Engineering Rock Mechanics

324 Design of surface excavations

The minimal volume of side slope excavation is required by the two
facing slopes whose dip directions are 045" and 225", which means that
the optimal trend for the road is either 135" or 315". If we examine
the quarry faces at these azimuths we see that they are 35" and 57",
respectively, and on this basis it makes sense to use the slope whose dip
direction is 135" for the access road.
We can undertake the same process for the case when the friction
angle is unknown, and the results of this analysis are given in the table
below. In this case, the minimal volume of side slope excavation is
required by the two facing slopes whose dip directions are 015" and 195",
which means that the optimal trend for the road is either 105" or 285".
The quarry faces at these azimuths have slope angles of 11" and 12",
respectively, and so either orientation may be used for the road.

Slopedipdirection 0 015 30 45 60 75 90 105 120 135 150 165

Slopeangle, $ 10 13 13 12 10 8 10 11 12 20 35 28

l/tan($) 5.67 4.33 4.33 4.70 5.67 7.12 5.67 5.14 4.70 2.75 1.43 1.88

1/2 tan($) 2.84 2.17 2.17 2.35 2.84 3.56 2.84 2.57 2.35 1.37 0.71 0.94

Slopedipdirection 180 195 210 225 240 255 270 285 300 315 330 345

Slope angle 24 22 20 20 22 23 15 12 10 9 8 9

l/tan($) 2.25 2.48 2.75 2.75 2.48 2.36 3.73 4.70 5.67 6.31 7.12 6.31

1/2 tan($) 1.12 1.24 1.37 1.37 1.24 1.18 1.87 2.35 2.84 3.16 3.56 3.16

Opposing slope 000, 015, 030, 045, 060, 075, 090, 105, 120, 135, 150, 165,

dipdirections 180 195 210 225 240 255 270 285 300 315 330 345

Combined exca- 3.96 3.40 3.54 3.73 4.07 4.74 4.70 4.92 5.19 4.53 4.27 4.10

vation volume

lineof'-.,

.

crest of slope --.

with @=30'

access road

quany floor access md .. *

crest of slope

with @ =O

We have included the case of zero angle of friction because, although

it is not a realistic case, it clearly shows how the amount of land required

by the quarry increases as the friction angle decreases: note that the scale

of this sketch plan is much smaller than that of the previous one.

The following diagram illustrates how the volume of excavation re-

quired at each slope dip direction varies with the friction angle. This plot

shows that, not only is the volume always much less with the higher fric-