Page 159 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

Chapter 9 Rock Mass Number 127

Non-Squeezing Ground Condition

ua ¼ Á H0:6 K0:35 % ð9:11Þ
a 28 N0:4 Á

Squeezing Ground Condition

ua ¼ H0:8 K0:6 % ð9:12Þ
a 10 Á N0:3 Á

where ua/a ¼ normalized tunnel closure in percentage, K ¼ effective support stiffness
(¼ pv Á a/ua) in MPa, and H and a ¼ tunnel depth and tunnel radius (half of tunnel width)

in meters, respectively.

These correlations can also be used to obtain desirable effective support stiffness so

that the normalized tunnel closure is contained within 4% (in the squeezing ground).

EFFECT OF TUNNEL DEPTH ON SUPPORT PRESSURE
AND CLOSURE IN TUNNELS

In situ stresses are influenced by the depth below the ground surface (see Chapter 28).
Support pressure and the closure for tunnels are also influenced by the in situ stresses.
Therefore, the depth of the tunnel, or the overburden, is an important parameter while
planning and designing tunnels. The effects of tunnel depth or the overburden on support
pressure and closure in a tunnel have been studied using Eqs. (9.9) through (9.12) under
both squeezing and non-squeezing ground conditions, which are summarized below.

1. Tunnel depth has a significant effect on support pressure and tunnel closure in
squeezing ground conditions; however, it has a lesser effect in non-squeezing ground
conditions (Eq. 9.9).

2. The effect of tunnel depth is higher on the support pressure than the tunnel closure.
3. The depth effect on support pressure increases with deterioration in rock mass qual-

ity, probably because the confinement decreases and the degree of freedom for the
movement of rock blocks increases.
4. This study would be helpful to planners and designers when deciding on realigning a
tunnel through better tunneling media or a lesser depth or both to reduce the antic-
ipated support pressure and closure in tunnels.

APPROACH FOR OBTAINING GROUND REACTION CURVE

According to Daemen (1975), the ground reaction curve (GRC) is quite useful for design-
ing the supports for tunnels in squeezing ground conditions. An easy-to-use empirical
approach for obtaining the GRC has been developed using Eqs. (9.10) and (9.12) for tun-
nels in squeezing ground conditions. The approach is explained in Example 9.2.

Example 9.2
The tunnel depth (H) and the rock mass number (N) have been assumed as 500 m and 1,
respectively, and the tunnel radius (a) as 5 m. The radial displacement of the tunnel is ua
for a given support pressure pv(sq).