1 06 Fractures and hemispherical projection(2) Sketch a circular arc around the northwest quadrant of the tracing
paper, centred on the centre of the tracing paper at a distance in from
the periphery equal to the plunge of the line of intersection, 38". This
arc represents the locus of lines with a pitch of 38" and trend in the
northwest quadrant.
(3) Rotate the tracing paper so that the intersection of the circular arc
and the great circle to plane 1 is on the east-west line, and mark a tick
on the periphery. This represents the trend of the intersection.
(4) Rotate the tracing paper
so that the strike of plane 2 is
on the north-south line, and
draw the great circle that
passes through the intersec- 30513
tion. This is the great circle to
plane 2. Mark a tick on the
periphery opposite the line of
maximum dip, measure the 256/50
dip of plane 2 and mark it next
to the tick. It is 80".
(5) Rotate the tracing paper SW plan
back to north and read off the ;z:
trend of the intersection and ~wpl~ne unknown plane
the dip direction of plane 2;
this gives the answers 305" and 222", respectively.
The orientation of plane 2 is then 222/80, and the orientation of the
line of intersection of plane 1 and plane 2 is 305/38.
The diagram shows that there is a second line of intersection between
the circular locus and the given plane, and the plane that passes through
this has an orientation of 222/38. However, the intersection is formed
in the SW quadrant, not the NW quadrant as stated, and so can be
discounted.
N47.6 Surveys have revealed that a rock mass contains 3 fracture
sets, the dip diredions/dip angles and fracture frequencies of which
are 161/23 and 7.72 m-', 218/58 and 3.07 m-', and 100/75 and
5.34 m-'.
(a) What will be the fracture frequency and mean length of the
recovered pieces of core in:
a vertical borehole;
0 a horizontal tunnel heading due north; and
0 an inclined borehole with a trend/plunge of 280/35?
Consider the engineering implications of this variation in terms of
site investigation procedures and subsequent engineering design.
(b) Using the frequency values determined above, compute the
theoretical RQD values that would be encountered in this rock mass
using the customary threshold value of 0.1 m. Adopting a threshold
value given by