Structural design verifications, structural analysis of tunnels - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 117

Structural design and safety
Loading cases
(5) The following three loading cases are differentiated in underground tunnelling:
- Loading case 1: Main loads and additional loads (without temporary loads during the
construction period according to Clause 14) in the least favourable combination in
each case,
- Loading case 2: Main loads and additional loads (including temporary loads during
the construction period according to Clause 14) in the least favourable combination
in each case,
- Loading case 3: Main loads and additional loads (without temporary loads during the
construction period according to Clause 14) and special loads in the least favourable
combination in each case.
In the determination of the least favourable combination of loads, the range or variation
of the individual loads is taken without safety factors. It should also be considered that in-
dividual loads do not act in certain cases or can reduce to almost zero. Particularly in the
case of two-layer lining, the case should be investigated for the inner layer that no actions
from the rock mass according to Clause 9 are present, unless this can be safely excluded.
Design of the lining
(18) The final lining is to be designed for loading cases 1, 2 and 3 (see Clause 5).
The temporary support is normally only to be designed for loading case 2, unless it is
considered as load-bearing and thus part of the final lining.
The outer layer may only be considered for design purposes as part of the final lining if its
durability is guaranteed.
Shield constructions are to be dealt with as a temporary support.
When pre-grouting is carried out before tunnelling below a railway line, the strength un-
der vibration is to be verified according to ATV worksheet 161 (DVGW bulletin GW 312).
The design of the preliminary support and the final lining should be performed under con-
sideration of the interpretation of the already available measurements.
Designing of the lining for loading case 2
(19) For short-term construction states lasting up to a few days in loading case 2, reduc-
tions of the safety factors of loading case 1 are permissible. The safety factors for concrete
and reinforced concrete may be
- reduced by 10 %, when the safety of rail traffic and other public safety are affected, and
- reduced by 20 %, when the safety of rail traffic and other public safety are not affected.
For the determination of the permissible stresses in steel for the general verification of
stresses, the minimum value of the yield limit according to the applicable quality standard
may be reduced by the safety factor.
- γ = 1.5; when the safety of the operational facilities of the DB network and other public
safety are affected and