Name: _______________________________
Student ID:________________________________
Signature:________________________________
UNSW Sydney
TERM 3 2021 FINAL EXAMINATION
CVEN3202: Soil Mechanics
1. TIME ALLOWED: 2.5 hours (10 minutes reading the questions, 2 hours for the exam,
and 20 minutes for uploading answers) – solutions to be uploaded to Moodle by
4:30 pm.
2. Upload your solutions in a single pdf file.
3. THIS EXAMINATION PAPER HAS 7 PAGES.
4. TOTAL NUMBER OF QUESTIONS – 4
5. TOTAL MARKS AVAILABLE – 100
6. ANSWER ALL QUESTIONS.
7. MARKS AVAILABLE FOR EACH QUESTION ARE SHOWN IN THE EXAMINATION PAPER.
ALL QUESTIONS ARE NOT OF EQUAL VALUE.
8. CANDIDATES MAY USE CLASS NOTES, TEXT BOOKS AND ELECTRONIC CALCULATORS.
page 1 of 7
Please read the following notes carefully.
NOTE 1: Use 𝐺𝑠 = 2.65 and 𝛾𝑤 = 9.81 𝑘𝑁⁄𝑚3 where needed.
NOTE 2: In all questions, 𝜴 is the last two digits of your student id.
Example: for a student with the id of z1234567, 𝛺 = 67. So if, for example, the volume
𝛺
of a particular soil sample is (3 + 10) 𝑚3 in a question, for this student, the volume is
3 + 6.7 = 9.7 𝑚3 , or if another parameter is (50 + 3𝛺), that parameter is 50 + 3(67) =
251.
So please complete the boxes below before continuing:
Your student id:
z_______
➔ value of 𝛺 for you:
𝛺=__
NOTE 3: A linear-linear graph and a log-linear graph are provided at the end of the
questions. You may use them for developing your plots. Alternatively, you can use Excel or
MATLAB or similar software for your plots.
***************************************************************************
QUESTION 1
(20 marks)
In a project, silty soil is to be excavated from a borrow area and used as fill. A test pit having
a volume of 0.5 𝑚3 is excavated in the borrow area. The total mass of the soil excavated from
𝛺
the test pit was 0.94 𝑡𝑜𝑛𝑛𝑒𝑠 and it had a moisture content of (4 + 8 ) %. The compacted fill
𝛺
is to have a void ratio of 𝑒 = 0.55 and degree of saturation of 𝑆𝑟 = (60 + 10) %.
(a) If (20,000 + 500𝛺) 𝑚3 of compacted fill is required in the project, what volume of the
silty soil must be excavated from the borrow area? (10 marks)
(b) How much water (in tonne) must be added to the excavated material prior to compaction
in the fill? (10 marks)
page 2 of 7
QUESTION 2
(25 marks)
The figure below shows a concrete dam with a sheet pile. The water content of the soil under
𝛺
the dam is 𝑤 = (15 + 8 ) %.
(a) Compute the amount of water flowing under the dam per meter of dam if the coefficient
of permeability is 4 × 10−5 𝑐𝑚/𝑠. (3 Marks)
(b) Compute the pore water pressure and the hydraulic gradient at points A and B shown in
the figure. (6 Marks)
(c) Compute the effectives stress at point B. (4 Marks)
(d) Evaluate the exit gradient and compute the factor of safety with respect to quick
condition. Comment on the acceptability of the factor of safety you have calculated. (5
Marks)
Hint: You need to develop a flow net first. (7 Marks)
page 3 of 7
QUESTION 3
(22 marks)
An undisturbed clay sample was
taken from the middle of the
(4
+
6m
𝛺
)𝑚
Gravel
25
𝐺𝑟𝑎𝑣𝑒𝑙
void ratio = 0.5
normally consolidated clay layer
shown opposite, resulting in the
4m
following average values:
12
12 𝑚
m
𝛺
𝐶𝑐 = 0.3 + 200
A
= 20
22%
% (𝑤𝑎𝑡𝑒𝑟
(water content)
𝑤w=
𝑐𝑜𝑛𝑡𝑒𝑛𝑡)
𝑐𝑣 = 6 × 10−4 𝑐𝑚2 ⁄𝑠
A 10𝑚 × 20𝑚 platform is to be
constructed on the soil profile
applying
an
average
stress
of
(300 + 𝛺)𝑘𝑃𝑎 on the surface of the gravel.
a) Determine the ultimate consolidation settlement under the centreline of the platform. Use
2:1 method for calculating the increase in vertical stress due to construction of the
platform. (7 Marks)
b) How much settlement would occur 6 𝑦𝑒𝑎𝑟𝑠 after construction of the platform? (4 Marks)
c) How much would be the effective stress at point 𝐴 (shown in the figure) 5 𝑦𝑒𝑎𝑟𝑠 after
construction of the platform? For simplicity, assume that the increase in the stress at point
𝐴 due to loading is the same as the stress increase for the layer you have calculated in part
(a). (5 Marks)
d) It is planned to remove the platform once the consolidation of the clay layer is complete.
If the final thickness of the clay layer a long time after removing the platform will turn up
𝛺
to be (11.6 − 200) 𝑚, what would be the value of the recompression index of the clay
(𝐶𝑟 )? (6 Marks)
page 4 of 7
QUESTION 4
(33 marks)
A consolidated undrained (CU) triaxial test was performed on a specimen of saturated overconsolidated clay with a cell pressure of 400 𝑘𝑃𝑎 and a back pressure of 150 𝑘𝑃𝑎. At
𝛺
failure, the total vertical stress on the sample was (1000 + 2 ) 𝑘𝑃𝑎. Assuming 𝑐 ′ =
𝛺
(25 + 10) 𝑘𝑃𝑎 and 𝜑 , = 28° for the sample,
(a) Plot the effective stress and total stress Mohr circles at failure, and also the failure line on
the same plot. (6 Marks)
(b) Determine the total pore water pressure in the sample at failure. (4 Marks)
(c) Calculate the effective normal stress and the shear stress on the failure plane. (4 Marks)
(d) At failure, what angle (between 0 and 90 degrees) would the failure plane make with the
horizontal? (3 Marks)
(e) How much is the Skempton parameter 𝐴 at failure (𝐴𝑓 ) for this test? (4 Marks)
(f) Plot the total stress path (TSP) and effective stress path (ESP) on a p-q diagram for this
test. (6 Marks)
(g) A sample of this clay is taken from a different depth where it is known that the undrained
shear strength is 𝑐𝑢 = 120 𝑘𝑃𝑎. The sample is then subjected to a direct shear test in which a
normal stress of (200 + 𝛺)𝑘𝑃𝑎 is applied to the sample, and then immediately after
application of this normal stress, the sample is sheared quickly. What is the pore water
pressure in the sample at failure? (6 Marks)
****************************
END OF QUESTIONS
page 5 of 7
***********************
page 6 of 7
q
p
****************************
END OF PAPER
page 7 of 7
****************************