Civil Engineering Reference
In-Depth Information
Figure 25.9
Influence of tunnel depth on surface settlement profile.
25.7 Load factors to limit ground movements
In Sec. 25.4 I investigated the ultimate limit state of tunnels and headings and calcu-
lated the tunnel support pressure at collapse
tc
for undrained and drained tunnelling.
However, if the tunnel support pressure approaches the ultimate limit state ground
loss and settlements will be relatively large. In order to limit ground movements so
nearby structures are not damaged it is convenient to apply a load factor to determine
an allowable tunnel support pressure. This approach is the same as the one used to
determine allowable bearing pressures for foundations in Chapter 22.
Figure 25.10(a) illustrates the volume loss
V
l
increasing as the tunnel support pres-
sure
σ
z
in the ground at the level of the axis the settlements will be negligible. As the tunnel
support pressure approaches the ultimate limit state
σ
t
decreases. If the tunnel support pressure is the same as the vertical stress
γ
σ
tc
the settlements become very
large as the tunnel and heading collapse. At the design point the allowable tunnel
support pressure
σ
ta
causes an allowable volume loss
V
la
and this causes allowable
Figure 25.10
Relationship between tunnel support and ground settlements.
