Abstract:The traditional slope stability analysis used the Factor of Safety (FS) from the Limit Equilibrium Theory as the determinant. If the FS was greater than 1, it was considered as "safe" and variables or parameters of uncertainty in the analysis model were not considered. The objective of research was to analyze the stability of natural slope, in consideration of characteristics of rock layers and the variability of pre-stressing force. By sensitivity and uncertainty analysis, the result showed the sensitivity for pre-stressing force of rock anchor was significantly smaller than the cohesive (c) of rock layer and the varying influence of the friction angle (φ) in rock layers. In addition, the immersion by water at the natural slope would weaken the rock layers, in which the cohesion c was reduced to 6 kPa and the friction angle φ was decreased below 14 • , and it started to show instability and failure in the balance as FS became smaller than 1. The failure rate to the slope could be as high as 50%. By stabilizing with a rock anchor, the failure rate could be reduced below 3%, greatly improving the stability and the reliability of the slope.

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A case study on an existing tunnel was investigated in Taipei city. Ground improvement was carried out prior to the excavation of a new underpass crossing over the tunnel. Monitoring systems such as strain gauges and convergent points were set up for the tunnel as excavation for the underpass might cause heaving of the tunnel. The real-time monitoring system has proved to be effective in warning and preventing disaster from happening during the excavation process. Finite difference code FLAC was used to simulate the whole construction sequence and good agreement was obtained between the numerical and field measured results.