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Tunnels in weak rock

Đào hầm trong đá yếu sẽ xảy ra một số thách thức đặc biệt để các kỹ sư địa kỹ thuật kể từ khi misjudgements trong thiết kế của các hệ thống hỗ trợ có thể dẫn đến thất bại rất tốn kém. Để hiểu được các vấn đề liên quan đến quá trình thiết kế hỗ trợ cho loại đường hầm đó là cần thiết để xem xét một số khái niệm rất cơ bản về cách thức một khối đá xung quanh một đường hầm biến dạng và làm thế nào các hệ thống hỗ trợ kiểm soát. | 12 Tunnels in weak rock 12.1 Introduction Tunnelling in weak rock presents some special challenges to the geotechnical engineer since misjudgements in the design of support systems can lead to very costly failures. In order to understand the issues involved in the process of designing support for this type of tunnel it is necessary to examine some very basic concepts of how a rock mass surrounding a tunnel deforms and how the support systems acts to control this deformation. Once these basic concepts have been explored examples of practical support designs for different conditions will be considered. 12.2 Deformation around an advancing tunnel Figure 12.1 shows the results of a three-dimensional finite element analysis of the deformation of the rock mass surrounding a circular tunnel advancing through a weak rock mass subjected to equal stresses in all directions. The plot shows displacement vectors in the rock mass as well as the shape of the deformed tunnel profile. Figure 12.2 gives a graphical summary or the most important features of this analysis. Deformation of the rock mass starts about one half a tunnel diameter ahead of the advancing face and reaches its maximum value about one and one half diameters behind the face. At the face position about one third of the total radial closure of the tunnel has already occurred and the tunnel face deforms inwards as illustrated in Figures 12.1 and 12.2. Whether or not these deformations induce stability problems in the tunnel depends upon the ratio of rock mass strength to the in situ stress level as will be demonstrated in the following pages. Note that it is assumed that deformation process described occurs immediately upon excavation of the face. This is a reasonable approximation for most tunnels in rock. The effects of time dependent deformations upon the performance of the tunnel and the design of the support system will be not be discussed in this chapter. 12.3 Tunnel deformation analysis In order to explore .