• Journal of the Korean Geotechnical Society
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• v.33 no.5
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• pp.5-13
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• 2017

The sand layer deposited in clay is called a sand seam, which is formed by inflow of sands due to river flooding or slope failure in the middle of sinking and sediment of clay. When the sand seam exists in clay layer, the drainage direction changes from one way to both ways, and the time of consolidation may be reduced. However, it is not clearly proved due to lack of studies of sand seam and currently is not reflected in the design of soft soil improvement. As a fundamental study about sand seam, the oedometer tests and constant rate of strain tests with Rowe cell were conducted on clay specimens with sand seam. For tests, a frozen method was specially designed for making the sand seam. It was concluded that the test results showed the sand seam affects the coefficient of consolidation of clay. If the thickness of sand seam exceeds 0.05 times of specimen height, the sand seam works as drainage layer of pore water horizontally as well as vertically, and consequently the consolidation is accelerated.

• Journal of the Korean Geotechnical Society
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• v.35 no.12
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• pp.59-67
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• 2019

With the increasing use of geotextile mats in dredging and reclaiming work and coastal construction, the assessment of bearing capacity in soft ground has become an important evaluation index for negligent accidents. The review of the allowable bearing capacity of soft ground consisting of inhomogeneous layers by laying geotextile mats and sand mat layers for soft ground improvement is generally compared with the equation of Meyerhof (1974) and Yamanouchi (1985). Mayerhof formula results in economic loss due to underestimation of bearing capacity, and Yamanouchi (1985) formula does not take into account negligent accidents for punching shear failure, so rather high bearing capacity is evaluated. It is considered that economic feasibility and stability will be ensured by proposing a modified formula to calculate the appropriate bearing capacity by applying the seam tensile strength of the geotextile mat to the design standard of soft ground improvement.

• Journal of the Korean Geotechnical Society
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• v.17 no.2
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• pp.41-49
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• 2001

본 연구에서는 2단계 확정예정부지의 일부 원지반에서 수행된 CPTu(piezocone penetration test)와 DMT(dilatometer) 및 실내시험 결과를 바탕으로 흙의 분류, 비배수전단강도 그리고 압밀계수 등의 공학적 특성을 살펴보았다. CPTu와 DMT를 이용한 흙의 분류 결과, 점토층 사이에 얇게 산재한 샌드심(sand seam)층을 보다 정확하게 판정할 수 있었다. 삼축압축시험의 비배수 강도($S_{u}$ )를 기준으로 산정한 콘계수는 CPTu의 경우 $N_{k}$ =18.2를, DMT의 경우 Roque(1988)의 제안식을 이용한 $N_{c}$=6.35로 추정한 비배수전단강도가 비교적 일치하는 것으로 나타났다. 또한 CPTu와 DMT를 이용한 수평압밀계수는 비교적 근사한 것을 알 수 있었다. 그러나, 해성점토사이에 실트, 샌드심이 존재하는 실트질 지반에서의 수평압밀계수가 연직압밀계수보다 상당히 크며, 압밀계수비($C_{h(Oedo, CPTu, DMT)}$ /$C_{v Oedo}$ )는 4.3~10.2로 큰 차이를 보이고 있다.

• Geomechanics and Engineering
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• v.36 no.5
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• pp.427-440
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• 2024

This study concentrates on the 301 comprehensive caving working face, notable for its considerable mining height. The roof model is established by integrating prior geological data and the latest borehole rock stratum's physical and mechanical parameters. This comprehensive approach enables the determination of lithology, thickness, and mechanical properties of the roof within 50 m of the primary mining coal seam. Utilizing the transfer rock beam theory and incorporating mining pressure monitoring data, the study delves into the geometric parameters of the direct roof, basic roof movement, and roof pressure during the initial mining process of the 301 comprehensive caving working face. The direct roof of the mining working face is stratified into upper and lower sections. The lower direct roof consists of 6.0 m thick coarse sandstone, while the upper direct roof comprises 9.2 m coarse sandstone, 2.6 m sandy mudstone, and 2.8 m medium sandstone. The basic roof stratum, totaling 22.1 m in thickness, includes layers such as silty sand, medium sandstone, sandy mudstone, and coal. The first pressure step of the basic roof is 61.6 m, with theoretical research indicating a maximum roof pressure of 1.62 MPa during periodic pressure. Extensive simulations and analyses of roof subsidence and advanced abutment pressure under varying working face lengths. Optimal roof control effect is observed when the mining face length falls within the range of 140 m-155 m. This study holds significance as it optimizes the working face length in thick coal seams, enhancing safety and efficiency in coal mining operations.

• Journal of the Korean Geotechnical Society
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• v.16 no.2
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• pp.115-123
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• 2000

Piezocone Penetration Test has been performed in the soft ground over the site preparation area at Inchon International Airport(IIA). With the pore pressure dissipation test results, the changes in the permeability and the coefficient of consolidation in clayey soil were checked and the differences of the undrained shear stength verified the soft ground improvement effect from vane test and piezocone test both before and after the improvement. From the results, thin sand seam was found and this caused a big difference in the coefficient of permeability and consolidation. The coefficient of consolidation was high in the upper marine deposit and relatively low in lower marine deposit. It was found that the reduction of void ratio by preloading resulted in the reduction of coefficient of consolidation after the ground improvement. In addition, there were some variations of undrained shear strength when the number of 15 or 18 was used as the coefficient of piezocone(Nkt). However, when the average value of undrained shear strength calculated using Nkt=10 was applied, the result indicated the similar average value with the result of vane test and the increasing rate of strength( Δsu/Δ$\sigma$≒0.38) also showed the similar distribution.

• Magazine of the Korean Society of Agricultural Engineers
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• v.39 no.1
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• pp.102-111
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• 1997

Experiments both in laboratory and field were performed to compare and analyze the characteristics of alluvial clay. The alluvial clay was sampled in test site in which large-scaled tests for the part of the site are under process to suggest the rational method for alluvial clay and the criterion for ground settlement monitoring system. The followings were observed through the experiments : 1. Natural water content, plastic limit, and liquid limit of alluvial clay composed of highly fine grains were 40~80%, 10~20%, and 30~55%, respectively. The values of these properties were relatively small at the ground surface, while the values showed maximum at G.L.- l0m and gradually decreased below the level. 2. Shear strength of alluvial clay was proportionally increased to the depth. Unconfined and triaxial compressive strengths were 0.2~0.6kgf/$cm^2$ and 0.1~0.3kgf/$cm^2$, respectively. 3. Compression index and secondary compression index showed maximum values at G.L.-l0m and gradually decreased below the level. The value of consolidation coefficient was relatively large at the ground surface, constant with decreasing the depth, and incresed when G.L. was below -20m. 4. Piezocone test appeared that alluvial clay with N value of 2~4 was uniformly distributed with 20~ 30m thickness from the ground surface, sand seam was nonuniformly distributed, and penetration pore pressure was 0.8 ~ 1 times of the hydrostatic pressure. Undrained shear strength and consolidation coefficient were 0.04 ~ 0.76kgf / $cm^2$ and $2.88{\times} 10{^-4}~1.3{\times} 10{^-2} cm^2/s$ respectively.

• Geomechanics and Engineering
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• v.28 no.6
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• pp.613-624
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• 2022

A study of the evolution of overburden fractures under the solid-fluid coupling state was conducted based on the geological and mining characteristics of the coal seam depth, weak strata cementation, and high-intensity mining in the mining areas of West China. These mining characteristics are key to achieving water conservation during mining or establishing groundwater reservoirs in coal mines. Based on the engineering background of the Daliuta Coal Mine, a non-hydrophilic simulation material suitable for simulating the weakly cemented rock masses in this area was developed, and a physical simulation test was carried out using a water-sand gushing test system. The study explored the spatial distribution and dynamic evolution of the fractured zone in the mining overburden under the coupling of stress and seepage. The experimental results show that the mining overburden can be vertically divided into the overall migration zone, the fracture extension zone and the collapse zone; additionally, in the horizontal direction, the mining overburden can be divided into the primary fracture zone, periodic fracture zone, and stop-fracture zone. The scope of groundwater flow in the overburden gradually expands with the mining of coal seams. When a stable water inrush channel is formed, other areas no longer generate new channels, and the unstable water inrush channels gradually close. Finally, the primary fracture area becomes the main water inrush channel for coal mines. The numerical simulation results indicate that the overlying rock breaking above the middle of the mined-out area allows the formation of the water-conducting channel. The water body will flow into the fracture extension zone with the shortest path, resulting in the occurrence of water bursting accidents in the mining face. The experimental research results provide a theoretical basis for the implementation of water conservation mining or the establishment of groundwater reservoirs in western mining areas, and this theoretical basis has considerable application and promotion value.

• Journal of the Korean Geotechnical Society
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• v.26 no.8
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• pp.15-25
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• 2010

The thin-layered sand seam in clay affects the soil behavior. Although the standard cone penetrometer (A: $10cm^2$) have been used to evaluate the thin-layered soil, the smaller diameter cone penetrometer have been commonly recommended because of the high resolution. The purpose of this study is the development and application of the Cone Resistivity Penetrometer (CRP), which detects qc, fs, and electrical resistivity at cone tip for the evaluation of thin layered soils. Two sizes of the CRP are developed for the laboratory and field test. The projected areas of CRP for the laboratory and field tests are $0.78cm^2$ (d: 1.0 cm) and $1.76cm^2$ (d: 1.5 cm), repectively. The length of friction sleeve is designed in consideration of ratio of the projected area to the friction sleeve area. The application tests are carried out by using the artificially prepared thin-layered soils in the laboratory. In addition, the field tests are conducted at the depth of 6 to 15 m in Kwangyang. In the laboratory test, the measured electrical resistivity and cone tip resistance detect the soil layers. Moreover, in the field test the CRP investigates the three thin-layered soils. This study suggests that the CRP may be a useful tool for detecting thin-layered in soft soils.