Journal of the Korean Geotechnical Society (한국지반공학회논문집)

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Effect of Groundwater Flow on Ice-wall Integrity

얼음벽 형성에 대한 지하수 흐름의 영향

  • 신호성 (울산대학교 건설환경공학부) ;
  • 김진욱 (울산대학교 건설환경공학부) ;
  • 이장근 (한국건설기술연구원)
  • Received : 2018.10.08
  • Accepted : 2018.11.12
  • Published : 2018.11.30
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Abstract

AGF (Artificial Ground Freezing) method is a temporary ground improvement method which can apply to all types of soil with the purpose of high stiffness and low hydraulic conductivity. However, the groundwater flow and the heterogeneity of the ground increase the uncertainty of the ice-column formation which hinders the reliability of this method. The effects of groundwater flow and layered heterogeneity on ice-wall integrity by AGF method were analyzed using finite element analysis program for a coupled thermo-hydro phenomena in the freezing ground. Groundwater flow changes circular ice-column into elliptical shapes and increases the time required for the formation of ice walls. The previous theoretical formula overestimated the completion time of the ice wall and the critical groundwater velocity by neglecting the thermal interaction between adjacent ice-columns. Numerical results presented the corrected formula and verified the proposed equation for the dimensionless ice-wall completion time. In the layered heterogeneous ground, the thickness of the layer with higher hydraulic conductivity and its relative magnitude were found to be important factors in the ice-wall completion time and critical velocity.

인공동결공법은 일시적으로 지반의 강성을 높이고 투수계수를 낮추는 지반개량공법으로 지반에 적용가능하다. 하지만, 지하수 흐름과 지반의 불균질성은 동결구근 형성을 불확실하게 하여 공법에 대한 신뢰성을 저해한다. 동결지반 대한 열-수리 유한요소 해석 프로그램을 이용하여, 인공동결공법에서 지하수 흐름속도와 지반의 층상 비균질이 얼음벽 형성을 미치는 영향을 분석하였다. 지하수의 흐름은 원형의 동결구근을 원형에서 타원형을 변형시키며 얼음벽의 완성 소요시간을 증가시킨다. 기존의 이론식은 인접 동결구근의 열적 상호작용을 무시하여, 얼음벽의 완결시간과 한계유속을 과대 평가하였다. 수치해석 결과를 바탕으로 수정식을 제시하였으며 무차원 얼음벽 완결시간에 대한 제안식을 검증하였다. 층상의 비균질 지반에서 투수계수가 큰 지층의 두께와 상대적인 투수계수비는 얼음벽 완결시간과 한계 유속에 중요한 인자인 것으로 나타났다.

Keywords

GJBGC4_2018_v34n11_43_f0001.png 이미지

Fig. 2. Ice column growth under no groundwater flow (u=0m/s) in the GWS (granite weathered soil). (a) Temperature distribution during freezing using brine (Tpipe = -30℃, freeze-pipe spacing S=1m), (b) Ice saturation around freeze-pipes

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Fig. 3. Ice wall closure time tclose,u=0 with adjacent pipes arranged in a row in the absence of groundwater flow (u=0m/s) (blue circle = numerical study; red-dot line = Eq. (6) (Sanger and Sayles, 1979); black-solid line: modified S&S equation (Eq. (7))

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Fig 4. Ice column growth under groundwater velocity u=0.17m/s (top→bottom). (a) Temperature distribution over time during freezing using brine (Tpipe = -30℃, freeze-pipe spacing S=1m), (b) Ice saturation around freeze-pipes

GJBGC4_2018_v34n11_43_f0004.png 이미지

Fig. 5. Ice column growth under groundwater velocity u=0.35m/s (top→bottom). (a) Temperature distribution over time during freezing using brine (Tpipe = -30℃, freeze-pipe spacing S=1m), (b) Ice saturation around freeze-pipes

GJBGC4_2018_v34n11_43_f0005.png 이미지

Fig. 6. Ice column growth under groundwater velocity u=0.37m/s (top→bottom). (a) Temperature distribution over time during freezing using brine (Tpipe = -30℃, freeze-pipe spacing S=1m), (b) Ice saturation around freeze-pipes

GJBGC4_2018_v34n11_43_f0006.png 이미지

Fig. 8. Comparison of critical groundwater velocity for ice wall integrity: blue circle = numerical study; red-dot line = Sanger and Sayles (1979); black-solid line = modified S&S equation (Eq. 9) (dpipe=10cm)

GJBGC4_2018_v34n11_43_f0007.png 이미지

Fig. 9. Comparison of ice-wall closure time between Eq. (10) and numerical results

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Fig. 10. Non-uniform ice column in the heterogeneous soil with groundwater flow (Andersland and Ladanyi, 2004)

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Fig. 11. Evolution of Ice column growth (blue, boundary of T=0℃) and temperature distribution (red plane) with no groundwater flow (i=0)

GJBGC4_2018_v34n11_43_f0010.png 이미지

Fig. 12. Evolution of Ice column growth (blue, boundary of T=0℃) and temperature distribution (red plane) under groundwater hydraulic gradient i=0.045 (thickness of yellow line represents relative groundwater velocity)

GJBGC4_2018_v34n11_43_f0011.png 이미지

Fig. 13. Comparison of ice-wall closure time between single homogenous systems (“Sand”, “WGS”) and WGS-Sand-WGS layered system (“Layer”)

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Fig. 1. (a) Temperature field around the tunnels from AGF with groundwater flow (Pimentel et al., 2012), (b) Effect of more permeable layer around the frozen body under groundwater flow (Sres, 2009)

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Fig. 7. (a) Effect of groundwater velocity (u) and freeze-pipe spacing (S) rate on ice wall formation time (dpipe=10cm). (b) Relationship between groundwater flow (u) and freeze-pipe diameter (dpipe) for ice wall formation time (S=1.0m)

Table 1. Soil properties for numerical analysis

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