Failure area change estimation and tunnel face pressure characteristics of large soil-covering tunnel tests

Estimating the tunnel face earth pressure is essential for determining the limit support pressure in shield tunneling and for urban mountain tunnels in Japan. To meet these demands, limit equilibrium methods for calculating tunnel face pressures remain extremely important methods of calculation in tunneling practice. In limit equilibrium calculations, the appropriate setting of the failure zone prism affects the accuracy. It is crucial to properly account for changes in the height of the failure zone prism due to the frictional resistance of the ground. In this study, an improved simple method is proposed for estimating the height of the failure zone prism. The method utilizes a versatile logistic equation to describe the relationship between the soil cover and the prism height. The proposed method is a model in which the height of the failure zone prism converges to a constant value in the logistic equation as the soil cover increases, indicating an increase in frictional resistance. To validate the proposed method, tunnel tests with large soil cover ratios up to C / D = 10 in a 1 g field are conducted, and changes in the failure zone prism and the nature of the soil pressure are determined. As a result of the tunnel tests, the width of the failure zone can be approximately expressed by the width of the wedge derived under the condition of active earth pressure. The face pressure at the initial stage of tunnel pulling in the tunnel tests is proportional to the soil cover pressure when the soil cover is small. On the other hand, as the soil cover increases, it shows a value between the earth pressure at rest and the active earth pressure. This is thought to be due to the fact that the frictional resistance of the ground increases in proportion to the soil cover. Furthermore, the height of the failure zone prism, identified by a PIV (particle image velocimetry) analysis, can be adequately represented by the proposed method. An example of how the proposed equation can be incorporated into a simple limit equilibrium calculation is described, and it is shown that the results of this calculation can properly evaluate the experimental results.