Page 105 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

Chapter 7 Tunneling Hazards 77

Prediction of Degree of Squeezing
Degree of Squeezing and Its Effect on Tunneling

The degree of squeezing can be represented by tunnel closure (Singh, Jethwa, & Dube,
1995) as follows:

Mild or minor squeezing Closure 1–3% of tunnel diameter
Moderate or severe squeezing Closure 3–5% of tunnel diameter
High or very severe squeezing Closure >5% of tunnel diameter

On the basis of the previous limits of closures, out of 29 squeezing cases, 14 cases

denote mild or minor squeezing, 6 cases represent moderate or severe squeezing, and 9

cases pertain to high or very severe squeezing ground conditions.
It may be added here that tangential strain ey is equal to the ratio of tunnel closure and

diameter. If it exceeds the failure strain ef of the rock mass, squeezing will occur. Mild
squeezing may not begin even if closure is 1% and less than ef in most cases (see the
section Critical Strain on Rock Mass in Chapter 13).

Considering the previously mentioned limits of closure, it is possible to draw two

more demarcation lines, DE and FG, in the squeezing zone in Figure 7.3. The equation

of line DE separating cases of mild from moderate squeezing ground conditions is

obtained as:

H ¼ ð450 N0:33Þ Á BÀ0:1 meters ð7:10Þ

Similarly, the equation of line FG (Figure 7.3) separating the moderate and high squeez-
ing conditions is obtained as:

H ¼ ð630 N0:33Þ Á BÀ0:1 meters ð7:11Þ

All of the equations obtained from Figure 7.3 for predicting ground conditions are sum-
marized in Table 7.6. The squeezing ground condition has not been encountered in tun-
nels where Jr/Ja was found to be more than 0.5.

It is important to know in advance, if possible, the location of rock burst or squeezing
conditions, because the support systems are different in each condition. Kumar (2002)
classified modes of failures according to values of joint roughness number (Jr) and joint
alteration number (Ja), as shown in Figure 7.4. It is observed that mild rock burst

TABLE 7.6 Prediction of Ground Condition Using N

S. No. Ground conditions Correlations for predicting ground condition
1 Self-supporting H < 23.4 N0.88 Á BÀ0.1 and 1000 BÀ0.1 and B < 2 Q0.4 m
2 Non-squeezing 23.4 N0.88 Á BÀ0.1 < H < 275N0.33 Á BÀ0.1
3 Mild squeezing 275 N0.33 Á BÀ0.1 < H < 450N0.33 Á BÀ0.1 and Jr/Ja <0.5
4 Moderate squeezing 450 N0.33 Á BÀ0.1 < H < 630N0.33 Á BÀ0.1 and Jr/Ja <0.5
5 High squeezing H > 630N0.33 Á BÀ0.1 and Jr/Ja < 0.25
6 Mild rock burst H Á B0.1 > 1000 m and Jr/Ja > 0.5 and N > 1.0

Source: Goel, 1994.