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ISSN : 2093-5145(Print)
ISSN : 2288-0232(Online)
Journal of the Korean Society for Advanced Composite Structures Vol.10 No.4 pp.16-23
DOI : https://doi.org/10.11004/kosacs.2019.10.4.016

Reduction of Seismic Behavior for Underground Concrete Structure Applied Embedded-Trench Installation Using EPS Geofoam

Mabel Catuira1, Jun-Suk Kang2, Jong-Sup Park3
1Graduate, Department of Civil Engineering, Sangmyung University, Cheonan-si, Chungnam, Korea
2Professor, Department of Landscape Architecture and Rural System Engineering, Seoul National University, Seoul, Korea
3Professor, Department of Civil Engineering, Sangmyung University, Cheonan-si, Chungnam, Korea

본 논문에 대한 토의를 2019년 09월 30일까지 학회로 보내주시면 2019년 10월호에 토론결과를 게재하겠습니다.



Corresponding author: Park, Jong-Sup Department of Civil Engineering, Sangmyung University, Cheonan-si, Chungnam 31066, Republic of Korea. Tel: +82-41-550-5314, Fax: +82-41-558-1201, E-mail: jonpark@smu.ac.kr
July 8, 2019 July 29, 2019 July 30, 2019

Abstract


During earthquakes, underground structures generally suffer less than the structures on the ground. However, significant and severe damages had been reported by many researchers who have been paying attention to the dynamic soil-structure interaction in order to prevent calamities. The construction technique called Embedded Trench Installation was adopted in this study to reduce the earth loads experienced in the tunnel. The dynamic soil-structure interactions with expanded polystyrene (EPS) geofoam around concrete structures were investigated using finite element program, ABAQUS. The structures were analyzed under variation of EPS geofoam types, namely EPS12, EPS15, and EPS19. The results from finite element analyses showed that the geofoam with low density(EPS12) was the most effective at reduction of earth loads.



EPS Geofoam을 사용한 ETI 공법이 적용된 지중콘크리트 구조물의 지진특성 감소효과 연구

마 벨1, 강 준석2, 박 종섭3
1상명대학교 건설시스템공학과 석사과정
2서울대학교 조경지역시스템공학부 부교수
3상명대학교 건설시스템공학과 교수

초록


일반적으로 지중구조물은 지상구조물보다 지진하중 작용 시 상대적으로 작은 영향을 받는다. 그러나 많은 연구자들 은 심각한 지중구조물 손상에 대해 보고하고 있으며 동적 흙-구조물 상호작용에 대한 지속적인 연구를 수행하고 있다. 본 연구 에서는 유한요소해석 프로그램을 활용한 흙-구조물 상호작용을 지중구조물에 적용하고 지중구조물 하중저감기법인 ETI의 지오 폼을 해석변수로 경감효과 및 최적 지오폼을 제안하고자 한다. 해석연구에 고려된 지오폼은 EPS 12, EPS 15, EPS 19이다. 해석 결과로부터 지진하중시 최대 50%까지 지중하중이 경감되었으며, 수평처짐은 26%, 수직처짐은 8%이 경감되었다. 본 해석연구를 토대로 ETI 공법을 적용한 지중구조물이 정적 및 지진하중 하에서도 하중의 영향을 경감시키는 것을 확인할 수 있었다.



    1. INTRODUCTION

    Underground structures have been prominently used to convey life lines such as water, gas, electricity and communication cables. The buried structures were less affected during earthquakes, however, few incidents of major earthquakes in the history, such as the well-known Kobe earthquake(Japan) in 1995, Chichi earthquake (Taiwan) in 1999 and Gujarat earthquake (India) in 2001 happened to be the underground structures. When underground structures were damaged, the supplies of life lines are significantly affected and disasters may occur. Researchers have been interested in investigating the soil-structure interaction, such as on various kinds of buried structures under seismic loadings. Abuhajar et al.(2015) conducted both experimental and numerical tests regarding the seismic soil-structure interaction involving box culverts, Byrne et al.(1996) have studied a numerical analysis of dynamic soil-structure interaction regarding a three-hinged large buried culvert structures using finite elements, and Wen et al.(2017) extended the dynamic soil-structure interaction with the inclusion of an electrical substation equipment using infinite elements.

    However, these literatures only focused in the behavior and performance of underground structure under seismic excitation and did not consider possible methods in reduction of loads experienced by the buried structures with dynamic loading. Kim et al.(2018) presented a construction technique called Embedded Trench Installation (ETI) which was used to reduce the earth pressure acting on the underground structure. The ETI technique used soft compressible material of the expanded polystyrene (EPS) geofoam which encapsulated the whole structure. In recent years, studies regarding the use EPS geofoam for ETI technique had been proven to induce positive soil arching and reduce earth loads acting on the buried structure. Numerous studies had been conducted by Kang and his colleagues for the application of EPS Geofoam in pipes on different materials such as, corrugated PVC pipe (Kang et al, 2007a), concrete pipes (Kang et al, 2007b), concrete box tunnel (Kang et al, 2007c), and corrugated steel pipe (Kang et al, 2007d, 2007e). Jeong et al.(2018) extended the study to ecological bridges. Kim et al.(2018) determined the optimum size of geofoam in load reduction for underground arch structures. However, these studies were limited to static loading and did not include seismic loadings. This study presents the behavior of underground structure with the application of the EPS geofoam subjected to seismic loading.

    2. BACKGROUND OF RELATED STUDIES

    2.1 Mechanism of soil with installed structures

    Soil arching happens when there is a difference stiffness between the culvert and the surrounding soil. Marston and Anderson(1913) investigated that buried pipes at great depth affects the relative settlements which play a vital role in earth pressure acting on the structure. The relative settlement created friction forces that could be either added or subtracted to resultant load acting on the pipe. Based from the study, the soil was divided into three rectangular prisms by projecting imaginary vertical lines which extend from both sides of the pipe up to the ground. If the relative settlement in the adjacent soil prisms was more than the central soil prisms, the earth load acting on the conduit was increased by the downward friction forces generated by the adjacent prisms (negative soil arching). On the other hand, when the relative settlement of the central soil prism was greater than of the adjacent, the generated frictional forces acted oppositely to the resultant load, thus, was subtracted resulting to decrease in earth pressure acting on the pipe (positive soil arching).

    In order to induce a positive arching, Marston (1922) established a construction technique which was called as Imperfect Trench Installation (ITI). The ITI technique was a method that placed a soft compressible layer above the structure shown in Fig. 1(b), that was proven effective to reduce earth pressures at the top of the underground structure.

    2.2 Embedded trench installation technique

    The Embedded Trench Installation(ETI) technique came from the principles and ideas of the Imperfect Trench Installation. Kim et al.(2018) had studied that the reduced earth pressure on the top part of the structure moved to the side of the structure. Thus, in order to counter the effect of ITI technique, soft zone or compressible lightweight materials was also applied at the side of structure, hence, encapsulating the whole structure with soft compressible material as shown in Fig. 1(a) and Fig. 2(a).

    2.3 Infinite elements

    Nielsen(2006) studied that the fixed boundary conditions in static problems could be used without sacrificing much in terms of accuracy. However, the fixed boundary conditions caused the reflection of waves which effectively trapped the energy inside the dynamic model. Therefore, the simulation of the unbounded soil medium in numerical methods has been a challenging topic in dynamic soil-structure interaction(SSI). The simplest solution to this problem was to create a large enough field so that waves reflected from the boundaries did not have time to return to the region of interest. In order to simulate the soil medium in a simple yet realistic manner, numerous researches were conducted since Alterman and Kara(1968) first put forward the concept of placing the artificial truncated boundary which was far away from the domain of interest.

    However, the simulation of far field boundary was not a practical solution since it created too many unnecessary elements and generated longer time for finite element analysis. Lysmer and Kuhlemeyer(1969) replaced the far field boundary conditions with viscous damping to simulate the elastic resilience of the soil, and provided the algorithm for first- and second-order infinite elements in ABAQUS for dynamic response.

    Some studies were made regarding the infinite element in ABAQUS like Ismail and Mullen(2000) and Yue et al.(2006). The studies presented the comparisons in the viscous boundary and the infinite element in ABAQUS and provided that the use of infinite element in ABAQUS had many advantages over the viscous boundary.

    3. FINITE ELEMENT MODELING

    The ABAQUS was chosen to simulate the model due to its wide material modelling capability and the provided algorithm for first- and second-order infinite elements based on the work of Zienkiewicz et al.(1983) for static response and of Lysmer and Kuhlemeyer(1969) for dynamic response. The simulated structure was buried at 3m depth and was analyzed under clay soil medium, then further more investigated by varying the type of EPS geofoam, namely EPS 12, EPS 15 and EPS 19 which significantly differs in density.

    Lou and Chen(1999) and Lou et al.(2000) suggested that the soil medium could be defined as five times the width of the structure in order to reduce the motion reflection on the boundary. The soil size of the finite element is 15m×15m, having a concrete tunnel structure of 3m×3m. Figs. 3 and 4 show the schematic diagram of the model and the cross-section of the used concrete tunnel model, respectively. The concrete tunnel and geofoam are modelled as C3D8R for hexahedral element, and the soil is composed of C3D8R and AC3D8 for hexahedral finite and infinite elements, respectively. In order to simulate the acoustic elements AC3D8, AC3D8 elements were manipulated into CIN3D8 elements by using ABAQUS input file, thus, creating infinite elements.

    Infinite elements were used for lateral borderline of the soil structure in order to simulate the artificial boundary conditions for seismic loadings as shown in Fig. 3. The use of infinite elements has been proven to have built in damping constants to eliminate the reflection of waves back to the medium (Yue and Li, 2007). Fixed boundary conditions were applied at the bottom of the structure for static analysis and released the horizontal restraint during the dynamic analysis. Figs. 5 and 6 shows three-dimensional model and all of the models were subjected to gravity loading. Time-history data of El-Centro North-South component, as shown in Fig. 7, were applied at the base of the structure which values were retrieved from vibrationdata.com.

    Table 1 lists the properties of materials used in the model and soil values retrieved from finesoftware.com (Raymond and James, 1985;Nixon and Child, 1989: Liu et al., 2010), Table 2 shows the properties of concrete material where Kc is the ratio of the distances between the hydrostatic axis and respectively the compression meridian and the tension meridian in the deviatoric cross section. The fb0/fc0 is the ratio of strength in biaxial state(fb0) to strength in uniaxial state(fc0). Table 3 presents geofoam materials from ASTM D6817(2002). Drucker-Prager failure criteria were used to model the plasticity of clay soil medium, Concrete Damage Plasticity (CDP) was adopted to simulate the plasticity of concrete and the values from the stress-strain curve of geofoam were used to simulate the plasticity of the geofoam. A surface-to-surface contact type option was used for the interaction of the concrete structure and backfill and/or geofoam. The soil-structure interaction had a rough friction formulation for the tangential behavior and a hard contact for the pressure-overclosure section in normal behavior while restraining separation after contact. Dynamic, Implicit analysis was performed to simulate the seismic loading.

    4. FINITE ELEMENT ANALYSIS RESULTS

    Finite element analyses results were investigated at top slab, side wall and bottom slab in the middle part of the concrete structures. The solid lines represent the structure without geofoam and the broken lines represent all of the structures with geofoam. After applying gravity and dynamic loading, results show the static loading as well as the minimum and maximum loads throughout the earthquake simulation. Fig. 8

    4.1 Effect of EPS geofoam

    Fig. 9 shows the parabolic distributions of earth pressures on the top slab, side wall, and bottom slab similar with the works of previous researchers such as Dasgupta and Sengupta(1991). The reason behind the parabolic shape of earth pressures is due to the earth loads moving to the corners of the structure. Fig. 9 also shows the comparison between the earth pressures acting on the model with and without geofoam and presents that the use of geofoam generally reduces the earth loads experienced by the model with clay medium.

    Fig. 10 shows the comparison for horizontal deflection of the structure with and without geofoam. It presents that the values for top slab and bottom slab of the structure are nearly forming a flat line and the values of the side walls showed deformation. The reason behind this behavior is that the horizontal deflection of the structure is mainly influenced by the earthquake excitation. The geofoam applied to the structure also reduces the horizontal deflection.

    Fig. 11 for the vertical deflection of the structure shows that the values of the side walls almost formed a flat line and the same parabolic shape is observed for the top and bottom slab of the structure. The parabolic shape occurs before with the earth pressure experienced by the model and the reason behind this is that the vertical deflection is heavily influenced by static loading. Fig. 11 also indicates the same trend that the use of geofoam is effective in reduction of vertical deflection.

    4.2 Percentage reduction considering geofoams types

    The average percentage reduction was computed for the top slab, side walls and bottom slab considering the static, minimum and maximum values produced by dynamic loading (DN; Dynamic Minimum values, DM; Dynamic Maximum values). From the Tables 4-6, it displays that the values of percentage reduction for earth pressure decreases as the geofoam varies from EPS 12 to EPS 19. The same trends were also observed for percentage reduction of horizontal and vertical deflection and it shows that the best type of expanded polystyrene geofoam is the geofoam of EPS 12. Table 5

    5. CONCLUSION

    This study presents the behavior of underground structure with the application of EPS geofoam subjected to seismic loading using finite element analysis program, ABAQUS. The behavior of the structure was compared between the different types of geofoam applied to the model, in terms of load reduction and deflection. Based on this study, the following conclusions have been made:

    • 1) The application of geofoam on underground structures generally reduces the earth loads acting on the structure, both in static and dynamic loading.

    • 2) It was confirmed that the use of ETI construction technique with the application of EPS geofoam, which is a highly compressible material, reduces the stress on the buried structure up to 47% in static, and 50% in dynamic, compared to case of buried structures without EPS geofoam reinforcement.

    • 3) EPS 12 geofoam type, which has the least value of density, provides the highest percentage reduction in earth pressure and deflection acting on the structure. The least density produced the highest compression in the soft zone, thus, the larger settlement in the central prism generated more frictional forces against the resultant load resulting to reduction of loads experienced by the structure.

    Figure

    KOSACS-10-4-16_F1.gif
    Mechanism of Soil
    KOSACS-10-4-16_F2.gif
    Movement of Earth Pressure
    KOSACS-10-4-16_F3.gif
    Schematic Design of Simulated Model
    KOSACS-10-4-16_F4.gif
    Cross-section of Concrete Structure
    KOSACS-10-4-16_F5.gif
    Front View of the Finite Element Model
    KOSACS-10-4-16_F6.gif
    Isometric View of the Finite Element Model
    KOSACS-10-4-16_F7.gif
    The Input Acceleration Time-history
    KOSACS-10-4-16_F8.gif
    Stress-strain Curve of Geofoam
    KOSACS-10-4-16_F9.gif
    Comparison of Earth Pressures During Static and Dynamic Loading
    KOSACS-10-4-16_F10.gif
    Comparison of Horizontal Deflection during Static and Dynamic Loading
    KOSACS-10-4-16_F11.gif
    Comparison of Vertical Deflection during Static and Dynamic Loading

    Table

    Material Properties of Clay
    Material Properties of Concrete
    Material Properties of EPS Geofoam
    Percentage Reduction in Earth Pressures (unit: %)
    Percentage Reduction in Horizontal Deflection (unit: %)
    Percentage Reduction in Vertical Deflection (unit: %)

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