Engineering Rock Mechanics

Strvctvrally-controlled instability mechanisms 343

tion

Figure 19.4 Inclination of the hemispherical projection to be coincident with the

excavation surface.

The inclination is then applied (taking care to ensure that both the

direction and magnitude are correct) to all normals by moving along

small circles and replotting the points.

The procedure is shown in Fig. 19.5 and explained in detail in the Appendix.
The inclined normals are labelled N~I, NhI, NII and NZI in Fig. 19.5. Nfl is,
by definition, coincident with the centre of the net. From these normals,
the associated great circles representing the inclined projection of the
various discontinuities are constructed, as shown by the solid great circles
in Fig. 19.5. Similarly, the horizontal plane is derived from NhI and is shown

GREAT CIRCLE

REPRESENTING

HORIZONTAL

D, =

D,=

ALWAYS INCLINE NORMALS

THEN DRAW ASSOCIATED GREAT CIRCLES

Figure 19.5 Construction of an inclined hemispherical projection.