Engineering geology aspects for design and classification - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 57

The classification should be performed as part of the design for the client. It is based on:
- the ground conditions recorded by geotechnical investigations and the evaluation of
their effect on tunnelling.
- The shape and size of the cavity.
- The tunnelling method, which is determined by the type of excavation and support.
After the classification into project-related tunnelling classes, a forecast of the propor-
tions of the various tunnelling classes along the tunnel is produced, with clearly different
ground conditions being delineated. On site, the tunnelling classes are agreed between the
contractor and the client based on the approved construction design in the form of sup-
port specifications before each excavation process. If the opinions differ, then the client
specifies the tunnelling class. The tunnelling classes that are finally executed are to be
documented in an appropriate drawing and compared to the forecast tunnelling class [43].
2.4.3.2 Classification in Switzerland (“Klassierung” according to the SIA
standard)
Excavation classes according to the Swiss engineers' and architects' association SIA
198 [218]. The standard SIA 198 - Underground Structures - was issued in 1993 and is part
of the tender documents and construction contract in Switzerland. It applies to the tendering
and construction of underground works like mined tunnels, caverns and shafts, and includes
drill and blast, excavation by roadheader or TBM and tunnels in loose ground. Section 5.2
of the standard concerning the payment of excavation works for drill and blast in rock and
is based on the idea that the costs of the construction of an underground cavity are directly
related to the type and extent of the necessary support works, but also with the point in time
that such measures have to become effective. The SIA 198 therefore chooses the support
measures as criterion for the excavation classes. Classification is therefore based, on one
hand, on the type and extent of support measures, specifically rock blots, shotcrete, steel
arches with or without lagging, and on the other hand on the location of implementing these
measures and thus indirectly their time of installation. The following areas are differentiated:
- face area L1,
- advance area L2 and
- rearward area L3.
The linking of type, extent and installation location of the support measures gives the
relevant excavation class. Support measures outside the designated working area have no
effect on the classification of the excavation. Table 2-21 defines the five excavation classes
used for drill and blast in rock for any cross-sectional form and size. Quantitatively, the
measures are not laid down rigidly in the standard in order that the various local conditions
can be taken into account. Guideline values are, however, given for the values to be taken
according to profile size (Table 2-22).
The main idea behind the firm definition of excavation classes should in every case by the
degree of obstruction to the tunnelling works from the support works, categorised as follows:
I. The excavation support causes a negligible obstruction of the excavation cycle.
II. The excavation support causes a slight obstruction of the excavation cycle.
III. The excavation support causes a considerable obstruction of the excavation cycle.