Engineering Rock Mechanics

334 Design and analysis of surface excavations

Take fixed values

Calculate F

Generate random values

Repeat

Figure 18.17 Mechanism of the Monte Carlo simulation procedure.

Define scope of work

and aims

Review existing geological and geotechnical data

Borehole investigation

and material testing

Examine surface exposures t

I

Rock type, RQD, condition of

discontinuities, water return,

Orientation, size, spacing and

condition of discontinuities

For each

rock type

Freq.k.& 1 (up to 10)

RMR

Iterate to obtain v2 "' I

Produce typical profiles

for intermediate and

final slopes and benches

J

Locate main

rock types, postulate

or calculate of water table position ~ Estimate c,@for

rock mass, locate

critical slip surface

see Hoek and Bray p.240)

Generate: slice data (8-20 slices)

A, - Slice width

h, - Height of rock in slice

y, - Unit weight of rock in slice

h, ~ Height of water in slice

y, - Unit weight of water

u' - Effective normal stress

Typical slice a - Base angle of slice

I on basepf slice

7, = %d* 2

7 = (u' - u3)dF

1st estimate: u' = y,h, COS'Q ~ y,h,

For entire slope 1 + (tan 0 tan a/F)

i""';ontinuit?\ c = 7 - u' tan 0,

J-2 ,p

1 $: }*J

forms slice use base of c, 0 Janbu correction factor

found "La"- from direct t"... fo-1.125 + 0.033 loe-

J in tension crack

Sum for

all slices

\ LcJL 1 Y"

Generate new F = f, HAx p tan a + Q

values of Iterate to obtain F - Re-calculate u'

stability J

If F does not differ from

previous estimate

1 A

analysis

Build up histogram

of F values

Freq. P(F<F)

Figure 18.18 Monte Carlo simulation applied to slope instability in poor rock masses
(from lecture notes by S. D. Priest).