Civil Engineering Reference
In-Depth Information
From the results of the three LFJ tests carried out in metamorphic rock, deformation
moduli ranging from 5000 MPa to 40,000 MPa were determined, yielding a mean value
of = 19,000 MPa (Fig. 22.8, upper). Almost linear relationships between applied
pressure and slot widening were obtained, not differing signifi cantly for initial loading,
unloading and reloading. This is shown in Fig. 22.8 (lower) by means of the results of
an individual test. The slots for the fl at jacks were arranged in one test horizontally and
in the other two tests vertically. However, the orientation of fl at jacks, that is the loading
direction, yielded no remarkable differences of test results. Thus, the metamorphic rock
can be assumed to be linear elastic with isotropic deformability below the rock mass
strength. The moduli determined for the granite are of the same order as those of the
metamorphic rock.
As expected (Section 15.2), the results of the dilatometer tests were subject to a larg-
er range of scatter than the results of the LFJ tests. In addition, a marked difference
between the moduli for initial loading and the moduli for unloading and reloading
could be observed resulting in mean values of 9000 MPa and 13,000 MPa, respectively
(Wittke & Soria 1983). Because of the scale effect (Section 18.1) the results of the LFJ
tests were considered to be more reliable than those of the dilatometer tests.
The results of the triaxial cell stress measurements and their interpretation are com-
prehensively described in Section 16.7.1. As the most important result, increased hori-
zontal in-situ stresses due to tectonics were derived with their maximum in the order of
8 MPa directed normal to the cavern axis (Fig. 16.20).
Because of this rather unusual result, the LFJ tests were also used for stress measure-
ments by conducting them as compensation tests (Section 16.4). As a result, the in-situ
stress component normal to the cavern axis yielded a signifi cantly smaller value of only
about 4 MPa. Thus, a considerable uncertainty regarding the in-situ stress state in the
area of the cavern was left over after the design phase prior to construction. As outlined
in Section 16.7.1, the large-scale in-situ stress state could be reliably evaluated only as a
result of back analyses of monitoring results obtained during excavation of the cavern
(Section 22.10).
22.3
Location of the Powerhouse Cavern
The location of the powerhouse cavern was selected on the basis of criteria resulting
from the overall layout of the project and from the geological conditions found in the
exploration adits and boreholes. The placing of cavern at the toe of a slope at the end of
the valley was determined by the locations of the upper and lower reservoirs. The min-
imum pressure head at the turbines was required to be 4.1 bar when the lower reservoir
is at its minimum level. The elevation of the cavern had to be selected correspondingly.
These conditions led to the selection of the area into which the exploration adits and the
drainage tunnel were driven. This location also had the advantages that the transformer
station could be positioned at the ground surface and the tailrace tunnels were compar-
atively short (Figs. 22.4 and 22.5).
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