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Research Paper

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

인공동결공법 적용 후 융해에 따른 해성 점토지반의 역학적 특성 평가

Hyun-Jun Choi1
,
Dongseop Lee2
,
Hyobum Lee3
,
Young-Jin Son4
,
Hangseok Choi5*
최 현준1
,
이 동섭2
,
이 효범3
,
손 영진4
,
최 항석5*
1Assistant Manager, Research Institute, Dongmyeong Engineering Consultants & Architecture Co., Ltd.
2Post-doctoral Research Fellow, School of Civil, Environmental and Architectural Engineering, Korea University
3Ph.D. Student, School of Civil, Environmental and Architectural Engineering, Korea University
4General Manager, Infra Engineering Team 2, SK Engineering & Construction
5Professor, School of Civil, Environmental and Architectural Engineering, Korea University
1비회원, (주)동명기술공단 부설연구소 대리
2비회원, 고려대학교 건축사회환경공학부 박사 후 연구원
3비회원, 고려대학교 건축사회환경공학부 박사과정
4정회원, SK건설 Infra Eng’g2팀 부장
5정회원, 고려대학교 건축사회환경공학부 교수
* Corresponding Author
31 January 2019. pp. 31-48
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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 2.12 m3. In addition, the strength and stiffness of the ground were reduced by 48.5% and 22.7%, respectively, after a freezing-thawing cycle.

Keywords
Artificial ground freezing method
Freezing-thawing
Marine clay
Piezo cone penetration test
Lateral load test

인공동결공법(artificial ground freezing method)은 연약지반 및 도심지에서의 지하구조물 시공에 적합한 차수 및 지반보강 공법이다. 인공동결공법은 동결관(freezing pipe)을 지중에 매설한 후 냉매(refregerant)를 순환시켜 대상 지반에 차수벽 및 지지체의 역할을 수행하는 동결벽체(frozen wall)를 형성한다. 그러나 간극수의 동결에 따른 간극수의 부피팽창은 지반의 변형을 야기시킬 수 있고, 시공완료 후 동결토의 융해에 따른 지반의 소성변형 및 입자의 재배치 등은 지반의 역학적 특성을 변화시킨다. 본 논문에서는 인공동결공법에 따른 해성 점토지반(marine clay)의 동결속도를 평가하기 위하여 인공동결공법 현장실증시험을 수행하였다. 현장실증시험은 지중에 3.2 m 깊이로 매설된 동결관 1공 내로 초저온 냉매인 액화질소를 순환시키는 방법으로 수행되었다. 또한, 원지반과 인공동결공법에 의해 동결/융해된 지반에 대한 피에조 콘 관입시험(piezo cone penetration test, CPTu) 및 공내재하시험(lateral load test, LLT)을 수행함으로써 동결/융해(freezing-thawing)에 따른 해성 점토지반의 강도 및 강성 특성의 변화를 평가하였다. 시험결과, 부피가 약 2.12 m3인 원기둥 모양의 동결체를 형성하는데 총 3.5일이 동안 약 11.9 ton의 액화질소가 소요되었다. 동결/융해에 따른 지반의 강도 및 강성 저하는 각각 48.5%, 22.7%로 산정되었다.

키워드
인공동결공법
동결/융해
해성 점토지반
콘 관입시험
공내재하시험
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Information
  • Publisher :Korean Tunneling and Underground Space Association
  • Publisher(Ko) :한국터널지하공간학회
  • Journal Title :Journal of Korean Tunnelling and Underground Space Association
  • Journal Title(Ko) :한국터널지하공간학회 논문집
  • Volume : 21
  • No :1
  • Pages :31-48
  • Received Date : 2018-10-04
  • Revised Date : 2018-10-23
  • Accepted Date : 2018-10-26
  • DOI :https://doi.org/10.9711/KTAJ.2019.21.1.031
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