Yeon-Jun Park

Influence of asperity degradation on the mechanical behavior of rough rock joints under cyclic shear loading

A cyclic shear testing system was established to investigate the mechanical behavior of rough rock joints under cyclic loading conditions. Laboratory cyclic shear tests were conducted for two joint types of Hwangdeung granite and Yeosan marble: saw-cut and split tensile joints. Prior to test, the roughness of each specimen was characterized by measuring the surface topography using a laser profilometer. Several important aspects of cyclic joint behavior, such as high peak shear strength and non-linear dilation in the first loading cycle, different frictional resistance for the reversed shear loading direction, and anisotropic shear behavior and its dependence on the normal stress level were identified from the cyclic shear test results. These features and their variations in the subsequent loading cycles are mainly due to the effect of second order asperities and strength of rock material. It was also observed from experimental results that degradation of asperities under cyclic shear loading also followed the exponential degradation laws for asperity angle and the mechanism for asperity degradation would be different depending upon the shearing direction and the type of asperities. Based on the experimental results an elasto-plastic constitutive model, which can consider the degradation of second order asperities, was proposed. Numerical simulations for the monotonic and cyclic shear loading indicated agreement with the laboratory test results.

Test running of an underground food storage cavern in Korea

Abstract This paper mainly reports the details of the f rst real test of food storage in the underground food storage cavern in Korea. Pear and apple were selected, and the change of indices such as the rate of soundness, moisture content, total acid, and hardness were monitored with the temperature and humidity in and around cavern. The results showed that the fruits were stored in better quality in the case of underground storage with cooling than natural underground condition or surface storage tank.

A pilot study on the design of an underground food storage cavern in Korea

Abstract This paper reports the background information and the initial results obtained from the pilot underground food storage cavern in Korea, where the seasonal variation of air temperature makes the running of on-ground food storage tanks uneconomical. A pilot cavern has been driven into a granite rock mass which is the typical massive rock in Korea. Basic mechanical properties, including the thermal properties, were measured. The change of rock strengths and the behavior of joints upon the change of temperature, were also identified. A series of monitoring systems have been setup to measure the temperature of the ground, the behavior of the rock mass during excavation and running (i.e. cooling) of the cavern. The results of this paper can provide useful information for the design of future commercial underground food storage caverns in Korea.

Characterization of joint profiles and their roughness parameters

Abstract Joint roughness coefficient, most widely used to express the joint roughness, is known to have some problems such as a possibility to involve subjective judgement of the researcher in determining the value. In this study, to express the joint roughness by the parameters other than JRC, joint specimens were produced artificially by a splitter, and the joint profiles were digitized by a coordinate measuring machine. Six roughness parameters, micro average i -angle, root mean square of i -angle, standard deviation of height, roughness profile index, standard deviation of i -angle and fractal dimension, were computed from the digitized data of the joint profiles and the correlations among the parameters were studied. The influence of sampling interval on the five parameters except for fractal dimension was investigated, and also the regression equations between joint roughness coefficient back-calculated from direct shear test and other roughness parameters were determined.

A Case Study on the Design and Construction of a 2-arch Tunnel with Varying Section

This paper describes the design and construction of a 2-arch tunnel with varying section. This new design has advantages of 2-arch tunnels, which is rather expensive, but is still economically competitive compared to parallel tunnels. Economic analysis was also conducted. To secure the stability of the varying section tunnel, excavated part was reinforced by tie-bolts and RRS, and 2-arch part was supported by EPS blocks and concrete walls. Stability of the pillar was theoretically analyzed and also examined by numerical simulations for various widths. Displacement monitoring was conducted and results were compared with numerical results. Economic analysis showed reductions in construction cost and period by 11% and 10 months respectively.

A Numerical Study on the Reinforcement Method of a Pillar Using Tension Bolts at the Connecting Part between 2-Arch Tunnels and Parallel Tunnels

Abstract Two-arch tunnels require minimal spacing between the 2 tunnels and thus occupy small amount of land compared to parallel tunnels. But it is rather expensive. The parallel tunnel is not as expensive, but it requires more land than 2-arch tunnels. This may cause a problem when there is a land shortage. The new design is to connect these two types of tunnels by another pair of parallel tunnels where the separation distance is gradually increasing. The applicability of this new design to the cases where rock condition is not even fair has to be verified since the width of the pillar between the connecting tunnels can be quite narrow. Therefore both two and three dimensional numerical analyses were conducted and pillar stability was examined for rock classes IV and V in two different ways. Results showed that this new design is still effective for poor rock conditions if central pillar is properly reinforced by tension bolts as long as overburden is less than five times of the tunnel diameter.Key words 2-arch tunnel, Parallel tunnel, Pillar 2-arch varying section tunnel, Tension bolt초 록 2-아치 터널은 터널의 간격이 매우 좁아서 병렬 터널에 비해 점유 면적이 작으나 경제적이지 못하다 . 병렬 터널은 경제적이긴 하지만 2-아치 터널보다 점유 면적이 커서 용지의 매입이 어려운 경우에는 문제가 될 수 있다. 새로운 설계는 이 두 터널을 터널의 간격이 점차 증가하는 제 3의 병렬터널로 연결시키는 것이다. 접속 터널의 필라는 폭이 매우 좁을 수 있기 때문에 이 새로운 설계를 불량한 암반에 적용하려면 검증이 필요하다 . 이에 4등급 및 5등급 암반에 대해 2차원 및 3차원 수치해석을 이용하여 2가지 방법으로 안정성 검토를 수행하였다. 검토 결과, 중앙 필라가 텐션 볼트로 적절히 보강된다면 터널 심도가 직경의 5배 이하인 경우에는 적용이 가능하였다.핵심어 2-아치 터널, 병렬 터널, 2-arch 변단면 터널, 필라, 텐션 볼트

Numerical Analysis on the Performance Evaluation of Cablebolts as Tunnel Supports

Cablebolts used to be employed as auxiliary supports where long or high capacity bolts are needed, but become competitive by the improvements in supportability and easiness in handling. Based on the test results obtained from various researches, the performance of the cablebolts was analyzed numerically while varying lengths and fixing conditions. The supporting effecte is assessed by monitoring displacements and stress taken place in shotcrete. When cablebolts are grouted without being tensioned, supporting effect was not as good as that of rockbolts. But, their supportability was good enough to substitute rockbolts if tensioned properly. Post grouting right after tensioning of the cablebolts shows reduction in supportability, but long term stability could be achieved without losing supportability if grouted when the bolt is far enough from the face. Further study is necessary including laboratory and in-situ tests under various conditions to use cablebolts as main support in tunnels.