**6. Weak rock tunnelling-soil tunnelling**

In the evaluations of rock mass classification systems, support details for weak rocks are usually detailed as cast lining or ring closure of the inverted section with rigid lining. However, in tunnels defined as weak rock or soil, dimensioning support details is often not possible according to rock mass classification systems. In these sections, the dimensioning of support systems can be detailed as a result of analytical solutions and numerical solutions. According to ISRM classification systems, sections with a value of less than 1 MPa are considered soil. In addition, special support system solutions emerge in weak rocks, especially in units with weak strength such as clay, claystone and schist. The squeezing mechanism that develops under high overburden affects the long-term performance of the support systems. For this reason, both squeezing and swelling potential are the most important factors in tunnels excavated in weak rocks and soils. Two different tunnel support system approaches are used for tunnels excavated in weak rocks and soils [19–22]. These are called the passive method and the active method. While the passive method is based on the principle of increasing the support pressure by allowing deformations in the tunnel, in the active method, the support systems are dimensioned without allowing such deformations. However, for the passive approach in squeezing ground, the long-term deformations that may occur in real projects were often unsuccessful and an active approach was adopted in the revisions made. For this reason, it is very important to determine and evaluate the squeezing mechanism, which is one of the most important factors in weak ground.

### **6.1 Squeezing in tunnels**

In the evaluations for the squeezing mechanism, Jethwa et al. [23], Sakurai [24], Singh et al. [25], Goel et al. [26], Hoek and Marinos [27], Aydan et al. [28] approaches are quite common. In these approaches, the uniaxial compressive strength of the rock mass, unit weight and overburden height appear as the main factors.

Singh et al. [25] defined according to the Q value. If the H value determined according to Eq. (15) is greater than 350 ∗ *Q*<sup>1</sup>*=*<sup>3</sup> , squeezing is expected, while if the H value is less than 350 ∗ *Q*<sup>1</sup>*=*<sup>3</sup> , squeezing is not expected (**Figure 9**):

$$H = \mathfrak{B50} \ast Q^{1/3} \tag{15}$$

Goel et al. [26], on the other hand, defined the squeezing state similarly according to the Q value. He stated the calculated value of the Q value according to the stress-free conditions as N (rock mass number). If the H value (Eq. (16)) calculated according to this value is greater than the thickness of the overburden height, squeezing will occur, and if it is small, there will be no squeezing. In **Figure 10**, Goel et al. [26] the squeezing condition is shown according to the N value:

$$H = \left(27 \text{\textquotedblleft} \text{\textquotedblright}^{0.33}\right) B^{-1} \left(\text{m}\right) \tag{16}$$

**Figure 9.** *Squeezing conditions regarding Q and H values [25].*

**Figure 10.** *Squeezing conditions according to Goel et al. [26].*

Sakurai [24] classified the compression mechanism according to the strain value calculated based on the uniaxial compressive strength of the rock mass. He proposed Eq. (17) to determine the strain value:

$$
\epsilon p c = 1.07 \text{\AA} \sigma c m^{-0.318} \tag{17}
$$

The relationship between the uniaxial compressive strength of the rock mass depending on the strain value is given in **Figure 11**.

Jethwa et al. [23] classified the compression according to the N coefficient. Nc coefficient (competency factor) is given in Eq. (18):

$$\text{Nc} = \sigma \text{cm} / \text{Po} \tag{18}$$

**Figure 11.** *Relationship between uniaxial compressive strength of strain and rock mass [24].*

*Po* is the in situ stress and *σcm* is the uniaxial compressive strength of the rock mass. The compression status according to the calculated Nc value is given in **Table 7**.

Hoek and Marinos [27] defined the squeezing according to the strain value depending on the relationship between the uniaxial compressive strength of the rock and the in-situ stress. The strain value is given in Eq. (19):

$$
\varepsilon = 0.2 \ast \left( \sigma c m / \text{Po} \right)^{-2} \tag{19}
$$

The squeezing mechanism is given in **Figure 12** depending on the calculated strain value and the ratio of the compressive strength of the rock mass to the in situ stress.

While evaluating the squeezing mechanism, the critical factors are the height of the overburden, the unit weight and the uniaxial compressive strength of the rock mass.

#### **6.2 Swelling in tunnels**

Swelling soils can cause failures in the support systems due to unexpected loads. Most of the time, it can cause failures and swell even in the sections where inner lining and invert concrete are completed in tunnels. For this reason, the swelling potential of the ground should be evaluated during tunnel design. Support systems should be designed to carry these new loads that may occur. Einstein and Bischef [29] suggested a design procedure for swelling soils. First of all, the process can be listed as


**Table 7.** *Squeezing degree according to Jethwa et al. [23].*


#### **Figure 12.**

*Approximate relationship between strain and the degree of difficulty associated with tunnelling through rock [27].*

determining the primitive stress state of the current situation, determining the swelling soils around the tunnel and performing the swelling tests (odometer). In addition, they suggested closing the ring with invert, draining the water in the tunnel and closing the surface with steel-wire shotcrete (SFRS) after excavation.

Komornik and David [30, 31] proposed Eq. (20) for the analytical determination of the pressure that will occur due to the swelling:

$$\text{logps} = 2.132 + 0.0208aL + 0.000665\eta d - 0.0269am \tag{20}$$

where.

ps = selling pressure (kg/cm<sup>2</sup> ) at zero swelling strain. *ωL*=liquid limit (%).

γd = natural dry density (kg/m<sup>3</sup> ) and. *ωn*=natural moisture content (%).
