Site Investigation - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 385

In a tunnel, the routine mapping of the temporary face during excavation also gives a
three-dimensional insight into the rock mass. Among others, this allows the detection
and reconstruction of fault zones and other structural features with regard to their ex-
tent and persistence (Gaich et al. 1999).

Remote sensing mapping techniques
The conventional mapping method using hand-held equipment such as Clar's com-
pass, a measuring tape and a roughness profile device is a time-consuming process.
In addition, it can be hazardous if the rock surface is unstable and often a large
part of the exposure is inaccessible. In such cases photogrammetry, which allows
discontinuity geometry to be measured from photographs of rock surfaces, is a safer
method and provides greater access to the exposure (ISRM 1978e, Harrison 1993).
By using digital images followed by digital image processing the photogrammetric
mapping technique could be automated (Reid 1998, Reid & Harrison 2000, Lemy &
Hadjigeorgiou 2003).
Further improvements of remote sensing mapping techniques were achieved by the
development of stereoscopic imaging and computer vision systems. They enable a
fast sampling of rock mass structural data. Using these methods, factors such as dis-
continuity orientations, spacing values and roughness profiles can be directly deter-
mined, represented and evaluated from three-dimensional images (Gaich et al. 1999,
Fasching et al. 2001, Pötsch et al. 2007).

Example of a surface mapping
Figure 13.27 shows a detailed mapping of a rock cut. The location and geometry of
the cut are illustrated by a block diagram. The mapping is represented as a developed
view and plotted on a grid of elevation (above sea level) versus chainage. The colors
and signatures selected for the encountered rock types (soil, claystone, massive lime-
stone, marl), the discontinuities and the fault zones are specified in a legend. The loca-
tion of encountered rock boundaries, discontinuities and fault zones including orien-
tation (dip direction ) and extent are also plotted. The traces of the

discontinuities and fault zones are represented with their apparent dip angles. It can
be recognized that the claystone exhibits almost vertical joints that are separated by a
massive limestone bench. In addition, approximately horizontal, bedding-parallel dis-
continuities are present in both the claystone and the limestone bench. Their extent is
greater than that of the vertical joints. The discontinuities in the marl are slickensided
and undulating with widely differing strike and dip angles.

α d and dip angle β