Engineering Rock Mechanics

422 Questions 7.7-7.10: fractures and hemispherical projection

256/50, and the strike of the other is 132". What is the trend of the line of
intersection and what is the dip of the second plane?

47.6 Surveys have revealed that a rock mass contains 3 fracture sets, the

dip directionsldip angles and fracture frequencies of which are 161/23

and 7.72 m-l, 218/58 and 3.07 m-l, and 100/75 and 5.34 m-l.

(a) What will be the fracture frequency and mean length of the

recovered pieces of core in:

a vertical borehole;

a horizontal tunnel heading due north; and

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 theor-

etical 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

will maximize the range of RQD values. How do the RQD values

computed with this threshold compare to the earlier values?

47.7 A rock mass is known to contain two sets of fractures, the orient-

ations of which are 265/42 and 003/69. What borehole orientations will

allow one to drill through the rock mass and intersect all of the fractures

at an angle of 45" (measured between the borehole axis and the normals

to the fractures)? What is the minimum angle that all fractures can be

intersected at?

47.8 A petroleum reservoir is known to contain numerous fractures

which are highly conductive. These fractures dip almost vertically in a

northeasterly direction. To maximize production from the reservoir, the

production wells are to be deviated to run sub-horizontally, so that they

intersect the fractures as close to perpendicular as possible.

In order to determine the optimal orientation of the production holes,

two test holes have been drilled to orientate the fractures. The orienta-

tions of these test holes within the reservoir are 011/09 and 087/03, and

impression packers which were run down these holes show that in both

holes the angle between the normal to the fractures and the hole axis is

40".

Determine the orientation of the fractures, and the required trend of

the production holes.

47.9 A length of core, from a borehole whose orientation is 143/68,

contains a fracture plane of 204/47. The core has rotated through a

clockwise angle (looking down the borehole) of 140" during retrieval.

What will be the apparent orientation of the fracture as it emerges from

the borehole?