**2. Rock mass classification system**

The most important factor when designing a tunnel is to define the geological conditions that the tunnel will pass through. Based on this definition, rock mass classification systems are made by determining the strength parameters of the rock mass and the geological conditions. With these methods, the units that the tunnel will pass through are classified providing detailed information before the tunnel project commences. Rock mass classification systems have been developed in detail since the 1940s, and this process has been refined by different researchers until the present day. A main one of these methods, developed by Terzaghi [1] in 1946 made a classification for steel supports depending on the rock load. In later processes, Lauffer [2] developed a classification based on unsupported standup time, Rabcewicz [3–5], Rabcewicz and Golser [6] introduced the principles of the New Austrian Tunnelling Method, Deere et al. [7] in 1967, the direction of Rock Quality Designation (RQD), and Wickham et al. [8] developed the Rock Structure Rating system. The Rock Mass Rating (RMR) system, which is one of the most well-known systems today, was developed by Bieniawski [9–11] and was last modified in 1989. Barton et al. [12–14] improved the rock classification system known as the Q system. In 1995, Palmstrom [15] developed the Rock Mass Index (RMI) system. All of these methods emerged as empirical studies in the field.

While making rock mass classifications, uniaxial compressive strength of intact rock, condition of discontinuities (spacing, orientation, roughness, etc.), in situ stresses, groundwater condition, tunnel size and rock quality designation are all evaluated. The biggest problem when working with rock mass classification systems is that they are made according to interpretations that vary from person to person, since they are empirical. While one researcher defines discontinuities in a particular way, another researcher may make different interpretations under the same conditions. In addition, incomplete data collection in studies is an important factor. Since the boreholes, from which the main input parameters are obtained, are often not done properly, problems are created in terms of the accuracy of the data. When drilling, the length of the run, sampling and the application of the correct drilling technique will directly affect the results. For this reason, although all rock mass classification systems contain very important and valuable information when describing the rock mass, the results should always be viewed with suspicion. No rock mass classification system specifies a definite support system for the design, it just gives a range. While designing, these support systems should be considered as an initial step in analytical and numerical solutions, and never be considered absolute truth.

### **2.1 Rock load theory**

Rock Load Theory, developed by Terzaghi [1] for sizing steel rib systems, started to be applied in railway tunnels opened in the United States in 1946. Terzaghi considered the pressure exerted by the loose soil (Hp) on the steel rib on the tunnel (**Figure 1**). For this purpose, Terzaghi [1] divided the rock mass into nine main categories. These categories range from solid-intact rock to swelling ground. He calculated the pressure coming to the steel supports depending on Hp Eq. (1). In **Table 1**, rock mass classes and rock load factors are given.

$$p = Hp\*\gamma\*H\tag{1}$$

#### **2.2 Stand-up time**

Lauffer [2] defined the relationship of stand-up time to rock mass quality depending on the tunnel diameter in unsupported condition. The rock mass is classified from very good rock (A) to very weak rock (G). Here, the unsupported period is 100 years in the A rock class, while it ranges from 1 minute to 10 minutes in the G rock grade (**Figure 2**). This classification is valid for a span of 5 m. Stand-up time indicates the time during which the tunnel remains stable in unsupported conditions. Lauffer applied the stand-up time approach to the Bieniawski RMR rock classification system.

#### **2.3 New Austrian tunnelling method (NATM)**

The principles of the New Austrian Tunnelling Method (NATM) were introduced by Rabcewicz [3–5] in the 1960s. The NATM method is based on the principle of increasing the carrying capacity of a mountain with a flexible outer belt by allowing deformations around the tunnel. It is divided into rock classes, from very solid rock to swelling soils. In subsequent years very serious criticisms were made of NATM and this process continues today.

According to NATM principles, the rock is divided into three main groups stable (A), friable (B) and stress failure, squeezing (C). These rock classes are again subdivided into subclasses (**Table 2**).

**Figure 1.** *Rock load concept [1].*


#### **Table 1.**

*Rock load in tunnels within various rock classes [1].*

**Figure 2.**

*Relationship between active span and stand-up time and rock mass classes [2].*

### **2.4 Rock quality designation**

Deere et al. [7] made a classification according to the core samples obtained from the borehole. Here rock quality is defined by dividing the core samples larger than 10 cm by


*Tunnels DOI: http://dx.doi.org/10.5772/intechopen.109608*

#### **Table 2.**

*NATM rock classes.*


#### **Table 3.**

*Rock mass quality classification classes [7].*

the run size. The RQD value remained as a preliminary approach but has continued to be used as one of the most important data inputs to the other developed rock classification systems. The RQD value is shown in Eq. (2). In the classification made according to the RQD value, the rock mass is divided into five main categories (**Table 3**).

*RQD* ¼ ð Þ *Total length core pieces* >10 *cm=total length of the core* ∗ 100% (2)

### **2.5 Rock structure rating (RSR)**

Wickham et al. [8] developed the RSR system based on their work in smalldiameter tunnels. They established a relationship between the parameters determined according to the geological conditions and the construction parameters. RSR value Eq. (3) is also given:

$$\text{RSR} = \text{A} + \text{B} + \text{C} \tag{3}$$

Parameter (A) depends on the rock hardness, geological structure and rock type origin.

Parameter (B) depends on the discontinuity pattern which derives from joint spacing, joint orientation and direction of the tunnel drive.

Parameter (C) depends on the joint condition and water inflow.

### **2.6 Rock mass rating system (RMR)**

Rock Mass Rating System is a method developed by Bieniawski [9–11] between 1973 and 1989 on the basis of mine galleries and road tunnels. Bieniawski classified the rock mass according to six parameters.


The RMR system was developed in line with the data obtained from horseshoe tunnels with a diameter of 5–12 m. In this method, there is a rating value for each parameter. The support system is determined according to the RMR score calculated according to these determined values (**Table 4**).
