Structure of Rock - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 33

Fig. 2.34 shows a discontinuity system with fi ve sets of a gneiss that is encountered
in the section Bodio of the Gotthard basetunnel which was driven using a TBM. The
joints of the sets KI and KII, and KIII and KV are steeply dipping and strike parallel
to and perpendicularly to the tunnel axis, respectively. The joints of the set KIV are also
steeply dipping and strike diagonally to the tunnel axis. Because the latter are mechani-
cally less important they are not represented in Fig. 2.34. The discontinuities of the set
KS, which are parallel to the schistosity, are more or less horizontal. Fig. 2.35 (upper)
shows an idealized structural model of this rock mass in which sets KI and KII as
well as KIII and KV are each combined into one vertical set. These sets can form rock
wedges which may fall into the tunnel (Fig. 2.35, lower).
As another example, Fig. 2.36 shows the structural model of the clay slate, which
was illustrated in Fig. 2.18 and encountered in the construction pit for the power-
house at the dam site of the Selingue project in Mali. This rock mass is separated by
the discontinuity sets Sch, D1 and D2. The intact rock has a planar grain structure
caused by the schistosity. The schistosity-parallel discontinuities, Sch, have a mean
orientation of and and exhibit rusty coatings and often also
fi llings of fi ne-grained soil. They are closely spaced with a mean spacing of less than
1 m. The average trace length exceeds 10 m. The joints of D1 have a mean orienta-
tion of and as well as an average spacing of 2.5 m. These dis-
continuities also often have rusty coatings, but their extent in general is signifi cantly
less than that of the schistosity-parallel discontinuities. The joints of set D2 are
more or less horizontal. They frequently exhibit fi llings of cohesive soil. Their av-
erage spacing and extent amount to 1 m and 4 m, respectively. In addition, various
joints with the same orientation but extending more than 20 m were encountered.
Since some of them are fi lled with cohesive soil they exhibit low shear strength and
therefore lead to a large reduction of the powerhouse block's safety against sliding.
Because of their large extent, in the order of magnitude of the size of the power-
house blocks, they need to be considered as individual elements (Wittke 1990). The
structural model and the most important rock mechanical parameters, also speci-
fi ed in Fig. 2.36, lead to the corresponding rock mechanical model.
Fig. 2.37 shows the structural model of another clay slate encountered at the dam site
for a planned drinking water reservoir near Wiesbaden in Germany (Wittke & Schetelig
1978, Wittke 1990). As in the case of the other clay slate, the intact rock has a planar
grain structure caused by the schistosity, leading to a pronounced anisotropic deforma-
bility and strength. As a result of exploration the schistosity-parallel discontinuities Sch
and the joint sets D1 and D2 strike perpendicular and parallel to the valley and form an
orthogonal discontinuity system. The mean orientations as well as the most important
rock mechanical parameters of these sets leading to the rock mechanical model are also
specifi ed in Fig. 2.37.