Engineering Rock Mechanics

Question and answers: foundation and slope instability mechanisms 297

417.5 A cross-section through a rock slope is shown below. The rock

is impermeable with a unit weight of 22 kN/m3 and the fractures AC

and DE have an aperture of 1 mm. A drainage gallery at E reduces

the water pressure at that point to the atmospheric pressure value.

The drainage gallery, the fractures and the slope face all have the

same strike.

A D c' = 10 kN/m3-

\ $' = 30"

Assuming conditions of heavy rainfall, determine the water pres-

sure at B, and hence the factor of safety against sliding of the block

DBC. Take the kinematic viscosity of water to be 1.3 x 1 0-6 m2/s and

assume block ABD is stable.

A17.5 We start by analysing the fracture network in order to determine
the water pressure distribution along the fractures BD and BC. The
first step is to determine the elevation of the nodes and the lengths of
the channels from the network geometry. This is shown in the sketch
below.
17/tan30

A D

45"

Node: A B CD E

Elevation (m) 17.0 17.0tan30 = 9.815 0.0 17.0 9.815 - 6.5 = 3.315

Channel: AB BC BD BE

Length (m) 17.0/sin 30 - 17.0/cos 30 17.0/cos 30 17.0 - 17.0 tan 30 6.5

= 14.37 = 19.63 = 7.185