This paper presents results of an investigation which was carried out in order to analyze the stability of two tunnels. The tunnels were excavated in jointed rock masses of Jahrom formation. Due to structural feature and geology of the area the distinct element method was used to analyze the stability of tunnels. The code utilized according to this method was UDEC. The results gained from the numerical analysis show that the tunnel with smaller span, no major block fall occurs and hence a thin layer of shotcrete stabilizes the tunnel. However block falls and larger displacements were evident in the tunnel with larger span. The stability of the latter tunnel is achieved by using rock bolts in the sidewalls and a thin layer of shotcrete in the roof and sidewalls.


When an underground structure is excavated in jointed rock masses, instability occurs as a result of roof fall or wall failures. The presence of discontinuities such as faults, joints and bedding planes in the rock mass dominates the response or jointed rock masses to loading. Hence the stability analysis of underground structures being driven in jointed rock mass differs greatly from those of intact rock. Numerical modeling in rock mechanics has developed significantly in recent years (Pande1990). There are various numerical methods available whichever has its own applicabilities regarding the rock mass and discontinuities encountered in underground structures. One of these methods is the distinct element method which as been developed for deformation and stability analysis of multiply jointed rock masses around underground structures. The two dimensional distinct element computer code was used to analyze the stability or tunnels being driven in jointed rack masses of Jahrom formation.


The tunnels are driven in the jointed rock masses of Jahrom formation which is situated south west of Zagross mountains in Iran. The main rocks encountered in this formation are Limestone, marly limestone and dolomitic limestone. The rock mass surrounding the tunnels are divided into two parts: part A(50 meters in thickness) consisting of marly limestone and part B (220m in thickness) consisting of limestone and dolomitic limestone. Field investigations were carried out and the data were then analyzed by the Dips soil ware. The analyzed data show that here are two joint sets and a bedding plane in the area. Table I sows the dip and dip directions of these joint sets.


The Universal Distinct Element Code (UDEC, Itasca 1992) is a numerical program where the rock mass is presented as an assemblage of distinct blocks having joints between blocks. Each block can be modeled as either rigid or deformable materials. The rigid blocks represents the medium as a set of distinct blocks which do not change their geometry as a result of applied loading. Deformable blocks are internally discretized into finite difference elements, whichever element behaves according to a prescribed linear or non-linear stress-strain constitutive relationship.

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