**2.7 Tunnelling quality index (Q)**

Barton et al. [12–14] proposed a Tunnelling Quality Index (Q) system for underground structures. The Q method was developed in Scandinavia in 212 tunnels


**Table 4.**

*Guidelines for excavation and support of 10 m span rock tunnels in accordance with the RMR system [11].*

excavated in solid rock. The Q value is logarithmic, ranging from 0.001 to 100. According to the Q value, the rock mass is divided into nine main categories between exceptionally good and exceptionally poor. The Q value depends on a total of six parameters. These are;

RQD: Rock quality designation. Jn: Joint set number. Jr: Joint roughness number. Ja: Joint alteration number. Jw: Joint water reduction factor. SRF: Stress reduction factor. According to these parameters, the Q value is calculated with Eq. (4).

$$Q = (RQD/\text{Jw}) \* (\text{Jr/Ja})/(\text{Jw/SRF})\tag{4}$$

The (RQD)/Jn) value used in the equation corresponds to the rock mass geometry and the block/wedge size. The Jr/Ja value refers to the inter-block shear strength condition and the Jw/SRF value to the active stress incorporation water pressures and flows condition.

In addition to the Q value, the equivalent dimension (De) value is needed for the tunnel support details. This value is determined by dividing the tunnel diameter by the excavation support ratio. The details of the support system are determined by crossing these two values in the chart in **Figure 3** and **Table 5**.
