Part 8Part 8

MAPPING MAPPING DISCONTINUITIES DISCONTINUITIES UNDERGROUNDUNDERGROUND

• Unlined tunnel excavations offer an unimpeded three-Unlined tunnel excavations offer an unimpeded three-dimensional view of the rock fabric and structure. The dimensional view of the rock fabric and structure. The floor (invert) of a tunnel is not commonly mapped floor (invert) of a tunnel is not commonly mapped because it is usually covered with tunnel muck, debris, because it is usually covered with tunnel muck, debris, and water. This shows a road header working on the and water. This shows a road header working on the East Minneapolis sewer interceptor tunnel.East Minneapolis sewer interceptor tunnel.

Common tunnel block failure modes caused by the intersection of Common tunnel block failure modes caused by the intersection of the tunnel opening with crossing jointsthe tunnel opening with crossing joints

• The discontinuities intersecting any underground The discontinuities intersecting any underground opening need to be recorded and described, as best opening need to be recorded and described, as best possible form exposures within and exterior to the possible form exposures within and exterior to the opening. opening.

• When working near the ground surface it is When working near the ground surface it is advantageous to employ impression packers to advantageous to employ impression packers to examine fractures out-of-view, but in close proximity to examine fractures out-of-view, but in close proximity to the opening, as shown at left (arrow). the opening, as shown at left (arrow).

Impression Impression packers usedpackers used

FractureFractureMapsMaps

• Fracture maps Fracture maps are 2D are 2D representations representations of 3D of 3D underground underground openingsopenings

• Record the Record the geometry and geometry and physical physical characteristics characteristics of individual of individual fractures and fractures and geologic geologic contactscontacts

• Typical fracture mapping within a circular Typical fracture mapping within a circular tunnel excavation. Note survey stationing tunnel excavation. Note survey stationing and absence of information collected from and absence of information collected from the tunnel invert.the tunnel invert.