Engineering Rock Mechanics

474 Questions 7 8.7- 7 8.7 0: design of surface excavations

418.3 A quarry is to be opened in a rock mass which contains four

fracture sets with dip directions and dip angles as follows:

Set 1 2 3 4

Dip direction/Dip angle 292164 151137 052176 020116

The rock mass can be considered dry,

and the angle of friction for all fractures

Consider the primary potential modes

of instability (plane, wedge and direct

toppling) at 15" intervals of dip direction

(i.e. O", 159 309.. ., 3459 360") and use

kinematic feasibility techniques to pre-

pare a table showing the steepest safe

slope and the respective critical failure

mode at each azimuth.

Plane is30".

sliding

Wedge

sliding

Toppling

418.4 Prepare a similar table as in Q18.3 but for the case when the
friction angle is not known.

418.5 Use the results from 418.3 to draw a plan of the slope crest around
the quarry excavation assuming that the floor of the quarry is circular.
Determine the best orientation for a radial access road to the quarry
floor (assume that the road can be constructed in any direction and
that the optimal orientation is for a road with the steepest possible side
slopes - so that excavation associated with the road is minimized).
Repeat part (b) for an unknown friction angle, as was the case in Q18.4.

4 18.6 Add toppling due to interlayer slip to the analysis of Q18.3.

418.7 A trial design for a proposed 100 m deep roadstone quarry can
be approximated in plan by a circle. The quarry has been designed with
a number of 10 m high benches, such that the overall slope angle is 45"
and the individual bench angles are 60".
Preliminary mapping of the site has established that the rock mass
contains four dominant sets of fractures, having the following mean
orientations:

Feature Cleavage Joint set A Joint set B Bedding

Dip direction 015"

36" 75" 45" 28"

330" 080" 260"

DiP