Site Investigation - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 372

13.6 Exploration Adits and Shafts

Explorations carried out in adits and shafts provide more reliable information than expo-
sures, test pits and drillings. However, the costs of the excavation of an exploration adit or
shaft, are normally quite high and the corresponding time frame must be considered in the
design stage. An exploration adit or shaft should therefore be excavated only after care-
ful planning. The scope, intention and goal of investigations to be carried out have to be
defined precisely. In addition to mapping, the investigations should include field tests and
geotechnical measurements (Chapters 15 to 17). It must also be ensured that the excavation
of exploration adits and shafts does not unfavorably affect subsequent construction work.
To avoid rock loosening and to minimize the disturbed rock zone around exploration
adits and shafts a non-disturbing excavation method such as drilling or smooth blasting
should be used. The dimensions of the cross-section of an exploration adit depend on
the investigation purposes and on the rock mass conditions. The height should amount
to at least 2 m, in order to enable core drillings to be carried out from the adit (DIN
EN ISO 22475-1 2007). During excavation, the encountered ground and groundwater
conditions must be documented in detail.
Where possible, an exploration adit should be supported by rock bolts only. If for sta-
bility reasons the use of shotcrete is required, at least a few windows in the walls of the
adit should be left to enable rock mass investigations (Wittke 1990).
Exploration adits can be arranged within or outside of the cross-section of a planned
tunnel. Adits located outside of a tunnel's cross-section should be arranged so that they
can later be used as access tunnels to the main tunnel and as emergency, rescue or escape
adits. Rescue shafts or ventilation shafts planned for a tunnel afford the opportunity to
be carried out as exploration shafts (Fig. 13.15).

Figure 13.15 Arrangement of exploration adits and shafts for tunnels (Wittke 1990)

Exploration adits for large underground openings such as caverns are often carried out in
the roof because the exploration of the rock mass in the roof area is of particular impor-
tance with respect to the stability and the support of the final structure (Section 22.7). How-