CN245 Soil Mechanics
s
School of the Environment
Semester 2 Examinations 2006-2007
CN245
SOIL MECHANICS
Instructions to Candidates:
Time allowed: THREE hours
Attempt ALL questions in Section A (25%) and THREE questions from
Section B (75%)
Formula sheets provided
Graph paper provided
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CN245 Soil Mechanics
Friday 8 June 2007, 9:30-11:30 hours
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CN245 Soil Mechanics
SECTION A
Answer ALL questions
Question 1
For the soil strata (shown below in Figure 1) draw diagrams indicating the
total stress, pore pressure and effective stress profiles to a depth of 10 m for
(a)
the water table at the ground surface
(2 marks)
(b)
the water table at 2.0 m below the ground surface.
(assume the sand is partially saturated above the water table)
(3 marks)
0
SAND
4
bulk= 18 kN/m3
sat = 21 kN/m3
CLAY
Depth (m)
sat = 17.8 kN/m3
10
w = 9.81 kN/m3)
Figure 1
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CN245 Soil Mechanics
Question 2
The data from a direct shear box test on a sand sample 10 cm x 10 cm x 3
cm, at a constant vertical force of 1200 N are shown in Figure 2.
Figure 2
(a)
Is the sand dense or loose? Explain your answer.
(3 marks)
(b)
Determine (i) the maximum or peak angle of shearing resistance and
(ii) the critical state angle of shearing resistance.
(4 marks)
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CN245 Soil Mechanics
Question 3
(a)
Briefly describe what you understand by the terms normally
consolidated and overconsolidated. Use a plot of void ratio or
specific volume to illustrate your answer.
(3 marks)
(b)
A soil profile at a site consists of a layer of sand 10m thick overlying a
layer of soft normally consolidated clay 2.0 m thick below which is a
deposit of coarse sand, with a water table 3 m below ground level.
A building to be constructed on the site will impose a vertical effective
stress increase of 150 kPa at the middle of the clay layer.
Estimate the primary consolidation settlement of the clay layer
assuming the soil above the water table is saturated and the unit
weights of the upper sand stratum and clay layer to be 19 and 18
kN/m3 respectively and the initial moisture content of the clay is 43%.
f
H0

Cc log( ' ) ; Cc = 0.3 and Gs = 2.71]
1  e0
z
'
[ NB.
 pc
(6 marks)
Question 4
TRUE
FALSE
All soils behave as frictional materials in the long-term
Total stress analysis is performed using undrained soil
parameters
In a drained response, effective and total stresses are the
same
The permeability of soil is a function of the grain size
(4 marks)
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CN245 Soil Mechanics
SECTION B
Answer THREE questions from this section
Question 5
(a)
Outline the rules governing the construction of a flow net in a
hydraulically isotropic soil.
(4 marks)
(b)
A sheet pile wall is driven to a depth of 6 m into a permeable soil which
extends to a depth of 13.5 m below ground level. Below this there is
an impermeable stratum, as shown in Figure 3 below.
(i)
For a depth of water of 4.5 m on one side of the sheet pile, draw
the flow net for seepage under the sheet pile wall.
(12 marks)
(ii)
Estimate the seepage rate in (m3/day/m run) taking the
permeability of the soil as 6 x 10-3 mm/sec.
(6 marks)
(iii)
Determine the pore water pressure at point X.
(3 marks)
4m
6m
3m
X
13.5 m
NB: all dimensions are in metres
Figure 3
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CN245 Soil Mechanics
Question 6
It is required to estimate the pressure exerted on a sewer pipe at a depth of
5m passing adjacent to a building. The footprint of the building, and location of
the pipe is shown below in Figure 4.
If the pressure exerted by the building foundation slab is uniform at 45 kPa
then estimate
(a)
the pressure exerted at point X on the top of the pipe by the foundation
using Fadum’s chart
(15 marks)
(b)
compare the result with that obtained using Newmark's chart.
(10 marks)
Pipe
3m
3m
2m
X
3m
1m
6m
Figure 4
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CN245 Soil Mechanics
Question 7
A 100mm cubical sample of sand is in equilibrium when subjected to the
forces as shown in Figure 5 below.
(a)
Determine the stresses on the soil element and construct the Mohr’s
circle of stress.
(10 marks)
Using the Mohr’s circle determine
(b)
The magnitude of the major and minor principal stresses
(4 marks)
(c)
The orientation to the horizontal of the major principal stress plane
(4 marks)
(d)
The stresses on a plane at an orientation of 30 degrees anti-clockwise
to the major principal stress plane.
(4 marks)
(e)
If the sample is at failure, what is the angle of shearing resistance of
the sand?
(3 marks)
4.5 kN
1.3 kN
1.3 kN
3.5 kN
3.5 kN
1.3 kN
1.3 kN
4.5 kN
Figure 5
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CN245 Soil Mechanics
Question 8
(a)
Using the simplified method of slices derive the following expression, in
terms of effective stress parameters (c' and '), for the limit equilibrium
stability of a soil slope assuming a circular arc failure surface.
FS 
c' l  (W . cos   u.l ) tan  '
W . sin 
where,
l
= length of arc at slice base

= angle of slice base to horizontal
W
= weight of slice
u
= pore pressure at base of slice
(8 marks)
(b)
The profile of an old railway cutting is given below in Figure 6, the
phreatic surface is indicated by the dashed line.
Estimate the factor of safety of the slope using the Ordinary Method of
Slices.
(17 marks)
7.0 m
R = 8.5 m
1.5 m
2.5
4.0 m
1
c' = 4 kPa
 = 20 degrees
Unit weight = 20 kN/m3
Figure 6
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