Engineering Rock Mechanics

234 Rock mechanics interactions and rock engineering systems

procedure will not prevent a disaster. Both the technical audit and the
QA are required, and they must be performed in that order.
We anticipate greater emphasis on both the technical auditing and QA
aspects in the years ahead. The questions and answers in this chapter are
designed to highlight the process of thinking about the total problem,
whether this is for rock mechanics modelling only or for modelling in or-
der to develop a practical rock engineering design. It is of critical import-
ance in engineering to have a predictive capability, and this can only be
achieved if all the appropriate factors have been taken into account.

14.2 Questions and answers: rock mechanics

interactions and rock engineering systems

414.1 Explain with a one-sentence answer how, in a rock mass,

(a) the rock stress can affect water flow in fractures,

(b) the water pressure in fractures can affect the rock stress,

(c) the fractures can affect water flow,

(d) water flow can affect the fractures,

(e) the fractures can affect rock stress, and

(f) the rock stress can affect the fractures.

A14.1

(a) Changes in rock stress open and close fractures, and the resulting

changes in fracture aperture affect the water flow through the frac-

tures.

(b) When water in fractures exerts a pressure on the fracture walls, the

rock stress acting normal to the fractures is reduced by the value of

the water pressure, which is the principle of effective stress.

(e) Water flows mainly through the fractures rather than the intact rock,

because the hydraulic conductivity of a fracture is usually much

greater than that of the intact rock.

(d) Continued water flow can either erode the fracture walls or cause

other materials to block up the fractures, resulting in either increased

or reduced water flow.

(e) Surfaces of open fractures are principal stress planes (because the

air or water between cannot sustain a shear stress) and so the stress

state in the rock adjacent to a fracture surface will be changed in both

magnitude and orientation.

(f) Rock stresses can cause normal and shear fracture displacements and

damage to fracture walls.

414.2 Draw an interaction matrix having Fractures, Rock Stress, and

Water Flow as the leading diagonal terms. Insert the letters repres-

enting the six mechanisms in 414.1 in the appropriate off-diagonal

boxes of the interaction matrix.

A14.2 The required matrix is shown below. This is one example of a

simple interaction matrix.