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Evaluation of mechanical characteristics of marine clay by thawing after artificial ground freezing method

인공동결공법 적용 후 융해에 따른 해성 점토지반의 역학적 특성 평가

  • Choi, Hyun-Jun (Research Institute, Dongmyeong Engineering Consultants & Architecture Co., Ltd.) ;
  • Lee, Dongseop (School of Civil, Environmental and Architectural Engineering, Korea University) ;
  • Lee, Hyobum (School of Civil, Environmental and Architectural Engineering, Korea University) ;
  • Son, Young-Jin (Infra Engineering Team 2, SK Engineering & Construction) ;
  • Choi, Hangseok (School of Civil, Environmental and Architectural Engineering, Korea University)
  • 최현준 ((주)동명기술공단 부설연구소) ;
  • 이동섭 (고려대학교 건축사회환경공학부) ;
  • 이효범 (고려대학교 건축사회환경공학부) ;
  • 손영진 (SK건설 Infra Eng'g2팀) ;
  • 최항석 (고려대학교 건축사회환경공학부)
  • Received : 2018.10.04
  • Accepted : 2018.10.26
  • Published : 2019.01.31

Abstract

The artificial ground freezing (AGF) method is a groundwater cutoff and/or ground reinforcement method suitable for constructing underground structures in soft ground and urban areas. The AGF method conducts a freezing process by employing a refrigerant circulating through a set of embedded freezing pipes to form frozen walls serving as excavation supports and/or cutoff walls. However, thermal expansion of the pore water during freezing may cause excessive deformation of the ground. On the other hand, as the frozen soil is thawed after completion of the construction, mechanical characteristics of the thawed soil are changed due to the plastic deformation of the ground and the rearrangement of soil fabric. This paper performed a field experiment to evaluate the freezing rate of marine clay in the application of the AGF method. The field experiment was carried out by circulating liquid nitrogen, which is a cryogenic refrigerant, through one freezing pipe installed at a depth of 3.2 m in the ground. Also, a piezo-cone penetration test (CPTu) and a lateral load test (LLT) were performed on the marine clay before and after application of the AGF method to evaluate a change in strength and stiffness of it, which was induced by freezing-thawing. The experimental results indicate that about 11.9 tons of liquid nitrogen were consumed for 3.5 days to form a cylindrical frozen body with a volume of about $2.12m^3$. In addition, the strength and stiffness of the ground were reduced by 48.5% and 22.7%, respectively, after a freezing-thawing cycle.

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Fig. 1. Application of AGF method for tunneling construction

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Fig. 2. Plan view and test bed layout in Sinan-gun

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Fig. 3. Geologic profile of test bed evaluated by boring investigations

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Fig. 4. Measurement of thermal conductivity through QTM-500

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Fig. 5. Specific configuration of freezing pipe used in AGF field experiment

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Fig. 6. Injection process of liquid nitrogen and arrangement of freezing pipe and temperature hole

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Fig. 7. Temperature change with time at each position by AGF method

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Fig. 8. Frozen soil formation by liquid nitrogen flow through freezing pipe

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Fig. 9. Results of piezo cone penetration test

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Fig. 10. Soil classification based on results of cone penetration test (Robertson, 1990)

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Fig. 11. Components of soil structure (Angin et al., 2016)

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Fig. 12. Grain size distribution curve according to freezing-thawing

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Fig. 13. Results of lateral load test

Table 1. Summary of fundamental physical properties and consolidation parameter of test bed

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Table 2. Summary of laboratory thermal conductivity test

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Table 3. Soil classification according to freezing-thawing by Robertson’s chart (Robertson, 1990)

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Table 4. Fundamental physical properties according to freezing-thawing

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Table 5. Summary of lateral load test according to freezing-thawing

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Acknowledgement

Grant : 고수압 초장대 해저터널 기술 자립을 위한 핵심요소 기술개발

Supported by : 건설교통과학기술진흥원

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