Questions and answers: rock mechanics interactions and rock engineering systems 243starts to slide is released through a fractureRock block. If, as the block begins to slide, the irregularities of the rock
block and rock slope surfaces tend to inhibit further movement, there
is negative feedback. If, on the other hand, the block movement tends
to remove such irregularities and reduce the interface friction, there is
positive feedback and the block will accelerate.
Failure around un excavation. Assume that the stresses around an ex-
cavation are causing rock failure. If the rock failure causes a reduction
in the stress concentrations and the energy released is absorbed in the
rock failure process, then there is negative feedback and the process is
self-stabilizing. If, however, the rock failure causes an increase in the
stress concentration and not all the energy released is absorbed in the
rock failure process, there is positive feedback which could lead to a
rockburst.
Water flowing through u fracture. When water flows through a fracture
over a period of time, it could deposit sediment in the fracture reducing
the water flow. This is an example of negative feedback. Conversely, the
action of the water could erode the fracture walls enabling more water
to flow through the fracture. This is an example of positive feedback
an increase in water flow in the rock fracture will enlarge the fracture
further, which in turn allows more water to flow, which could enlarge
the aperture even further....
Most rock mechanics mechanisms are of the attenuating or self-stabil-
izing type with negative feedback or a small positive feedback; if they
were not, the actions of rock engineers would lead to mayhem because
any type of engineering work would immediately run out of control.
The art in rock engineering awareness is recognizing when positive
feedbacks might occur and then avoiding them.(b) The key engineering action
is to identify the interactions as-
sociated with such positive feed-
backs and to mitigate their effects.
For example, if the mechanism
path in the matrix to the right
is associated with positive feed-
back, the mechanism path can be
blocked by eliminating any of the
mechanisms marked with an 'x'.
In the case of the specific mechan-
ism path in A14.5, the best option
would probably be to grout the
fractures (the black box). Other in-
hibiting actions, such as reinforce-