Engineering geology aspects for design and classification - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 72

The round length in the top heading is 1.50 m to maximum 2.50 m and in the bench 2.50 m
to maximum 5.00 m.
The temporary support consists of:
- steel arches with distances according to the round length,
- a shotcrete layer altogether 20 cm thick,
- one layer of Q 257 mesh reinforcement externally and
- systematic anchoring in the top heading and invert.
The entire support is to be installed around the entire perimeter before the next round.
The following may also be necessary according to a decision on site:
- Anchoring of dangerous sequences of strata and
- shotcrete sealing directly after the round.
Tunnelling class (VKL) 4.4 (Fig. 2-18). VKL 4.4 applies in sections in unstable to slight-
ly squeezing rock mass.
The strength of the rock mass is exceeded locally. The rock mass loosens with heavy
rockfalls.
The short stand-up time of the rock mass requires sealing (from the muck pile) of the tun-
nel sides immediately after blasting.
The round length in the top heading is 1.00 m to maximum 1.50 m and in the bench 2.00 m
to maximum 3.00 m.
The temporary support consists of:
- sealing of the tunnel sides with 3 to 5 cm shotcrete,
- steel arches with distances according to the round length,
- a shotcrete layer altogether 20 to 25 cm thick,
- two layers of mesh reinforcement Q 257 and
- systematic anchoring in the top heading and invert.
In case of heavy water ingress, see special measures.
Sealing of the tunnel sides with 3 to 5 cm shotcrete is required immediately after blasting.
The following may also be necessary according to a decision on site:
- Widening of the top heading arch foundation (elephant foot) d = 60 to 80 cm,
- face support and
- pre-support with spiles dia. 25 mm.
Tunnelling class (VKL) 6 (Fig. 2-18). VKL 6 applies for sections in very unstable or
squeezing rock and for fault zones with pronounced karst features.
The severe loosening can lead to large rockfalls, which makes pre-excavation support
necessary. Fractures in the rock mass and the rock pressure demand an early ring closure
and lead to a considerable limitation of the timing of the advances of top heading, bench
and invert.