Abstract
Face collapse due to water seeping into sandy ground constitutes a big factor to decrease the safety of tunnel construction. This paper reports a case study of accumulated tunnel construction data to develop a new method of estimating the tunnel face stability. In the case study, I investigated the conditions that determine the occurrence of wash-out, factors interrelated with water inflow and physical properties of the face, in a multivariate analysis. The results of the analysis are as follows.
1. The critical amount of water inflow (Q) at the tunnel face, to cause a wash-out face is 200 to 300l/min. When Q is over 500l/min, wash-out invariably occurs no matter how favorable other indices are.
2. Physical properties closely related with the face stability include the fine grain content, uniformity coefficient, specific granular gravity of soil and 60% grain diameter.
Although these factors are closely related to the face stability, they don't provide the degree of face stability. This is because the face wash-out must be considered to occur when the power of ground water penetrating into the tunnel inner section exceeds the power of the tunnel face holding its integrity, or the ability that depends essentially on the conditions of ground. This means that the integrity of face should be evaluated in terms of the balance between these two types of power.
