地下水圧および飽和度による粘着力の変化を考慮した切羽安定評価法

Abstract

A ground water has a great influence on the tunnel face stability. This paper describes an evaluation method of tunnel face stability affected by ground water pressure and variation of sandy layer's cohesion due to a degree of saturation. A rigid plastic finite element analysis (RPFEA) was applied to simulate the face collapse in the study. The method pays attention only to the plastic state and uses the upper bound theorem. Body forces in the analysis were self-weight of soil and water-pressure. The subject is a railway tunnel of Tohoku-Shinkansen, constructed in soft ground by the shotcrete tunneling method, or the currently so-called NATM. At the construction site, the ground water level is high throughout the whole section, and the ground near the tunnel face is sandy soils with a clayey layer, whose position at the face varies by the face advance. The ground water flows both in the lower and upper sandy layers separated by the clayey layer. Therefore, deep well method was applied to lower the ground water level, since the ground water level normally has significant influence on the stability of the tunnel face. However, it is difficult to lower the ground water level in the upper sandy layer. Therefore, the authors analyzed the tunnel face stability by measurements of the groundwater level, results of face observation and RPFEA. As a result, it has been known that the face stability is affected strongly by value of soil cohesion c and it is necessary to estimate strictly the value of soil cohesion c. For this reason, the authors studied on the relation between degree of saturation and cohesion of sandy layer, and applied this relation to the evaluation of tunnel face stability. Furthermore, they developed the indicator for selection of auxiliary method to stabilize the tunnel face by the evaluation method.