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Theoretical and experimental studies on influence of electrode variations in electrical resistivity survey for tunnel ahead prediction

터널 굴착면 전방조사를 위한 전기비저항 탐사에서 전극의 변화가 미치는 영향에 대한 이론 및 실험연구

  • Hong, Chang-Ho (Dept. of Civil and Environmental Engineering, KAIST) ;
  • Chong, Song-Hun (Dept. of Civil and Environmental Engineering, Sunchon National University) ;
  • Hong, Eun-Soo (Dept. of Civil and Environmental Engineering, KAIST) ;
  • Cho, Gye-Chun (Dept. of Civil and Environmental Engineering, KAIST) ;
  • Kwon, Tae-Hyuk (Dept. of Civil and Environmental Engineering, KAIST)
  • 홍창호 (한국과학기술원 건설환경공학과) ;
  • 정성훈 (국립순천대학교 건설환경공학과) ;
  • 홍은수 (한국과학기술원 건설환경공학과) ;
  • 조계춘 (한국과학기술원 건설환경공학과) ;
  • 권태혁 (한국과학기술원 건설환경공학과)
  • Received : 2019.01.17
  • Accepted : 2019.02.22
  • Published : 2019.03.31

Abstract

Variety of tunnel ahead prediction methods have been performed for safe tunnel construction during tunnel excavation. Pole-pole array among the electrical resistivity survey, which is one of the tunnel ahead prediction method, has been utilized to predict water-bearing sediments or weak zone located within 5 times of tunnel diameter. One of the most important processes is the estimation of virgin ground resistivity and it can be obtained from the following process: 1) calculation of contact area between the electrodes and the medium, and 2) assumption of the electrodes as equivalent spherical electrodes which have a same surface area with the electrodes. This assumption is valid in a small contact area and sufficient distance between the electrodes. Since the measured resistance, in general, varies with the electrode size, shape, and distance between the electrodes, it is necessary to evaluate the influence of these factors. In this study, theoretical equations were derived and experimental tests were conducted considering the electrode size, shape, and distance of cylindrical electrodes which is the most commonly utilized electrode shape. Through this theoretical and experimental study, it is known that one should be careful to use the assumption of the equivalent half-spherical electrode with large ratio between the penetrated depth and radius of the cylindrical electrode, as the error may get larger.

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Fig. 1. Equipotential surface shape between two cylindrical electrodes

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Fig. 2. Test setup

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Fig. 3. Theoretical & experimental resistance and boundary effects (ρ = 52.16 Ωm)

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Fig. 4. Theoretical & experimental resistance and boundary effects (ρ = 29.97 Ωm)

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Fig. 5. Equipotential surface shape of spherical electrode

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Fig. 6. Effect of electrode geometry on the theoretical resistance

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Fig. 7. Electrical resistance of cylindrical and its equivalent spherical electrodes (ρ = 52.16 Ωm)

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Fig. 8. Electrical resistance of cylindrical and its equivalent spherical electrodes (ρ = 29.97 Ωm)

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Fig. 9. Error of the electrical resistance value from the cylindrical electrodes and its equivalent half-spherical electrodes

Acknowledgement

Supported by : 한국전력공사

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