Assessment of Discontinuities-Induced Overbreak by Tunnel Advancement using Discrete Fracture Network (Case study: Alborz Tunnel, Iran)

Discontinuities are known as a primary factor in instability of tunnels and underground excavations. To prevent potential damage and overbreak by underground advancement, it is essential to provide a model, which considers both the geometrical and mechanical characteristics of discontinuities. Discrete Fracture Network (DFN) is a conceptual model to represent and analyse the complex system of discontinuities within the rock mass. Combined DFN with analytical or numerical methods can be employed as a scientific tool to analyse generated rock blocks, and their stabilities under different loading conditions. This paper aims to investigate the created overbreak by tunnel advancement in the Alborz tunnel located in the Tehran-North freeway in Iran. First, the geometrical characteristics of discontinuities were surveyed by tunnel advancement in ۲۰۰ meters. Four major joint sets and a bedding plane were identified and their statistical characteristics were measured. The DFN model was generated and its validity was investigated through a comparison against field data. The average volume of generated blocks in the studied area was measured ۰.۲۲ m۳. The stability of generated blocks around the opening was kinematically evaluated. The volume of formed blocks around tunnel in the DFN model prone to instability due to static or dynamic loads was estimated ۲۶۰۵ m۳ while the measured overbreak in field was ۲۷۳۵ m۳. The depth of overbreak in DFN model showed a good agreement with field measurements.  The results show that DFN model combined with kinematic stability analysis can provide a scientific tool to investigate geological overbreak in underground excavations.

کلیدواژه ها:

Discrete Fracture Network ، Tunnel advancement ، Overbreak ، Kinematic stability analysis ، Alborz tunnel