## 3 Applying the Methodology to the Example of Dabar Power Tunnel
### Instabilities around the Contour Leading to Overbreaks  
#### 3.2.1 Deterministic Analysis  
The results of deterministic evaluation are critically dependent on the shear strength parameters of discontinuities. According to the BB criterion [19] defined by Equations (3) and (4), the main factors affecting the shear strength of discontinuities are friction on the surface, compressive strength, geometry of the surface (roughness), and the presence of filling and/or water pressure. Figure 3: Longitudinal cross-section along tunnel axis in Cretaceous limestone with major geological units. A total of 10 samples (taken from the gallery) were tested to determine the shear strength along the rock joints. In addition, four samples from outcrops were taken for laboratory shear testing. However, after detailed evaluation, the outcrop data were excluded from further analysis as the rocks were heavily affected by weathering and were regarded as non-representative of the geological conditions in the vicinity of the tunnel. Laboratory investigations [28] to determine the shear strength along the discontinuities were conducted according to valid standards [29], as well as recommendations of the International Society for Rock Mechanics [30]. Prior to performing the tests, all joints and samples were inspected and characterised as appropriate. Joint compressive strength (JCS) was defined using a Schmidt hammer and Barton's profilometer to determine the joint roughness coefficient (JRC). Both parameters were used as control parameters in the evaluation of the shear strength envelope according to the BB criterion [19]. The direct shear test device (Hoek's shear box), which is shown in Figure 5, was used to determine the shear strength along the joints. The original design of the apparatus was adapted to accept samples extracted from drill cores with a diameter of up to 145 mm. The shear box consists of two halves: the upper connected to two arms for reversible shearing action and the lower connected to a ram for normal load application. The sample preparation, which is also presented in Figure 5, included engineering-geological inspection and evaluation of joints, measuring of the openings, and appropriate cementing in the mould. The shearing plane in all tested samples was placed parallel to the direction of the appliance of the tangential load. After the cement hardened, each sample was subjected to a single shear testing procedure in several steps. First, vertical load was applied, which was maintained as constant for the duration of the shearing, for which tangential load was applied in steps, with interval readings of applied load and displacement. Shearing was continued after peak shear strength was achieved to obtain the value of residual shear strength. The interpretation considered the applied load with respect to the area of the surface of the discontinuity for which shear strength parameters were tested. Figure 4: Initial TBM (round shape) and changed NATM tunnel cross-section of implemented tunnel support type. On the basis of the test results, the parameters for the BB criterion envelope [31] were evaluated with considerations for the peak shear stress of all tested samples using the RSData software package (Roccscience, Inc., Toronto, ON, Canada). The test results and failure envelope are shown in Figure 6, together with the envelope of shear strength scaled to the natural size of joints at dimensions observed in the tunnel (see Equation (5)). After the repeatability of the results was established and a detailed inspection of the discontinuities of the samples was conducted, it was concluded that the measured shearing resistance could be regarded as representative of most joints and joint systems in the rock mass. Due to the relatively high homogeneity of the Cretaceous limestone, it was further assumed that this would be valid for a given rock mass and required for the entire tunnel section within the realistic low margin of discrepancy.