Han-Kyu Yoo

Evaluation of Engineering Properties of Geogrids for Soil Retaining Walls

This study focuses on evaluating the engineering properties of three types of geogrids (warp knitted, woven, and membrane drawn geogrid). Short-term tensile properties of the membrane drawn geogrid showed very low elongation results compared to the other types. However, estimated long-term creep deformation indicated that the 30 % of Tult loading level was the optimum value that satisfied the creep criteria in the case of membrane drawn geogrid (60 % in warp knitted geogrid and 65 % woven geogrid). The durability test results showed that the membrane drawn geogrid had good resistance to installation damage, chemical, biological and ultraviolet compared with the textile type geogrids. The total reduction factors that considered creep, installation damage and durability of the warp knitted, woven, and membrane drawn geogrid were estimated as 2.0, 1.9, and 3.6, respectively.

Evaluation of pullout and drainage properties of geosynthetic reinforcements in weathered granite backfill soils

The main aim of this research is to systematize the reinforced-earth wall system using the geosynthetic reinforcements in the weathered granite backfill soils containing relatively large amount of fines with low permeability. As a staged endeavor to accomplish this purpose, laboratory pullout tests and finite element modeling are carried out focusing on the analysis of friction characteristics associated with interaction behaviors of the geosynthetic reinforcements composed of geogrid with outstanding tensile resistance and geotextile with a sufficient drainage properties. In addition, drainage effects of the geotextile laid below the geogrid are examined based on the analysis of finite difference numerical modeling. From the present investigation, it is concluded that the geosynthetic reinforcements in the weathered granite backfills can be used to achieve combined effects of the reduction of deformations and the increase of tensile resistance, together with drainage effects.

Evaluation of rock load based on critical shear strain concept on tunnels

After studying the characteristics of three different evaluation methods of rock load; namely theoretical method, empirical method and numerical method, there were too many limitations for them to be applied on tunnels. Therefore, in this research paper, the method based on numerical analysis is selected to use as this method is the most reasonable one since it considers all parameters that are necessary for rock load estimations, and it also considers the interaction between ground and tunnel support. The critical shear strain concept formulated by Sakurai (1981) was used in order to measure exact rock load values based on numerical analysis. Evaluation on a Level 1 rock load height, which is depicted by the stable region in the graph shows that rock load is not affecting between ground grade 1~3, and it was evaluated that the fourth and fifth grades show less values of rock load height which led to the conclusion of a more economical design of concrete lining.

Stability analysis of an existing utility tunnel due to the excavation of a divergence tunnel emerging from double-deck tunnel

Government plans to construct a double-deck tunnel under a portion of Gyeongbu Expressway that will solve traffic problems and could also be used as a flood storage facility. Divergence tunnels connect the main tunnel to the urban areas and their construction effects on adjacent structures at shallow depth need to be analyzed. This study primarily includes the numerical analysis of construction effects of divergence tunnels on utility tunnels. The utility tunnel was analyzed for three cases of volume loss applied to the divergence tunnel and two cases of the angle between main tunnel and divergence tunnel (36° and 45°). The results show that the more the volume loss was applied and the shorter the distance was between utility tunnel and divergence tunnel, the more the utility tunnel was affected in terms of induced displacements, angular displacement and stability. The worst scenario was found out to be the one where the angle between main tunnel and divergence tunnel was 36° and the distance between divergence tunnel and utility tunnel was 10 m, resulting in the largest displacement and differential settlement at the bottom of the utility tunnel. A relationship between the angular displacement and the distance to diameter ratio was also established.

3-Dimensional numerical analysis on support performance of early-high-strength shotcrete

Now-a-days, the trend in constructing tunnels is to build more deeper, more longer tunnels of greater cross-sections. That’s why, the demand of “Early-high-strength shotcrete” is very high because of their advantage of attaining higher strength immediately after excavation, which controls the ground subsidence. So, this study reveals the supporting phenomena of early-high-strength shotcrete, using three-dimensional numerical analysis. The crux of this study can be applied practically in construction sites also. Support Performance of two different qualities of shotcrete was checked out, by keeping the general shotcrete’s thickness constant and comparing it with early-high-strength shotcrete’s thickness decreasing it gradually in five steps, and analysing/comparing the support performance in all cases. Effect of using early-high-strength shotcrete was analysed to save the cost of steel sets, which are widely used for supporting the ground before the hardening of general shotcrete. The results of numerical analysis on the performance of early-high-strength shotcrete show that, it behaves more effectively under worse ground conditions and it can support the ground more conveniently than steel sets, before the shotcrete is hardened.

Dynamic Response of Underground Three-layered Pipeline Subjected to Pile Driving Loads : I. Distance

This study presents the behavior characteristics of buried three-layered pipeline subjected to pile driving loads. The analysis considered the driving energy caused by 7 tonf of ram weight and 1.2m of stroke. Also the distance from vibration resource to pipeline varies in 5m to 30m. The vibration velocity and stress are investigated at the center of pipeline in longitudinal direction. In the same cover depth, attenuation ratio of vibration velocity and von Mises stresses for distance increment has shown a decreasing trend. The maximum stress occurs at the top and bottom for the inner pipe, however, an irregular stress distribution is found for the outer pipe.

Weatherability Assessment of Woven Geotextiles for Soft Ground Improvement

In this study, polypropylene and polyester woven geotextiles having 5, 8, and 12 ton/m design strength were used. The parameters which affect the weatherability of woven geotextiles were examined through the index and field exposure tests. The reduction factors of these exposure conditions were also determined to interpret the long-term design strength of woven geotextiles.