristics of the consolidation constants by SCT and CRSC methods. We have studied also on consolidation chracteristics and application for weathered granite soil using SCT and CRSC methods for undisturbed and disturbed samples. As the result, values of pre-consolidation stress, compression index, excessive pore pressure, pore water pressure ratio of the marine-clay were different due to different test methods(SCT and CRSC) and sampling directions(vortical and horizontal directions). Disturbed and undisturbed samples of the weathered granite soil have showed similar change aspect like marine clay during over-consolidatied and normally consolidatied stages.

• Journal of the Korean GEO-environmental Society
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• v.15 no.5
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• pp.67-74
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• 2014

In this study, a constant rate of strain test (CRS) and oedometer test was performed in order to study the consolidation characteristics to the liquid limit using a re-shaped clay which was preconsolidated at a constant pressure. Consolidation samples were made of kaolinite which was mixed with bentonite of 6 %, 9 %, 12 % and 15 % of it by weight and the test value of liquid limit of samples were appeared in 77 %, 84 %, 88 % and 91 % respectively. And then consolidation samples which were agitated sufficiently adding distilled water 2 times of liquid limit were preconsolidated in the condition of a constant pressure of 0.2 MPa. The oedometer test which is commonly used recently and the consolidation test of constant rate of strain which were applied in 0.001 %/min, 0.004 %/min, 0.01 %/min rate of strain according to ASTM, D4186-82 were performed and the preconsolidation pressures were compared and analyzed at 0.2 MPa preconsolidated pressure. As a result, in the case of low value of liquid limit, preconsolidation pressure was appeared same as 0.2 MPa preconsolidated pressure at the high speed strain rate, and in the case of high value of liquid limit, preconsolidation pressure was appeared same as 0.2 MPa preconsolidated pressure at the low speed strain rate.

• Journal of the Korean Geotechnical Society
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• v.32 no.7
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• pp.25-34
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• 2016

Recently, in Korea, problems related with unstability of slope or sinkhole in urban area due to erosion of compacted granite soil which was used as a backfill or embankment material have been treated as important issues. Small hole might develop inside of backfill area due to erosion of not only weathered granite soil but also clay, silt, fine sand size particles when underground water flows. Once erosion starts in a soil mass, erosion rate increases gradually to cause rapid destruction. In this study, a rotating cylinder test (RCT) was performed to evaluate the hydraulic resistance characteristics of compacted weathered granite soil under various relative densities and preconsolidation pressures. Meanwhile, an image analysis method was introduced to analyze radius of irregularly eroded sample. It was found that image analysis is an effective means of minimizing the error in calculating a critical shear stress and threshold shear stress on the irregularly eroded sample. Furthermore, in general, hydraulic resistance capacity increases with the increase of relative density and preconsolidation pressure.

• Journal of the Korean Geotechnical Society
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• v.17 no.2
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• pp.5-12
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• 2001

고유기질토의 정규압밀과 과입밀에 대한 압밀특성을 연구하기 위하여 전북지역에서 채취한 2종류의 고유기질토 IS와 YJ를 사용하여 일련의 실내 표준압밀시험과 반복압밀시험을 수행하였다. 전북지역 대표적인 고유기질토의 물리적 특성은 유기물함량이 각각 58%와 72%, 자연함수비 487%와 635%, 초기공극비 7.02와 9.72이었다. 표준압밀시험결과 K, $M_{v}$ , $C_{v}$ 는 압밀하중이 증가함에 따라서 선행하중을 경계로 일반 점토보다 크게 감소하는 특성을 보였다. 또한 2단계, 3단계 반복압밀 시험결과 K, $M_{v}$ 는 압밀하중이 증가함에 따라서 완만하게 감소하였으며 표준압밀시험결과보다는 작게 나타났다. 2단계 반복압밀의 경우 $C_{v}$ 는 정규압밀영역에서 표준압밀시험결과보다 약 10-12배정도 크게 나타났다. $C_{c}$ , $C_{a}$ 는 압밀하중이 증가함에 따라서 각각 선행하중의 약 2.5배, 3배 하중을 정점으로 크게 증감하는 고유기질토 고유의 특성을 나타냈다. 고유기질토의 $C_{c}$ , $C_{a}$ 는 일반 점토보다 압밀압력 의존성이 큰 특성을 나타냈으며 최대값도 각각 2.9-3.8, 0.3-0.4로 일반 점토보다 큰 값을 보였다. 2단계, 3단계 반복압밀시험결과 $C_{c}$ , $C_{a}$ 는 압밀하중이 증가함에 따라서 약간 증가하였다. $C_{a}$ 는 OCR이 증가함에 따라서 감소하였으며 특히 OCR이 1-2 범위에서 감소폭이 크게 나타났다. 고유길토의 제압밀계수는 반복회수 및 하중의 크기 등에 따라서 큰 영향을 받으며 또한 과압밀 크기의 대소는 고유기질토의 $C_{a}$ 에 큰 영향을 미치는 것으로 생각되어 이 분야에 대한 더 많은 연구가 필요하다.

• Journal of the Korean Geotechnical Society
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• v.19 no.4
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• pp.311-320
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• 2003

The coefficient of consolidation of clay deposit is one of the most important properties in the design of ground improvement. The in-situ value of $c_h$ is generally estimated by pore pressure dissipation using piezocone. Many researchers have suggested theoretical formula for its estimation. This study attempts to find out the validity of the existing theoretical formula in Korea and to find out the characteristics of $c_h$ related to the mechanical properties of clay. Piezocone tests and laboratory tests were performed at the site of pilot project of ground improvement at Yangsan-Mulgeum, Gyeongnam. Comparison of the estimated values of $c_h$using piezocone tests results and those from laboratory consolidation tests are carried out. Results show that Torstensson(cylindrical cavity theory) and Teh & Houlsby solutions derive similar values of $c_h$. And $c_h$ from oedometer test shows values similar to the above two methods. The value from either of the above two methods[Torstensson(cylindrical) or Teh & Houlsby] is recommended to be used as $c_h$.

• Composites Research
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• v.23 no.6
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• pp.61-66
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• 2010

In this paper, the variation of mechanical properties of T800 carbon/epoxy composites according to the forming pressure, which was referred to previous studies on a filament winding process, were investigated. The specimens of all the tests were fabricated by an autoclave de-gassing molding process controlling forming pressure (absolute pressures of 0.1MPa, 0.3MPa, 0.7MPa including vacuum) and water jet cutting after fabricating composite laminates. Various tensile tests were performed for in-plane properties and interlaminar properties were also measured by using Iosipescu test jig. Fiber volume fraction was measured to correlate the property variation and the forming pressure. This properties are expected to be utilized in the design of Type III pressure vessel for hydrogen vehicles which uses the same carbon fiber (T800 carbon fiber) for the filament winding process.

• Journal of the Korean Geotechnical Society
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• v.25 no.6
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• pp.17-29
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• 2009

There has been an increase in the investigation of deep sea sediments with a consequent increase in the amount of energy required to undertake these investigations. The geotechnical characteristics of Ulleung Basin sediment are explored by using depressurized specimens following methane production tests carried out on pressured core samples obtained at 2,100 m water depth and 110 m below sea floor. Geotechnical index tests, X-ray diffraction, and scanning electron microscope are conducted to identify the geotechnical index parameters, clay mineralogy, chemical composition, and microstructure of the sediments. Compressibility, and elastic and electromagnetic wave parameters are investigated for two samples by using a multi sensing instrumented oedometer cell. The strength chatracteristics are obtained by the direct shear tests. The dominant clay minerals are mostly kaolinite, illite, chlorite, and calcite. The SEM shows a well-developed flocculated structure of the microfossil. Void ratio, electrical resistivity, real permittivity, conductivity, and shear wave velocity show bi-linear behavior with the effective vertical stress: as the vertical effective stress increases. The friction angle obtained by the direct shear test is about $21^{\circ}$, which is similar to the value observed in the Ulleung Basin sediments. This study shows that the understanding of the behavior acting on the diatomaceous marine sediment is important because it often maintains the useful energy resources such as gas hydrate and so will be the new engineering field in the next generation.