A proposal for simple design method of the reinforcement of elephant foot for tunnelling in the soft ground

Jaehwan Lee; Ju-Young Oh; Seokbue Chang

doi:10.9711/KTAJ.2026.28.3.245

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2026 Vol.28, Issue 3 Preview Page

Technical Paper

A proposal for simple design method of the reinforcement of elephant foot for tunnelling in the soft ground 토사터널의 숏크리트 단부 보강에 대한 간편 설계방법 제안

31 May 2026. pp. 245-266

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Abstract

It is necessary to take care of tunnelling in the soft ground since severe tunnel deformation occurs due to the settlement of the elephant foot (enlarged foot of shotcrete), leading to the collapse of the tunnel, causing damage to life and property. The primary reason for tunnel displacement is that the bearing capacity on the elephant foot of shotcrete is insufficient during the upper excavation and at the beginning stage of the lower excavation stage. In this case, reinforcement design is required to make sure of sufficient support of the elephant foot, but so far, detailed design method that can be applied to design practice has not been presented. Therefore, in this study, the design method from the design load to the bearing capacity calculation according to the size of the elephant foot and reinforcement of leg pile was proposed. Accordingly, sensitivity analyses were performed based on factors such as design load, pile drilling diameter and ground conditions etc., and the effect on the leg pile design was confirmed through the simple design method. There are some assumptions to make the proposed design method. That is why additional research like numerical analysis or field test on whether it is suitable is needed. However, it is judged that the uncertainty is somewhat reduced through a conservative approach, and the proposed simple design method is considered a meaningful study in that it focuses on formulas that can be effectively integrated into practical reinforcement design process for the elephant foot in tunnelling in the soft ground.

Keywords

Tunnel

Soft ground

Elephant foot (enlarged foot of shotcrete)

Simple design method

Leg pile

토사지반에서의 터널은 천장부에 대한 사전보강에도 불구하고 상반 굴착 및 하반 굴착의 초기 단계에서 숏크리트 단부의 지지력이 부족하면 단부의 침하가 발생할 수 있다. 이러한 침하는 터널 변상으로 이어지며, 심한 경우 터널 붕괴를 초래하여 인명 및 재산 피해를 유발할 수 있다. 따라서 숏크리트 단부의 지지력 확보를 위한 보강설계가 필요하나, 설계실무에 직접 적용할 수 있는 구체적인 설계방법은 아직 충분히 제시되어 있지 않은 실정이다. 이에 본 연구에서는 토사터널 숏크리트 단부 보강을 위한 간편 설계방법을 제안하였다. 제안된 방법은 설계하중 산정, 숏크리트 단부의 확폭에 따른 지지력 평가, 그리고 leg pile 보강을 위한 허용 하중 산정 절차로 구성하였다. 또한 터널 토피고에 따른 이완하중 산정 방법을 수치해석과 비교하였으며, pile 천공경과 지반조건 등의 변수에 대한 민감도 분석을 수행하여 각 인자가 leg pile 설계에 미치는 영향을 검토하였다. 본 연구에서 제안한 설계방법은 일부 가정과 불확실성을 포함하므로, 향후 수치해석 또는 현장시험 등을 통한 추가 검증이 필요하다. 하지만, 보수적인 접근 방법을 적용함으로써 설계상의 불확실성을 일정 부분 줄인 것으로 판단되며, 제안한 간편 설계방법은 토사터널 숏크리트 단부 보강설계 실무에 활용할 수 있는 수식을 체계적으로 정리하였다는 점에서 의의가 있다고 사료된다.

키워드

터널

토사지반

숏크리트 단부

간편 설계방법

Leg pile

References

Chang, S.B., Moon, H.K. (1998), “A study on the quantitative evaluation of the load distribution factors considering the design conditions of tunnel especially for the ring-cut excavation method”, Magazine of Korea Geotechnical Society, Vol. 14, No. 5, pp. 5-15.

Cho, C.H. (2010), Piling Engineering Practice, ENG Book, Seoul, pp. 99-103, 665-690.

Das, B.M. (1984), Principles of Foundation Engineering, Brooks/Cole Engineering Division, Monterey, CA, pp. 101-107.

FHWA (2005), Micropile design and construction reference manual, Publication No. FHWA-NHI-05-039, Federal Highway Administration, U.S. Department of Transportation, Washington, D.C., pp. 5-1~5-85, 5-22.

Kim, N.Y., Park, G.T., Baek, S.C., Lee, K.H., Choi, J.W., Her, Y. (2017), “Analysis of displacement behavior in fractured fault and groundwater flow under tunnel excavation”, Journal of Korean Tunnelling and Underground Space Association, Vol. 19, No. 1, pp. 71-82.

10.9711/KTAJ.2017.19.1.071

Lee, G.Y., Oh, H.M., Cho, K.H., Oh, J.H., Kim, J.J., Kim, Y.S. (2018), “A case study on the effect of elephant foot method considering the rate of changes in tunnel cross section”, Journal of the Korean Geosynthetics Society, Vol. 17, No. 1, pp. 111-118.

10.12814/JKGSS.2018.17.1.111

Lee, G.Y., Oh, J.H., Cho, K.H., Lee, D.S. (2016), “A case study on elephant foot method for railway tunnelling in large fault zone”, Journal of the Korean Society of Civil Engineers, Vol. 36, No. 6, pp. 1161-1167.

10.12652/Ksce.2016.36.6.1161

Moon, C.Y. (2014), “A study on displacement of tunnel in the brittle facture zone under excavation construction”, Journal of the Korean Geo-Environmental Society, Vol. 15, No. 2, pp. 45-52.

10.14481/jkges.2014.15.2.45

Park, C.M., Lee, H., Park, J.H., Yoon, C.K., Hwang, J.D. (2009), “A case study on elephant foot method for tunnelling in the soft ground”, Proceedings of the International Symposium on Urban Geotechnics, Incheon, pp. 863-874.

Rose, D. (1982), “Revising Terzaghi’s tunnel rock load coefficients”, Proceedings of the 23rd US Symposium on Rock Mechanics, Berkeley, California, pp. 953-960.

Terzaghi, K. (1943), Theoretical Soil Mechanics, John Wiley & Sons, New York, pp. 66-76.

10.1002/9780470172766

Terzaghi, K., Peck, R.B., Mesri, G. (1996), Soil Mechanics in Engineering Practice (3rd Edition), John Wiley & Sons, New York, pp. 313.

Tomlinson, M.J. (1957), “The adhesion of piles driven in clay soils”, Proceedings of the 4th International Conference on Soil Mechanics and Foundation Engineering (ICSMFE), Vol. 2, London, pp. 66-71.

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 : 28
No :3
Pages :245-266
Received Date : 2026-04-10
Revised Date : 2026-04-30
Accepted Date : 2026-04-30
DOI :https://doi.org/10.9711/KTAJ.2026.28.3.245

Journal of Korean Tunnelling and Underground Space Association ISSN:2233-8292(Print) 2287-4747(Online) 한국터널지하공간학회 논문집

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