Engineering Rock Mechanics

Questions 74.7-74.70: rock mechanics interactions 457

P1. Geology P2. Folds P3. Faults P4. Rainfall

P5. Freeze P6. Previous P7. Intact rock P8. Weathering
and thaw instability strength

P9. Number P10. Orientation P11. Aperture P12. Persistence
of sets

P13. Spacing P14. Mechanical P15. Rock mass P16. Hydraulic
properties strength conditions

P17. Slope P18. Slope P19. In situ P20. Potential
orientation dimensions stress instability

The 20th parameter, 'Potential instability', was added in a similar way to
the CONSTRUCTION term in Q14.4 in order to consider the effects of the
parameters on potential instability.
An interaction matrix was generated using these 20 parameters as the
leading diagonal terms. The interactions in the off-diagonal terms were
then assigned a value according to the same scheme as in Q14.8:

0 - no interaction; 1 - weak interaction; 2 - medium interaction;

3 - strong interaction; 4 - critical interaction.

This gave (C, E) co-ordinates and values of C + E and C - E as follows.

P1 P2 P3 P4 P5 P6 P7 P8 P9 P10

C 24 34 52 17 21 22 15 9 24 28
E 54 3 4 17 42 8 27 13 12
C+E 29 38 55 21 38 64 23 36 37 40
C-E 19 30 49 13 4 -20 7 -18 11 16

P11 P12 P13 I'14 P15 P16 I'17 P18 P19 P20

C 27 26 18 9 9 21 21 8 18 0
E 22 19 20 26 40 29 17 19 16 60
C+E 49 45 38 35 49 50 38 27 44 60
C-E^5 7 -2 -17 -31 -8^4 -11^2 -60

Plot the 20 parameters using C-E axes, and hence illustrate for the
natural slope system which three parameters are the most interactive,
which three are the most dominant, and which three are the most
subordinate.

Q74.10 One of the most important aspects of the systems approach is
establishing when positive feedback in the system can occur, because
positive feedback loops can cause instabilities.
(a) Explain the meaning of the terms 'negative feedback' and 'positive
feedback', and give examples of each in engineering rock mechanics.
(b) Also explain how engineering actions can mitigate or eliminate
positive feedback (equivalent to altering the mechanisms in the off-diag-
onal boxes of the interaction matrix).