Engineering Rock Mechanics

1 00 Fractures and hemispherical projection

N 017/76 Finally, rotate the tracing paper

back to N, and read off the trend

of the bisector. The orientation of

298138. 055/72 the bisector is therefore found to

321 I66

be 321/66.

t---3

(d) Orientation of a borehole which is perpendicular to the two holes already

drilled

298/38 017l76

(d) Orientation of a borehole which is perpendicular to the two holes already

drilled

298/38

I1 4

19711 4'

We know the orientation of the

plane on which BHl and BH2 lie,

and so a borehole which is per-

pendicular to BH1 and BH2 is ori-

ented such that it is normal to this

plane. We find the normal to the

plane by placing the plane on its

great circle, counting across the E-

W line 90" from the line of max-

imum dip, and marking the nor-

mal. Also mark a tick and write

down the plunge of the normal.

Rotate the tracing paper back to

N, read off the trend of the nor-

mal, and write it next to the tick.

The orientation of the borehole is

therefore 197/14.

7.2 Questions and answers: fractures and

hemispherical projection

Q7.1 The overall RQD for 134 m of borehole core was found to be

58%.

(a) Compute estimates for the total number of pieces of core, and

the total length of those pieces of core that could be expected to be

greater than 0.1 m long.