1) Compressive strength of intact rock

The value of this parameter was determined based on the strength characteristic of outcropped masses and on performing tests.

2) Rock quality designation

The rating of 8 may be given depending on the state of the discontinuity system.

3) Joint spacing

The rating for this parameter was the least value of spacing. In the study region, there were three sets of discontinuities (two sets of dominant discontinuities together with bedding), and the minimum discontinuity spacing was less than 10 centimeters. Considering the geomechanical classification parameters, the rating of 8 - 10 was given to this parameter.

4) Condition of discontinuities

The joints in the rock masses in unit one of the Atamir Formation, K (at) 1, were 10 - 20 meters long, clay, calcite, and few traces of iron oxide were the main filling, the joints were smooth to a little rough and, with respect to euhidrality, the rock masses were planar and moderately weathered. Considering the existing parameters, the rating given to this parameter was 10.

5) Groundwater

Presence of groundwater in a region is undoubtedly very important, especially if permeability is high. Considering the conditions of the rock masses in the region, the seepage state was determined and given the rating of 4.

6) Discontinuity orientations

This parameter is determined by the mode of transgression of the structure towards the jointed rock mass, by the dips of the layers (and of the discontinuities), and by the orientation of the structure (which has been used by Feenstra & Wickham in 1975 [1] and suggested by Bieniawski 1973) [2] .

The presented parameters 2 - 1 to 2 - 5 are used for determining the value of RMR that, for the rock masses in unit one of the Atamir Formation, K (at) 1, was 34 - 36. Based on this, these rock masses were placed in the poor class; pertaining data are shown in Table 3.

The index Q for the rock masses in unit one of the Atamir Formation, K9 (at) 1, was estimated based on the following 6 parameters.

1) Rock quality designation (RQD)

Considering the condition of the rock masses, the value of 43 - 48 was given to the RQD parameter of unit one in the Atamir Formation, K (at) 1.

2) Number of joint sets (J_n)

Based on the data obtained from the geological field surveys, this parameter received the value of 12 for unit one of the Atamir Formation, K (at) 1.

3) Discontinuity surface roughness (J_r)

This parameter shows the degree of roughness of the discontinuities and, based on Barton’s theory, the weakest set of critical discontinuities or discontinuities filled with clay materials in the related zone must be determined. Considering the conditions, the value of one was given to this parameter for unit one of the Atamir Formation, K (at) 1.

4) Joint alteration number (J_a)

Considering the joint filling and the apparent conditions of discontinuity surfaces, this parameter received the value of 4 for unit one of the Atamir Formation, K (at) 1.

5) Joint water reduction factor (J_w)

This parameter measures water pressure, which inversely affects shear strength of the discontinuity. Based on data collected in the study region, the value of 0.66 was considered for this parameter.

6) Stress reduction factor (SRF)

This parameter is determined based on the characteristics of the study region including the existing stresses, the tectonic conditions, the shear zones, etc. Since the necessary tests were not performed, the estimated value of one was used for this factor.

Considering the ratings of the six parameters, and using the presented relationship, the expected value determined for the Q index was 0.592 - 0.66 for the rock masses in unit one of the Atamir Formation, K (at) 1, which places them in the class of very poor rocks. Data are shown in Table 4.

Using the following three methods, the value of the geological strength index for the rock masses in unit one was determined.

1) Geological strength index of the rock mass (GSI) (Figure 8)

The GSI system is the only classification system that is directly related to engineering parameters such as Mohr-Coulomb and Hoek-Brown parameters. Conventional methods for determining strength parameters and rock mass modulus in Iran and other countries include plate-loading tests and block shear in situ. However, these tests can be performed only if exploration drilling has been carried out and, in addition to that, they are very costly. Therefore, many studies have been conducted and numerous attempts made for developing and introducing methods to indirectly determine engineering parameters of rock masses, one of which is the GIS system [3] . In the following sections, the GSI of rock masses in unit one will be discussed.

2) Determining GSI based on field observations

Based on field studies and observations, the value of geological strength index of rock masses in unit one of the Atamir Formation, K (at) 1 was 35 - 45.

To determine these parameters, the GSI was considered 35 - 45. Since cohesive strength and angle of internal friction are among the input parameters in most methods of analysis, the Mohr-Coulomb and Hoek-Brown criteria had to be fitted to determine these two parameters. Results are listed in Table 5 and shown in Figures 9-11.

Analysis of wedge instability was performed using SWEDGE. This software, which was developed by the ROCSCIENCE Company, analyzes wedge stability in rock slopes, degree of cohesion, geomechanical characteristics, etc., studies stability from various aspects, analyzes areas with rock fall potential using the StereoNet network, follows discontinuities and determines and analyzes intersection points of wedges, and plots the StereoNet using input data. Figure 12 shows stereographic images of the joint sets. Applying the primary parameters, the characteristics of the largest wedges that can be formed in the walls are determined. According to the performed analysis, wedge slide will happen along the interface between two discontinuities, and the factor of safety (FS) will be 0.07743. Figures 13-18 show other views.

Although water is not directly involved in mass (slope) movement, it is an important factor because:

1) Increased water content of slopes caused by rain and snowmelt makes them heavy. Water can penetrate into pores and fractures and replace air and, since water is heavier than air, weights of rocks increase. Because weight is the same as force, and force applies stress on surfaces, increased stress can cause instability.

2) Water can alter slope angle (angle of the slope at which the slope is stable).

3) Water can be absorbed or repelled by soil mineral substances. Water absorption causes the electrical poles of water molecules to stick to the surfaces of mineral materials and penetrate into them. Therefore, increased water content makes rocks heavier, leading to reduced rock strength. In general, wet clay soils have less strength compared to dry clay soils. Therefore, water absorption reduces strength and, since clay is the material that fills the spaces between the joints in this unit, increased water content gains more importance.

4) Groundwater is present almost everywhere under the surface of the ground. This water fills the empty spaces between rock particles and even the cracks within rocks. Groundwater level changes due to rainfall and rises in wet seasons leading to greater water penetration. In dry seasons, groundwater level falls causing less water penetration. These changes in groundwater levels can be an important factor in slope stability.

If water penetrates through joint surfaces, the factor of safety will become zero (FS = 0) (Figure 19).