Journal of the Korean Geotechnical Society (한국지반공학회논문집)

Korean Geotechnical Society (한국지반공학회)
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Numerical Analysis and Exploring of Ground Condition during Groundwater Drawdown Environment in Open-cut Type Excavation

개착식 굴착공사시 지하수위 저하로 인한 지반상태 탐사 및 해석기법 연구

  • Han, Yushik (Geotechnical Engrg. Research Institute, Korean Geotechnical Society)
  • 한유식 (한국지반공학회 지반공학연구소)
  • Received : 2018.11.06
  • Accepted : 2018.11.28
  • Published : 2018.11.30
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Abstract

Precise investigation and interpretation of the ground subsidence risk factors needed to predict and evaluate the settlement problems of the surrounding ground due to the ground excavation. There are various geophysical exploration methods to investigate the ground subsidence risk factors. However, there are factors that influence the characteristics of the underground medium in these geophysical methods, and the actual soil contains complex factors affecting geophysical exploration. Therefore, it is necessary to analyze the effects on the geophysical methods based on the understanding of the geotechnical properties of soil. In this study, a test bed was constructed to consider various complicated factors in the complex ground and the ground behavior was analyzed by numerical analysis. In addition, we analyzed the limitations on investigating the ground subsidence risk factors through ground penetration radar (GPR) survey. As a result, ground subsidence of Open-cut Type Excavation is caused by various factors. Especially, in the case of soft ground condition, it was found that it was greatly influenced by the flow change of groundwater level. At the center frequency of GPR of 250 MHz, the attenuation of the electromagnetic wave is severely attenuated in the clay with high electrical conductivity, making it difficult to penetrate deeply into the ground (4 m below the surface). As the electromagnetic waves pass through the groundwater level below the groundwater, the attenuation of the electromagnetic waves becomes severe.

지반 굴착에 따른 주변 지반의 침하 문제를 예측하고 평가하기 위해서는 지반침하 위험인자의 정확한 조사와 해석이 필요하다. 지반침하 위험인자를 조사하기 위한 비파괴조사방법에는 여러 가지 물리탐사방법이 있다. 하지만 이러한 물리탐사 방법에는 지하매질 특성에 영향을 미치는 요인들이 있으며, 실제 지반의 토질은 물리탐사에 영향을 주는 복잡한 요소들이 내포하고 있다. 따라서 흙의 공학적 성질에 대한 이해를 바탕으로 한 물리탐사 방법에 미치는 영향을 분석하는 접근이 필요하다. 이 연구에서는 실제 복합지반에서 다양하고 복잡한 요소들을 고려할 수 있도록 Test Bed를 구축하고, 수치해석을 통해 지반거동을 분석하였다. 또한, 지표투과레이더(GPR) 탐사를 통해 지반침하 위험인자 조사를 위한 한계성을 분석하였다. 그 결과, 개착식 굴착공사에 있어서 지반침하는 여러 가지 요인에 따라 발생 할 수 있으며, 특히 연약지반 조건일 경우에는 지하수위 흐름 변화에 큰 영향을 받는 것으로 나타났다. 또한 GPR의 중심주파수 250MHz인 경우, 전기전도도가 높은 점토 지반에서는 전자기파의 감쇠가 심하게 일어나서 땅속 깊은 곳(지표아래 4m)까지의 투과를 어렵게 만들며, 지하수위 아래에서는 지하수면을 전자기파가 통과함에 따라, 전자기파의 감쇠가 심하게 일어나는 것으로 나타났다.

Keywords

GJBGC4_2018_v34n11_93_f0001.png 이미지

Fig. 2. Stress-strain curve showing strain hardening

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Fig. 1. Normal consolidation line and overconsolidation curves

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Fig. 3. Critical state line and yield curve in p'-q space (Roscoe and Burland, 1968)

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Fig. 4. Design for open-cut type excavation

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Fig. 5. Modeling of FEA

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Fig. 6. Numerical analysis due to variation of critical state line (M)

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Fig. 7. Numerical analysis due to variation of Hydraulic Conductivity

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Fig. 8. As a result of numerical analysis

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Fig. 9. Groundwater changes by excavation stage

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Photo. 1. Subsidence for surrounding ground of excavation site

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Fig. 10. GPR exploration mode (Reflection mode)

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Fig. 11. GPR exploration before excavation

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Fig. 12. GPR exploration after excavation

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Photo. 2. Measuring the electrical properties of clayey sand

Table 1. Relationship with variables of underground electrical characteristics, GPR Exploration and Frequency

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Table 2. Estimated initial parameters of soils and materials

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Table 3. Changes of clayey sand parameters for the sensitivity analysis

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Table 4. Electrical properties of clayey sand

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