Engineering Rock Mechanics

Questions 9.1 -9.1 0:

permeability

49.1 How long does it takes for water, subjected to a 10-m head

difference, to pass horizontally through

(a) a 5 m length of intact granite which has an isotropic hydraulic

(b) through a 5 m length of fractured limestone with an isotropic

conductivity, K, of 1 x 10-l2 m/s, and

hydraulic conductivity of 1 x m/s?

49.2 The hydraulic conductivity of an array of parallel fractures (Hoek
and Bray, 1977) in the direction parallel to the plane of the fractures is
given by the equation, K = hge3/12u where h is the fracture frequency,
g is gravitational acceleration, e is the fracture aperture, and v is the
kinematic viscosity of the fluid.
For a rock mass with a fracture frequency of one fracture per metre
and with fracture apertures of 0.01 mm, the hydraulic conductivity is
8.3 x lo-'' m/s.
(a) What is the hydraulic conductivity of a second rock mass which
has ten fractures per metre and fracture apertures of 1 mm?
(b) What is the main factor contributing to the difference in the
hydraulic conductivity of the two rock masses?

49.3 In question Q9.2, the hydraulic aperture was used. For a fracture
with planar and parallel sides, this aperture is the perpendicular distance
between the two sides (see Fig. Q9.1).
(a) Explain the meaning of fracture aperture when the fracture surfaces
are rough.
(b) Do you think that the mechanical aperture and the hydraulic
aperture of a fracture have the same value? If not, which is greater?

49.4 What is meant by the term 'transmissivity'?

49.5 The horizontal section below shows two sets of rock fractures in
a 10 m square rock block of unit thickness. On the left-hand side, there
is a hydraulic head of 3 m; on the right-hand side, the head is 1 m; and