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).