• Journal of the Korean Geotechnical Society
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• v.25 no.12
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• pp.5-12
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• 2009

As rock bolts become one of the main support systems in tunnels and underground structures, the integrity of the rock bolts affects the safety of these structures. The purpose of this study is the evaluation of rock bolt integrity using wavelet transforms of the guided ultrasonic waves by using transmission test in the field. After several rock bolts with various defect ratios are embedded into a large scale concrete block and rock mass, guided waves are generated by a piezo disk element and measured by an acoustic emission (AE) sensor. The captured signals are analyzed in the time-frequency domain using the wavelet transform based on a Gabor wavelet. Peak values in the time-frequency domain represent the interval of travel time of each echo. The energy velocities of the guided waves increase with an increase in the defect ratio. The suitable curing time for the evergy velocity analysis is proposed by the laboratory test, and in-situ tests are performed in two tunnelling sites to verify the applicability of rock bolt integrity tests performed after proposed curing time. This study proves that time-frequency domain analysis is an effective tool for the evaluation of the rock bolt integrity.

• Journal of the Korean Geotechnical Society
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• v.26 no.7
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• pp.93-106
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• 2010

In performing a series of in-situ thermal response tests, the effective thermal conductivities of six vertical closed-loop ground heat exchangers were experimentally evaluated and compared one another, which were constructed in a test bed in Wonju. To compare thermal efficiency of the ground heat exchangers in field, the six boreholes were constructed with different construction conditions: grouting materials (cement vs. bentonite), different additives (silica sand vs. graphite) and the shape of pipe-sections (general U-loop type vs. 3 pipe-type). From the test results, it can be concluded that cement grouting has a higher effective thermal conductivity than bentonite grouting, and the efficiency of graphite better performs than silica sand as a thermally-enhancing addictive. In addition, a new 3 pipe-type heat exchanger provides less thermal interference between the inlet and outlet pipe than the conventional U-loop type heat exchanger, which results in superior thermal performance. Based on the results from the in-situ thermal response tests, a series of economic analyses have been made to show the applicability of the new addictives and 3 pipe-type heat exchanger.

• Journal of the Korean Geotechnical Society
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• v.23 no.5
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• pp.77-88
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• 2007

Lateral earth pressure and horizontal displacement of the diaphragm walls constructed in multi-soil layers were analyzed by the field instrumentation from six building construction sites in urban area. The distribution of the developed earth pressure of the anchored diaphragm walls during excavation shows approximately a trapezoid diagram. The maximum earth pressure of anchored diaphragm walls corresponds to $0.45{\gamma}H$ and the earth pressure acts at the upper part of the walls. The maximum earth pressure is two times larger than the empirical earth pressure of flexible walls in sands suggested by Terzaghi and Peck(1967), Tschebotarioff(1973), and Hong and Yun(1995a). The horizontal displacement of diaphragm walls is closely related with supporting systems such as struts, anchors, and so on. The horizontal displacement of anchored walls shows less than 0.1 percent of the excavated depth, and the horizontal displacement of strutted walls shows less than 0.25 percent of the excavated depth. Therefore, the restraining effect of horizontal displacement to the anchored diaphragm walls is larger than the strutted diaphragm walls. In addition, since the horizontal displacement of the diaphragm walls is lower than the criterion, $\delta=0.25%H$, used for control the anchored retention wall using soilder piles, the safety of excavation sites applied with the diaphragm walls is pretty excellent.

• Journal of the Korean Geotechnical Society
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• v.23 no.7
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• pp.87-98
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• 2007

A series of dilatometer test, field vane test, and $CK_0U$ triaxial test were performed for clayey soils of Busan new port site to develop the relationships between undrained shear strength and the DMT results. Normalized undrained shear strength is turned out to be $S_{u(CKU)}/{\sigma}'_v=0.30{\sim}0.35\;for\;CK_0U$ triaxial test and ${\mu}S_{u(VST)}/{\sigma}'_v=0.20{\sim}0.22$ for vane shear test. By comparing the undrained shear strength estimated from DMT indices with the results measured by in-situ vane test or $CK_0U$ triaxial test, two methods to predict the undrained shear strength from DMT results are suggested. One is based on the relationship between $S_u/{\sigma}'_v$ and horizontal stress index (KD) while another method comes from $N_c-I_D$ and $N_c-E_D$ correlation. It was observed that the method based on $N_c-I_D\;or\;N_c-E_D$ relation shows slightly better accuracy than the one based on $K_D$ although all of the methods suggested in this study provided comparable values of predicted undrained shear strength. Since the definitions of $I_D\;and\;E_D$ contain $p_1-p_0$, in which soil condition is reflected, it is believed that the prediction method using $N_c$ is capable of taking a material type into consideration.

• The Mathematical Education
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• v.61 no.1
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• pp.1-28
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• 2022

The purpose of this study is to explore how students' fractional knowledge is related to their solving of proportion problems. To this end, 28 clinical interviews with four middle-grade students, each lasting about 30~50 minutes, were carried out from May 2021 to August 2021. The present study focuses on two 7th grade students who exhibited their ability to coordinate three levels of units prior to solving whole number problems. Although the students showed interiorization of three levels of units in solving whole number problems, how they coordinated three levels of units were different in solving proportion problems depending on whether the problems required reasoning with whole numbers or fractions. The students could coordinate three levels of units prior to solving the problems involving whole numbers, they coordinated three levels of units in activity for the problems involving fractions. In particular, the ways the two students employed partitioning operations and how they coordinated quantitative unit structures were different in solving proportion problems involving improper fractions. The study contributes to the field by adding empirical data corroborating the hypotheses that students' ability to transform one three levels of units structure into another one may not only be related to their interiorization of recursive partitioning operations, but it is an important foundation for their construction of splitting operations for composite units.

• Asia-pacific Journal of Multimedia Services Convergent with Art, Humanities, and Sociology
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• v.7 no.5
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• pp.961-969
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• 2017

Today, the world is experiencing a variety of natural disasters due to the extreme weather. Drought that occurred throughout Southeast Asia from February to May 2016 is also a form of abnormal climate. As a result of this drought, five countries, including Cambodia, Thailand, Vietnam, Laos and Myanmar, faced food shortages, food shortages, as well as rice yields for export. In this study, remote sensing technique was applied to the vicinity of Tonlé Sap, the largest lake in Southeast Asia, to quantitatively analyze the damage caused by drought. As a result, the change of land cover caused a drastic decrease in the water system (132.582㎢) and greenery (706.937㎢) in February 2016, and the reduced water system and greenery changed to dry land and paddy field. It was also found that the temperature rise of 6℃ ~ 8 ℃ compared to the previous year due to the drought from February to April 2016 due to the change of the surface temperature. And it was found that the function of the lake was deteriorated in April due to continuous drought.

• Journal of the Korean Institute of Rural Architecture
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• v.24 no.4
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• pp.85-95
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• 2022

This study is about the story of 'Saemi', existing in the Sacheon Alluvial fan area. Saemi is a local word for Dumbeong, which is the traditional water irrigation facilities in this area that could be formed according to the geographical characteristics of a Alluvial fan site. In the meantime, although Saemi has been an important source of water, related research has been mainly done from an ecological point of view. Accordingly, the researcher paid attention to the functional aspects of Saemi itself, grasped its location, distribution status, and usage including the construction method, and considered its intrinsic value through classification and characteristic analysis of Saemi. As a result of five field surveys from September 2021 to October 2022, 129 Saemies remained in the Sacheon alluvial fan area. According to the structure and shape, Saemi could be divided into basic type, complex type, and buried type. The basic type was subdivided into bucket-type and stairs-type along with the complex type, and the buried type was subdivided into all buried-type and some buried-type. Saemies were mainly distributed at the distal end of the Sacheon alluvial fan site, individual Saemies were built on farmland, and common Saemies were usually built along roadsides adjacent to villages. The reason why the Saemies are concentrated at the distal end is the geographical characteristics of the alluvial fan where the water underflows. Saemi was an important multifunctional water supply source equivalent to the main water source for people at the distal end of the pond who did not receive a stable supply of water from the reservoir. Saemi was at the center of the underground water irrigation network agricultural system in the Sacheon alluvial fan area according to the principles of 'bbaeim(drop out)' and 'gaepim(pooling)' It has provided a foundation for establishing itself as an appropriate technology in this area. Such Saemi contributed to the rural landscape and agricultural biodiversity through its own system and served as a public interest function. It is necessary to know, conserve, manage, and continuously utilize the value of this Saemi as an agricultural heritage.

• Journal of Korean Tunnelling and Underground Space Association
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• v.11 no.3
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• pp.287-301
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• 2009

The novel cut-and-cover tunnel construction method using rib-reinforced pre-cast arch segments has been recently developed and applied for practice to secure a structural stability of high covering and wide width section tunnels. Cut-and-cover tunnels are usually damaged by the seismic behavior of backfill grounds in case of a low covering condition. Seismic analyses are performed in this study to characterize the dynamic behavior of rib-reinforced pre-cast arch cut-and-cover tunnels. Seismic analyzes for 2 lane cast-in-place and rib-reinforced pre-cast arch cut-and-cover tunnels are carried out by using the commercial FDM program (FLAC2D) considering various field conditions such as the covering height embankment slope and excavation slope. It can be concluded that the amplification of seismic wave is reduced due to an increase in the structural stiffness induced by rib-reinforcement. The results show that the rib-reinforced pre-cast arch cut-and-cover tunnels are more effective against the seismic loading, compared to the cast-in-place cut-and-cover tunnels.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.43 no.1
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• pp.9-20
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• 2023

For highway concrete structures, the deterioration of the structure is accelerated due to the increase in the use of deicing materials, and sectional repair work is being frequently carried out to restore performance. However, after the repair work, re-damage such as cracks, delamination, and poor bond performance is exhibited in the repaired sectional area. In this study, overseas repair material requirements were first analyzed, and present domestic requirements were improved repair material performance through field surveys of common concrete structures, laboratory experiments, and test construction on a disused concrete bridge. In addition, performancebased quality requirements were presented so that all materials that meet the required performance can be applied, and different test methods for each material were unified into concrete test methods for consistent test results analysis. The considered performance requirements were compression strength, bending strength, and bond strength for structural properties, and length change rate, crack resistance, thermal expansion coefficient, and elasticity coefficient were for dimensional behavior. For resistance to chloride penetration resistance and freeze-thaw resistance were presented as durability. The proposed requirements for concrete repair materials are expected to contribute to the improvement of the quality of concrete sectional repair work in Korea.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.30 no.1C
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• pp.37-44
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• 2010

Recently, urban retrofit and extension, development of new buildings and facilities, and construction of underground structures like subway tunnels in urban areas give rise to significance of site investigation at urban areas. However, ambient electric noises, traffic vibrations, embedded objects work as obstacles to high-quality and accuracy in site investigation at urban areas. In this paper, a new technique called the pseudo-DC resistivity survey (in brief, PDC-R) was proposed to minimize the adverse effect of ambient electrical noises in resistivity survey. PDC-R technique utilizes an AC current with frequency range of 0.1 to 1.0 Hz rather than DC current, which is used for conventional resistivity survey. The motivation of using low-frequency AC current is to avoid 60-Hz components or its multiples in the resistivity survey which ambient noises are mostly composed of. The implementation of PDC-R technique also included the parametric study on skin effect, frequency effect and current-level effect, which led to the determination of optimal values of frequency and current level for PDC-R survey. The reliability and feasibility of PDC-R technique was verified through field tests, accompanied by the comparison with DC resistivity survey and CapSASW tests.