Engineering geology aspects for design and classification - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 89

heavy excavation support being necessary in the machine area in excavation class 5. But if
the excavation support consists of an immediate and continuously installed ring of segments,
the excavation classes no longer apply; excavation class T is provided for this case.

The consideration of the installation location is more complex for TBM drives than for drill
and blast. The working areas for boring with a TBM are described as follows in Switzerland:
- L1 machine area.
- L2 backup area.

- L3 rearward area up to 200 m behind the backup.

Within the areas L1, L2 and L3, the working zones L1*, L2* and L3* are defined, in which
the temporary support is installed according to the conditions on the project and the con-
straints of the machine type (Fig. 2-31).

Figure 2-31 Working areas and working zones for a TBM [218].

It is the task of the contractor to include with the tender details of the lengths of the work-
ing areas L1, L2 and L3 and the working zones L1*, L2* and L3* on a plan of the intended
tunnel boring machine, and also to state which support works are possible for the tendered
works; he has to take into account the conditions regarding installation location of the
temporary support specified in the tender documents.
The combination of type, extent and installation location of the support measures gives the
relevant excavation class. Support measures outside the designated working area have no
influence on the classification of excavation.

The main principle for the firm definition of the excavation classes should be in every case
the degree of obstruction due to support works (Table 2-32). Exact assignment is accord-
ing to Table 2-34.

Table 2-32 Excavation classes according to Swiss standard SIA [218].

Excavation class Obstruction of excavation

AK I. The excavation support causes an insignificant obstruction of the excavation cycle.

AK II. The excavation support causes a slight obstruction of the excavation cycle.

AK III. The excavation support causes a considerable obstruction of the excavation cycle.

AK IV. The excavation support causes an interruption of the excavation cycle (immediate

support after every stage of excavation).

AK V. The excavation support is installed continuously with the excavation and requires

immediate buttressing of the face or pre-excavation support.

AK T. The excavation support consists of a closed segment ring, which is installed

immediately and continuously.