Site Investigation - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 397

13.10 Evaluation of a Structural Model

When setting up a structural model the mapping results should be summarized for ho-
mogeneous rock mass units. Because of existing constraints in fi eld exploration it is
necessary to extract as much information as possible from the collected data.
As an example, the setting up of the structural model of the clay slate illustrated in
Fig. 2.36 that was encountered at the dam site of the Selingue project in Mali is consid-
ered (cf. Section 2.7.3).

The structural model has been primarily established on the basis of two detailed map-
pings where a total of 218 discontinuities were recorded. The vast majority of these
could be assigned to three sets Sch, D1 and D2. By mapping the construction pit's
invert, mainly the steeply dipping schistosity-parallel discontinuities of set Sch and the
joints of set D1 could be encountered, while the mapping of the northern wall of the
construction pit, illustrated in Fig. 13.32, allowed the schistosity-parallel discontinuities
of set Sch and, to a larger extent, the shallowly dipping joints of set D2 to be sampled.
The grouping of discontinuities into sets Sch, D1 and D2 has been carried out with the
aid of the pole plot of all recorded discontinuities (Fig. 13.33). Discontinuity traces of
sets Sch and D2 were then labeled in accordance with this grouping in Fig. 13.32. Al-
though the rock mass is separated to a far greater degree parallel to the schistosity, only
a relatively small number of schistosity-parallel discontinuities of set Sch were meas-
ured. The mapping is therefore not representative in this respect. Mappings of other
exposures, however, allowed us to record additional schistosity-parallel discontinuities.
These revealed that they dip steeply in north-easterly and south-westerly directions as
in the case of the few discontinuities of Sch represented in Fig. 13.33.
Because of the bias with respect to set Sch, only the orientations of the discontinuities
of sets D1 and D2 could be statistically evaluated on the basis of the mappings of the
construction pit's invert, which is not represented here, and the construction pit's north-
ern wall represented in Fig. 13.32. As a result, it was found that the angles of strike and
dip of the discontinuities of both sets after being transformed according to (13.10) to
(13.13) follow beta distributions with the exception of the transformed strike angle of
D2 that is uniformly distributed (Wittke 1990).
A beta distribution exhibits four parameters that can be adapted to distributions of
different shapes such as triangle, U and J shapes (Harr 1977). The probability that the
orientation of a discontinuity of set D1 lies within the area enclosed by the green line
in Fig. 13.33 and that the orientation of a discontinuity of set D2 lies within the area
enclosed by the blue line in Fig. 13.33 was found to be 65% (Wittke 1990). The mean
orientation of schistosity-parallel discontinuities was determined as

Sch = 160° and

Sch = 90° (Fig. 13.33).