Questions 7 7.7 - 7 7.7 0: foundation and slope instability mechanisms 471
Q 17.5 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.
3Assuming conditions of heavy rainfall, determine the water pressure
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 lop6 m2/s and assume block
ABD is stable.417.6 Assume the same circumstances in 417.5 and consider the case
when the block DBC has moved downhill a limited amount, resulting
in a considerable increase in the aperture of DB. Calculate the factor of
safety against sliding of the block DBC.417.7 A rectangular foundation base, 6 m x 10 m in plan, carries a
vertical load of 180 MN which it transmits to the rock as a uniform
bearing pressure. The rock can be taken to be CHILE, with a modulus of
elasticity E = 800 MPa and Poisson's ratio u = 0.2.
Ignoring the weight of the rock, evaluate the vertical settlement at the
centre, C, of the base and the vertical stress at a point 5 m below C.
Q 17.8 The expression for the radial stress induced beneath a line load of
magnitude P acting normal to the surface of a semi-infinite half space is5, = -.
2P sin8
xr
(a) Show on a sketch the geometry of this problem, indicating clearly
the angle 8 and the orientation of the stress components 00 and 5,. What
are the magnitudes of 50 and trO in this problem?