Cork Institute of Technology
Bachelor of Engineering (Honours) in Structural Engineering- Award
(NFQ Level 8)
Summer 2006
Geotechnical And Foundation Engineering
(Time: 3 Hours)
Answer five questions Examiners: Mr T Corcoran.
Prof. P. O’Donoghue
Mr J P Murphy
Q1. (a) A retaining wall with a smooth vertical back supports a 12 m depth of soil. Assume the
water table is well below the base of the wall. Using the Rankine method, determine the
magnitude and position (height above the base of the wall) of the resultant active thrust on
the back of the wall. The soil has the following properties: surface horizontal, no surcharge,
two layers, 0-4 m below surface: c
′
= 0;
φ′
=30°and
γ
=19.6 kN/m3; below 4 m: c
′
= 25
kN/m2;
φ′
=15°and
γ
=18.2 kN/m3.
(10 marks)
(b) Show that for a wall retaining an unsurcharged horizontal soil surface, using Coulomb’s
method, the active thrust is given by:
P
A =
)tan(
)sin(
)cos(
W
φθ
δα
δα
′
−
−
+−
(5 marks)
(c) Calculate the active thrust acting on a smooth vertical wall of height 6 m due to a wedge of
homogeneous soil (Use a wedge angle of 62°) having an unsurcharged horizontal surface
and the following properties: c
′
= 0;
φ′
=30°and
γ
=19.0 kN/m3.
(5 marks)
Q2. An anchored sheet pile is required to support an excavation of depth 9 m. The anchor will
be attached at a point 1.5 m below the ground surface. The water table is well below the toe
of the wall. There is no surcharge, wall friction may be ignored and no allowance for
overexcavation need be made. The soil properties are c
′
= 0;
φ′
=28°and
γ
=20.0 kN/m3.
(a) Determine the design embedment depth by the factor of safety on embedment method
using Fd = 1.2. (Hint: initial trial depth =4.0 m)
(15 marks)
(b) Determine the force in the anchor per metre run and the maximum shear force in the sheet
pile if FT = 2.0.
(5 marks)
