Chang-Won Kwak

Modified Cyclic Shear Test for Evaluating Disturbance Function and Numerical Formulation of Geosynthetic–Soil Interface Considering Chemical Effect

Geosynthetics have been broadly used in waste landfill sites for filtration, drainage, and separation. Geosynthetics contact soil directly, creating a geosynthetic–soil interface corresponding to the external forces and conditions. The differences in the intrinsic material characteristics at the interface induce complicated stress–strain behaviors and strain-softening processes. Recent studies have presented the behaviors of geosynthetic–soil systems as depending on the interface shear strength degradation, which is affected by ambient factors such as the water content, chemical condition, etc. In this study, the disturbed state concept (DSC) and a disturbance function are introduced to explain the cyclic shear stress behavior of the interface. The degree of interface damage can be expressed by the disturbance function, and the shape of the disturbance function curve represents the intrinsic characteristics of the material. Massive sets of cyclic shear tests have been performed to investigate the effects of the pH values of leachates on the shear behavior of the geosynthetic–soil interface. Both geosynthetics and Jumunjin sand have been submerged in acid, neutral, and basic solutions for 200 days. A multi-purpose interface apparatus that can simulate the cyclic shear conditions of a geosynthetic–soil interface has been newly manufactured and modified for better performance. Test results display remarkable distinction in chemical degradation trends according to the pH values. New disturbance function parameters that determine the characteristics of the shear strength of the interface were estimated according to the chemical conditions as well. Furthermore, it was discovered via focused ion beam electronic microscopy that the different patterns of damage on the surface of soil particles with different pH values induce variation in the disturbance phase at the geosynthetic–soil interface. For the numerical formulation of the disturbance function, the constitutive equations of the DSC were modified using the Mohr–Coulomb model. Based on the modified DSC equations, verification and numerical implementation shall be performed for further study.

Development of Modified Interface Apparatus and Prototype Cyclic Simple Shear Test Considering Chemical and Thermal Effects

Geosynthetics are widely used at waste landfill sites due to their mechanical and physical advantages. Geosynthetics inevitably involve interface problems corresponding to the parametric conditions, which affect the shear behavior of the geosynthetic-soil interface. In this research, chemical and thermal effects were considered in an investigation of the cyclic shear behavior of the geosynthetic-soil interface. The modified multipurpose interface apparatus (M-PIA), which was able to consider thermal condition, was introduced, and major modifications and improvements of the structure and units were described. Displacement capacity to reach peak strength subjected to the specimen size was verified by the static tests. Prototype cyclic simple shear tests were performed, and the disturbed state concept (DSC) was utilized to estimate quantitatively the cyclic shear stress degradation of the interface. The pH values of the solutions represented the chemical condition of the leachate because the pH value changed dramatically due to complicated biochemical reactions in the leachate (Bilgili et al. 2007, “Metal Concentrations of Simulated Aerobic and Anaerobic Pilot Scale Landfill Reactors,” J. Hazard. Mater., Vol. 145, Nos. 1–2, pp. 186–194), and pH was considered to be the most significant parameter affecting leachate concentration (Rafizul and Alamgir 2012, “Characterization and Tropical Seasonal Variation of Leachate: Results From Landfill Lysimeter Studied,” Waste Manage., Vol. 32, No. 11, pp. 2080–2095). Specimen temperatures of 20 and 60°C were applied and maintained during the tests to approximate the thermal conditions inside of the waste landfill. As a result, shear stress degradation with an increasing number of cycles was observed, and the greatest interface disturbance appeared under the acidic condition. The rapid increase in the disturbance was observed at a high temperature in acid and basic conditions; however, the disturbance function curves under the neutral condition were nearly identical despite the different temperatures. Therefore, it was deduced that an elevated temperature can degrade the shear resistance of the geosynthetic-soil interface under severe acid and basic conditions. An elevated temperature under a neutral condition did not affect the shape of the disturbance function curves.

Seismic response characteristics according to the supporting conditions of middle slab of double-deck undersea tunnel using the centrifuge testing

Due to the concentration and congestion of traffic in Seoul metropolitan area, effective utilization of underground space is required, and construction of various underground structures such as a double deck tunnel is increasing. Double deck tunnels are divided into upper and lower runways, and the most important part is middle slab. To investigate seismic behavior of middle slab, experimental study is required because of the complexity of the load and the mechanism of earthquake. In this study, centrifugal model tests were conducted to investigate the response characteristics of earthquake response according to the support conditions of the middle slab of a double deck tunnel. Artificial, Ofunato (short period) and Hachinohe (long period) seismic waves were employed in the experimental study. As a result, it was confirmed that the acceleration attenuation of elastomeric bearings condition was 10.6% in artificial earthquake, 13.6% in Ofunato earthquake, and 10.3% in Hachinohe earthquake. The results indicate that elastomeric bearings have some advantages in the viewpoint of seismic behaviors.

Study on flexible segment efficiency for seismic performance improvement of subsea tunnel

Underground structures that have recently become larger are required to be stable not only during normal times but also during earthquakes. Especially, it is very important to maintain the stability of the subsea tunnels during the earthquake. The objective of this paper is to verify the effectiveness of the flexible segment, which is one of the breakthrough facilities to maintain the stability of the subsea tunnel during the earthquake using the shaking table test. Another goal of this paper is to propose the optimum position of the flexible segment through 3D dynamic numerical analysis based on the verified results from shaking table tests. The 1g shaking table test considering the similarity ratio (1:100) to the cross section of the selected artificial subsea tunnel was carried out considering the Geongju and Artificial seismic waves, longitudinal and lateral wave, and with/without flexible segments eight times or more. As a result of the shaking table test, it was confirmed that the flexible segment is effective in improving the seismic performance of the undersea tunnel in all the experimental results. In addition, 3D dynamic numerical analysis was performed to select the optimum position of the flexible segment which is effective for improving seismic performance. As a result, it was confirmed that the seismic acceleration is attenuated when the flexible segment is installed adjacent to the branch section in subsea tunnel.

Seismic response analysis of virtual honam-jeju subsea tunnel

Underground structures such as subsea tunnel having large section should be stable against seismic loads. In general, underground structures show more stable behavior due to the limited dynamic motion and force, and considerable energy dissipation; however, severe damage was reported from recent earthquakes. Therefore, more sophisticated and analytic approach is required to investigate the seismic response of underground structure like subsea tunnel. In this study, seismic analysis of virtual Honam-Jeju subsea tunnel are performed. Consequently, stresses and forces of tunnel lining increased at fractured and/or weak rock zones. Stresses and forces of tunnel lining also increased at large section under axially deformed condition; however, decrease under transversely deformed condition.

Dynamic shear behavior of geosynthetic-soil interface considering thermalchemical factors

ABSTRACT: The needs for the utilization of space in the urban ara due to the increasing population and traffic volume. A Double-deck tunnel can be an appropriate solution. Geosynthetics are inevitably installed between ground and tunnel lining, therefore, geosynthetic-soil interface is also comprises. Dynamic shear behavior of geosynthetic-soil interface affects the dynamic behavior of tunnel, and experimental study is required since the behavior is very complicated. In this study, chemical factors such as acid and basic element in the groundwater and temperature are considered in the laboratory test. Multi-purpose Interface Apparatus(M-PIA) is utilized and submerging periods are 60 and 960 days. Consequently, dynamic shear degradation of geosynthetic-soil interface considering chemical and thermal factors are verified.Keywords: Chemical factors, Cyclic simple shear tests, Soil-geosynthetic interface, Dynamic shear behavior, Waste landfill 초 록: 최근 제한된 국토 면적에 비해 늘어나고 있는 인구와 그에 따른 교통량 증가에 따라 효율적인 공간 이용의 필요성이 급증하고 있다. 이를 위하여 여러 가지 지하구조물 건설이 증가하고 있으며, 그 중 대표적인 것이 복층터널이다. 복층터널 건설시 터널 라이닝과 주위 지반이 접촉하게 되고, 그 사이에 차수, 보호 또는 지반보강의 목적으로 토목섬유를 시공하게 된다. 따라서 필연적으로 지반-토목섬유 접촉면이 형성되어 터널의 전단 거동에 큰 영향을 미치게 되며, 특히 지진과 같은 동적하중 재하시 그 거동은 매우 복잡해지므로 실험적 접근이 필수적이다. 본 연구에서는 터널 주위지반의 지하수 내 산성 및 중성과 같은 화학적 성분이 반복 전단하중 상태에서 지반-토목섬유 접촉면의 전단강도 감소에 미치는 영향을 실내 시험을 통하여 분석하였다. 또한 여러 가지 요인에 의하여 형성되는 지중 온도차를 고려하기 위하여 시료의 온도를 20°C와 40°C로 설정하여 온도에 의한 접촉면의 동적 전단특성을 고려하였다. 이를 위하여 접촉면 동적전단시험기를 이용하고 60일, 960일간 수침시킨 토목섬유와 흙 시료를 이용하여 반복 단순전단시험을 수행하였고, 그 결과를 교란상태개념을 이용하여 지반-토목섬유 접촉면의 전단강도 감소 특성을 확인하였다.주요어: 복층터널, 반복단순전단시험, 토목섬유-흙 접촉면, 동적전단거동, 열-화학적 인자Copyright ⓒ2016, Korean Tunnelling and Underground Space Association

Characteristics of Dynamic Shear Behaviors of the Interface in Waste Landfill Sites

The number of the waste landfill site increases continuously in recent years. Various types of geosynthetics have been utilized in the waste landfill site for encapsulating, isolating, protecting, and so on. Geosynthetics compose the interface with soil and affect the shear behaviors of waste landfill system. The chemical aggressors in the leachate also play a critical role in the interface and it is known that the dynamic loading is able to accelerate manifestation of the effects of chemical attack. In this research, numerical analysis was conducted considering shear stress degradation in the geosynthetic-soil interface due to dynamic load with chemical effect. Cyclic simple shear test results was reviewed and applied to the numerical modeling as well. Consequently, the stability of waste landfill site could be overestimated without considering the shear stress degradation of interface.