s
School of the Environment
Examinations 2009-2010
CN127
SOIL MECHANICS
Instructions to Candidates:
Time allowed: TWO hours
Answer ALL questions in SECTION A
and TWO questions from SECTION B
Question
Mark
Section A
1
2
3
4
5
All answers are to be written on the exam paper
6
7
8
9
Section B
Student number
Seat location
Total mark
June 2010, 2 hours
Page 1 of 11
CN127 Soil Mechanics Exam 2010
Section A: Answer ALL questions from this section
Question 1
Using sketches, briefly describe the following retaining structures:
(a) Crib wall
(b) Counterfort wall
(c) Sheet pile wall
(9 marks)
Page 2 of 11
Section A
CN127 Soil Mechanics Exam 2010
Question 2
Two pumping tests are performed in an area with high permeability soil and an area
with low permeability soil. Draw characteristic water tables for each area during a
pumping test into the sketches below.
water level in borehole during pumping test
original water table
high permeability soil
impermeable rock
water level in borehole during pumping test
original water table
low permeability soil
impermeable rock
(4 marks)
Question 3
What type of foundation should be used for a small domestic building in the following
ground conditions?
Peat overlying dense
gravel in 1.5m depth
Dense gravel and sand
Stiff clay (medium
swelling potential)
Soft clay gradually
becoming stiffer in 20m
depth
Loose sand and clay (15
m thick) over hard rock.
(5 marks)
Page 3 of 11
Section A
CN127 Soil Mechanics Exam 2010
Question 4
Match the following types of soils against the environments where the soils were
formed by writing the correct letters into the empty boxes:
Layered silty clay
A
Glacial sediment
Angular cobbles and boulders
B
River sediment
Well rounded fine gravel
C
Wind-blown sediment
Well graded clayey sandgravel
D
Sediment from the bottom af
a lake
Uniformly graded silt
E
Scree from the bottom of a
steep slope
(5 marks)
Question 5
Determine the Plasticity index of samples 1-3; classify each sample using the graph
below and write the class symbol in the table.
Upper plasticity range
Low
70
Intermed.
High
Very high
CV
Extremely high
CE
Plasticity Index (%)
60
CH
50
CI
Clay
ME
40
CL
30
MV
20
Silt
10
MI
ML
0
0
10
20
30
40
MH
50 60 70 80
Liquid limit (%)
Sample Plastic limit
(%)
Liquid limit
(%)
1
15.4
32.8
2
34.2
95.7
3
39.7
58.6
Plasticity Index
(%)
90 100 110 120
Classification
Symbol
(6 marks)
Page 4 of 11
Section A
CN127 Soil Mechanics Exam 2010
Question 6
A soil slope of fine sand is purely friction controlled. The slope angle is  = 30.
The sand has a unit weight of 19.7 kN/m3 (dry) and 22.3 kN/m3 (saturated).
The angle of friction of the sand is  = 38. The volume element V is representative
for the whole slope.
water level during flood
2.5 m
1.8 m
Volume V

a)
Calculate the normal stress and the shear stress on the sliding plane
and determine the factor of safety of the dry slope.
(5 marks)
b)
During a flood the slope is completely submerged in water.
Calculate the normal stress and the shear stress on the sliding plane
and determine the factor of safety of the submerged slope.
(5 marks)
Page 5 of 11
Section A
CN127 Soil Mechanics Exam 2010
(Question 6 continued)
c)
After the flood has drained, the sand in the soil is still saturated, i.e. the water
table is at the slope surface and the water seeps parallel to the slope surface.
Determine the factor of safety at a plane 1.8m vertically below the slope surface
as shown in the figure above.
(8 marks)
Question 7
Indicate whether the following statements are true or false by circling “T” or “F”:
a)
Loose soil is more likely to liquefy than dense soil.
T
F
b)
Volume lost by shrinking when a clay soil dries out will not be fully
recovered by swelling when wetting the soil again.
T
F
c)
Quartz gives soil a reddish-brown colour.
T
F
d)
Water pressure can destabilise slopes.
T
F
e)
Glacial deposits are very inhomogeneous.
T
F
f)
The Liquid Limit of a soil is measured by rolling a 3mm thread.
T
F
g)
Loose soil dilates during shear.
T
F
h)
Water pressure is the same in each direction, but stresses in soils
may be different in each spatial direction.
T
F
(8 marks)
Page 6 of 11
Section A
CN127 Soil Mechanics Exam 2010
Question 8
Using sketches, briefly describe how replacement piles (made of reinforced
concrete) are installed into the ground.
(7 marks)
Question 9
Give a short definition of “strain” in soil mechanics:
(2 marks)
Page 7 of 11
Section A
CN127 Soil Mechanics Exam 2010
Section B: Answer TWO questions from this section
Question 10:
constant
head tank
manometers
h
timer
Q
A
soil
B
L
The permeability of a soil was measured in the laboratory by carrying out three
constant head permeameter tests as shown in the figure.
The following results were established in the tests:
Distance between manometer taps:
Diameter of cylindrical test sample:
120 mm
90 mm
Test 1
Test 2
Test 3
Duration of test, t:
10 min
12 min
15 min
Quantity of water collected, Q:
450 ml
134 ml
486 ml
Head difference in manometer, h:
84 mm
22 mm
63 mm
Give the values of the parameters listed in the table below for each test.
Determine the value of permeability k for each test and the average value of k.
Area of sample, A
Test 1
Test 2
Test 3
Flow rate, q
Apparent velocity, v
Gradient, h/L
Permeability, k
Average value for k:
(18 marks)
Page 8 of 11
Section B
CN127 Soil Mechanics Exam 2010
Question 11:
Mass
Volume
Air
Proportions
VA
e
VV
MW
MS
Water
VW
V
Solid
VS
VS
1+e
1
Proportions:
n = e / (1 + e)
e = n / (1 – n)
v = 1 / (1 – n)
v=e/n
Moisture content = MW / Ms
n = VV / V
e = Vv / VS
Sr = VW / VV
g = 9.81 m/s2
rW = 103 kg/m3
A fully saturated soil sample has a moisture content of 24%; the volume of the soil
grains is VS = 4.53×10-3 m3; the mass of the soil grains is MS = 11.80 kg.
Determine the following:
The density of solids in the
sample
The mass of the water in the
sample
The total volume of the sample
The void ratio of the sample
A moist soil sample has a mass of 20.82 kg and a volume of 9.40×10-3 m3.
After drying in an oven the sample has a mass of 16.75 kg, and the volume is
unchanged. Determine the following:
Moisture content of the sample
Volume of water in the sample
Density of dry sample
Density of moist sample
Unit weight of moist sample
(18 marks)
Page 9 of 11
Section B
CN127 Soil Mechanics Exam 2010
Question 12:
During a site investigation a 4m thick layer of sand (=18.4 kN/m3) was found above
a thick layer of silty clay (=20.5 kN/m3). The water table is at a depth of 2.0 m below
ground level. Above the water table the sand is saturated due to capillary action.
Take the unit weight of water equal to W = 9.81 kN/m3.
Complete the table with the values for total vertical stress, pore water pressure and
effective stress at 2.0m, 4.0m and 8.0m depth. Plot the total stress, water pressure
and effective stress profiles in the graph.
Depth
2.0 m
4.0 m
8.0 m
Total vertical
stress, V
Pore water
pressure, u
Vertical effective
stress, V’
A structure has been built on the soil surface applying a uniform load of 55 kN/m2
on the soil. Determine the effective stresses at 2.0m, 4.0m and 8.0m depth a few
days after building. Plot the effective stress profile into the graph.
Vertical effective
stress, V’
Pressure, Stress (kN/m2)
-20
0
20
40
60
80
100
120
140
0
1
2
Depth (m)
3
4
5
6
7
8
(18 marks)
Page 10 of 11
Section B
160
CN127 Soil Mechanics Exam 2010
Aide Memorie
Slope stability
'v  tan 
 v  tan 
Slope stability (friction)
Fos 
Slope stability (cohesion)
c ur 2
Fos 
W d
Depth of tension crack
hc  2c u / 
Slope stability
(friction and cohesion)
Fos 
c' r  N  tan 
T
Hydraulic permeability
Apparent velocity
v
Q
At
Flow rate
q
Q
t
Seepage velocity
vs 
Darcy’s law
v k
Page 11 of 11
 vA
v
n
h
L
q kA
h
L
Aide memoire