Experimental study on the relaxation zone depending on the width and distance of the weak zone existing ahead of tunnel face

터널 굴진면 전방에 위치한 연약대 폭과 이격거리에 따른 이완영역에 대한 실험적 연구

  • Ham, Hyeon Su (Dept. of Civil and Transportation Engineering, Ajou University) ;
  • Lee, Sang Duk (Dept. of Civil System Engineering, Ajou University)
  • 함현수 (아주대학교 일반대학원 건설교통공학과) ;
  • 이상덕 (아주대학교 건설시스템공학과)
  • Received : 2018.07.12
  • Accepted : 2018.08.09
  • Published : 2018.09.30


When a weak zone exists ahead of tunnel face, the stress in the adjacent area would increase due to the longitudinal arching effect and the stability of the tunnel is affected. Therefore, it is critical to prepare a countermeasure through the investigation of the frontal weakness zone of the excavated face. Although there are several researches to predict the existence of weak zone ahead of tunnel face, such as geophysical exploration, numerical analysis and tunnel support, lack of studies on the relaxation zone depending on the width or distance from the vulnerable area. In this study, the impact of the weak zone on the formation of the relaxation zone was investigated. For this purpose, a series of laboratory test were carried out varying the width of the weak zone and the separation distance between tunnel face and weak zone. In the model test, sand with a water content of 3.8% was used to form a model ground. The model weak zone was constructed with dry sand curtains. The tunnel face was adjusted to allow a sequential excavation of upper and lower half part. load cells were installed on the bottom of the foundation and the tunnel face and measuring instruments for displacement were installed on the surface of the model ground to measure the vertical stress and surface displacements due to tunnel excavation respectively. The test results show that the width of weak zone did not affect the ground settlement while the ground subsidence drastically increased within 0.25D. The vertical stress and horizontal stress increased from 0.5D or less. In addition, the longitudinal arching effect is likely within the 1.0D zone ahead of the tunnel face, which may reduce the vertical stress in the ground following tunneling direction.


  1. Bang, J.H., Kim, K.Y., Jong, Y.H. (2007), "Development of 3D absolute displacement monitoring system and its application at the stage of tunnel construction", Journal of Korean Tunnelling and Underground Space Association, Vol. 9, No. 3, pp. 229-240.
  2. ITA Working Group No.2 Report (2011), Monitoring and Control in Tunnel Construction, pp. 23.
  3. ITA-CET (2009). Training Course Material - Tunnelling in Hot Climate Country, Monitoring of Tunnels, Riyadh.
  4. Kim, Y.S., Kim, C.D., Jung, Y.C., Lee, J.S., You, K.H. (2004), "A study on analysis method for the prediction of changes in ground condition ahead of the tunnel face", Journal of Korean Tunnelling and Underground Space Association, Vol. 6, No. 1, pp. 71-83.
  5. Kwon, O.Y., Choi, Y.K., Lee, S.D., Kim, Y.G. (2004), "Longitudinal arching characteristics around the face of a soil-tunnel with crown and face-reinforcement", Journal of the Korean Geotechnical Society, Vol. 20, No. 9, pp. 133-144.
  6. Lee, S.D. (2017), Soil mechanics, CIR Publication, Seoul, pp. 77-92.
  7. Lee, S.D. (2013), Tunnel mechanics, CIR publication, Seoul, pp. 826.
  8. Ribeiro e Sousa, L. (2004), "Learning with accident and damage associated to underground works", Geotechnical Risk in Rock Tunnels, Campos e Matos, A., Ribeiro e Sousa, L, Kleberger, J., and Lopes Pinto, P. (Editors), Talyor & Francis.
  9. Schubert, W., Steindorfer, A. (1996), "Selective displacement monitoring during tunnel excavation", Felsbau, Vol. 14, No. 2, pp. 93-98.
  10. You, C.S., Park, J.G. (2014), "Deformation behavior of tunnels crossing weak zone during excavation - numerical investigation", Journal of Korean Tunnelling and Underground Space Association, Vol. 16, No. 4, pp.373-386.