**2.8 Section conclusion**

The main purpose of all rock mass classification systems developed empirically is to describe the rock mass. There are many parameters as inputs in all studies.While making the evaluations, the comments that each engineer will make in the field may be different.

**Figure 3.** *Q excavation and support system chart [14].*


#### **Table 5.**

*Q support system details [14].*

Since these evaluations are empirical, they directly affect the determination of the rock class. Another important factor in rock classification is the evaluation of boreholes, which is one of the basic input parameters and perhaps the most important one. Consideration should be given to the sampling and run lengths required while drilling. Errors made during these processes will directly affect the design. Since the wrong calculation of the RQD value will be a direct input parameter in the rock mass parameters, borehole quality is extremely important. The number and location of drillings is another important factor. The insufficient number of boreholes will significantly increase the uncertainty in determining the tunnel rock classes. The uniaxial compression test or point load test, also appears as direct input parameters. During laboratory experiments, sampling, transportation or errors seriously affect the rock mass classification systems.

It is unwise to accept the results of the rock classification system as invariably correct, even when there are great uncertainties in determining the rock parameters.

Each of the researchers, on the other hand, aimed to make rock mass classifications according to the support systems that were successful in the field. Each method has its own limitations. Terzaghi [1] only offered suggestions for steel supports for tunnels excavated in solid rock. Bieniawski determined the intervals as 20 units in the determination of the RMR system. This situation often brings with it uncertainties. When the RMR value is calculated as 41, the rock class is medium rock, and when it is 39, it is weak rock. According to this situation, one should be very careful in recommending support. As stated, in cases where the rock mass parameters are purely empirical and relative, specifying and applying the support elements directly according to the determined RMR values may lead to wrong results.

Since similar limitations are valid for all rock mass classification systems, taking the rock mass classification systems as invariable correct will give extremely inaccurate results. Rock mass classification systems are, however, extremely important for the preliminary evaluation and preliminary design of the unit that the tunnel will pass through. In light of these obtained data, it is necessary to dimension the support systems with analytical and numerical methods.

Because the support systems determined according to all rock mass classification systems are given within a range a clear and precise sizing cannot be given. For this reason, these approaches are only a guide in terms of tunnel design.
