Engineering geology aspects for design and classification - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 76

This type of microcrack formation has been demonstrated by microscopic examination of
thin sections, particularly in dense granite or granitic gneisses:
- at the Amsteg hydropower project in the Central Aare granite,
- the Piora investigation tunnel in the Leventina granite-gneiss,
- the Paute project in Ecuador in granodiorites,
- the Lötschberg Tunnel in Gastern granite.
In the central Aare granite at the Amsteg hydropower station, the reduction of wave veloc-
ity in the drill core through microcrack formation averaged about 15 % within four days
of extraction. It is evident that the compression, shear and tension strengths are reduced
by such microcracks. Within the range of tolerance, it may be assumed that the strength
reduction is proportional to the wave velocity reduction. In some zones of the Aare granite
at the Amsteg power station, there is a clear although not very pronounced orientation
of the mica minerals. This slight foliation should lead to compressive strengths that are
much higher at right angles to the orientation plane of the mica minerals than in the plane
of foliation. The anisotropic factor is then always greater than one. Measurements on
drill cores have however also shown a contradictory result, which can only be explained
through the formation of microcracks. Thin section investigations have confirmed these
assumptions; instead of the logical anisotropy values of about 1.2, values of 0.91 to 0.75
were determined.
When the face is bored, rock has to be excavated that has not yet been able to relax. The
strengths that have to be overcome in such cases are considerably higher than the mate-
rial properties determined using conventional methods. If preliminary investigations show
such variable anisotropy values, additional investigations should be carried out to clarify
the uncertainty.
As long as the tension strength remains in a normal relation to the compression strength, a
statement about the compression strength is adequate for rough dimensioning. If the rock
mass is of extreme toughness - which is categorised as hard to bore - detailed investiga-
tions, even including thin section observations, are essential. If these investigations are
omitted, the necessary penetration will not be achieved and the tunnel drive can then only
be completed after changing the disc cutters to button cutters, with the associated very
high cost.
2.5.3 Influences on the machine bracing
The bracing (or gripping) forces have a magnitude of about double the thrust forces (see
[148, Chap. 4]). The large-diameter disc cutters used today lead to correspondingly high
thrust forces and thus to remarkable stresses, which have to be transferred into the rock
mass through the gripper unit (Fig. 2-21).
The values recorded for various TBMs in use on construction sites show loadings of 2 to
10 MPa on the side of the cavity. Related to the primary stress in the rock mass, this is a
significant extra loading.