156 PermeabilityOne can imagine that the normal stress component across the fracture
is indeed reduced by the jacking effect of the water.
(b) Unfortunately, the situation is not as simple as may be thought
because the fracture has already disturbed the in situ stress field, rotating
and altering the principal stress components. Also, as the water pressure
is steadily increased, the ratio between the amount of shear stress
and the amount of normal stress that can be transmitted across the
fracture changes, until eventually no shear stress will be transmitted
when the fracture surfaces are separated, and only normal stress can be
transmitted across the fracture, now the same value, u, along the whole
fracture.
In A8.8, we explained that the main difference between soil mechanics
and rock mechanics is that soil is a particulate medium in which the
grains are much smaller than the engineered structure. The effective
stress concept holds in rock mechanics, providing a sufficiently large,
strongly fractured rock mass is being considered, but often the dimen-
sions of the engineered structure are of the same order as the sketch
above and it is the local, not global, conditions that govern engineering
success or failure in rock mechanics.
Q9.10 In the photograph below, the water pressure in a borehole
in marble has been steadily increased. In addition to the normal
pressure created at the rock surface, the water pressure in this
configuration also creates a circumferential tensile stress of the
same magnitude in the rock, as in 44.6. A crack developed and
was propagated under servo-controlled conditions from A to B. The
water not only penetrated the crack but also the intact marble,
forming the dark inverted V visible in the photograph. How do we
deal with the effective stress under these circumstances?