Site Investigation - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 357

Figure 13.3 Test pit (Wittke 1990)

13.5 Boreholes

13.5.1 Core Drilling
Core drilling is the standard exploration method in rock engineering. The informa-
tion obtained from one exploration borehole is limited. When carried out in sufficient
number, however, drillings can reveal stratigraphy, rock type and, to a limited extent
and with a considerable degree of uncertainty, also discontinuities. A number of rock
mechanical tests and measurements have been developed to be carried out in boreholes
(Sections 13.5.2 to 13.5.4, 15.2, 15.8, 16.2, 16.3, 16.5, 17.4, 17.7 and 17.8). In addition,
rock samples are taken from cores for subsequent laboratory testing (Chapter 14).
For the purpose of exploration, rotary core drilling is carried out. The drilling equip-
ment consists of the drilling rig, the drill rod, the core barrel being a cylindrical tube
or double tube to cover and protect the core. The drilling rig is the power and control
station of the drilling process (Fig. 13.4). The drill rod linked to the core barrel takes the
core barrel to the depth required. Thrust, revolution and torque are brought from the
drilling rig to the top of the core barrel. The drill bit, cutting an annular space between
the core and the borehole wall, consists of a short steel cylinder mounted at the end of
the core barrel. Reaming shells are placed behind the bit on the core barrel to maintain
the correct borehole diameter and to stabilize the core. Depending on the rock mass, the
face of the bit is coated with natural or synthetic diamonds (Fig. 13.5, left) or tungsten
carbide inserts (Fig. 13.5, right). Hard rock requires small diamond grain size, for soft
rock bigger diamond grain size is an advantage. Carbide inserts are most suitable for