Engineering geology aspects for design and classification - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 56

During the construction phase, the excavation class to be used is agreed between client
and contractor in each case. The contractor proposes the excavation classes. Repeated
changing of the excavation classes has to be expected, although there is no special pay-
ment for the changeover itself [263].
Recommendations of the working group “Tunnelling” of the Deutsche Society for
Geotechnics (ETB) [52]. This also recommends the use of the general classification
scheme according to DIN 18312, which should be used as a basis for the project-related
classification of a tunnel in each case. The purpose of this is:
- before the construction phase, to implement the design work into tunnelling classes to
enable performance-related price formation for tunnelling in various ground conditions,
- during the construction phase, to regulate on site the selection of the excavation class
and support measures that are appropriate for the ground conditions and
- after the completion of construction, to provide a basis for payment and comparability
for later tunnel projects.
The excavation and support and in some cases support to the face are the works, which
determine the advance rate. For this reason, this recommendation also selects the descrip-
tion tunnelling class, which corresponds to the DIN 18312 (draft 4/1995).
A division into sub-classes can achieve a refinement of the general tunnelling classifica-
tion, for example by consideration of additional obstructions. The following special condi-
tions are given for universal cases and also for TBM and shield machine tunnels:
- Exceptional geological-hydrogeological conditions.
- Change of tunnelling class (which is not considered through the tunnelling class itself,
but through an appropriate item in the bill of quantities).
- Effects on the groundwater or measures to limit them.
- Difficulties caused by groundwater or formation water (generally considered through
obstruction items in the bill of quantities and not in the tunnelling classes).
- Data given by the bidder about advance rates for the relevant tunnelling classes.
- Tunnelling beneath buildings/facilities that are susceptible to settlement.
Definite special situations related to a specific project are only included for universal tun-
nelling (blasting, roadheaders, excavator, manual) as follows:
- Quantities for the type and extent of support in bands (shotcrete thicknesses, number
and length of rock bolts, type of arches and spacing, type of reinforcement: one or two
layers. A certain overlap between the bands in adjacent tunnelling classes is useful).
- Details of round lengths and the free unsupported span.
- Details of the installation location and sequence of support for all stages of excavation.
- If appropriate, limitation of the advance rate to achieve the load-bearing capacity of the
support (shotcrete/bolt grouting).
- Restrictions due to building or facilities when the overburden is shallow.
- If appropriate, restrictions taking blasting vibration into consideration.
- Consideration of extensive advance measures to consolidate or waterproof the rock
mass (grouting, freezing) and if appropriate special classes.
- Consideration of the special features of shafts and caverns.
The special features in the ETB for tunnel boring machines (TBM) and shield machines
are dealt with in Section 2.5.6.1.