Page 1 of 7
CONFIDENTIAL EXAM PAPER
This paper is not to be removed from the exam venue
Civil Engineering
EXAMINATION
Semester 2 - Main, 2019
CIVL2410/CIVL9410 Soil Mechanics
EXAM WRITING TIME: 3 hours
READING TIME: 10 minutes
EXAM CONDITIONS:
This is a RESTRICTED OPEN book examination - specified materials permitted
During reading time - writing is not permitted at all
MATERIALS PERMITTED IN THE EXAM VENUE:
(No electronic aids are permitted e.g. laptops, phones)
Calculator - non-programmable
Sydney Uni Soil Mechanics Data Sheets. These will be provided
MATERIALS TO BE SUPPLIED TO STUDENTS:
1 x 16-page answer book
Type G (Centimetre)
INSTRUCTIONS TO STUDENTS:
Candidates should attempt FOUR questions.
Question 1 is compulsory and is worth 40%. The remaining questions are each
worth 20%.
Please tick the box to confirm that your examination paper is complete. 
Room Number ________
Seat Number ________
Student Number |__|__|__|__|__|__|__|__|__|
ANONYMOUSLY MARKED
(Please do not write your name on this exam paper)
For Examiner Use Only
Q Mark
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Total ________
Page 2 of 7

Question 1
(a) Determine the bulk unit weight of a soil specimen which has dry mass, ms = 100g, mass of water,
mw = 20g, specific gravity Gs = 2.5 and degree of saturation, S = 50%.

(b) Determine the vertical effective stress at point A, 5 m below the centre of a flexible circular
foundation, 10 m in diameter, applying a stress of 100 kPa to the surface of the clay, as sketched
in Figure 1. The water table is 1 m below the surface of the clay and the clay has dry unit weight,
dry = 15 kN/m3 and moisture content = 20%.





Figure 1

(c) A 5 m thick layer of clay is undergoing 1-D consolidation as a result of a sudden increase in
stress of 60 kPa. Measurements are taken to determine the excess pore pressure at the centre
of the clay layer. It is found that the excess pore pressure is 20 kPa when the settlement of the
layer is 65 mm. If the clay layer can only drain from its lower boundary, determine the final
settlement when consolidation is complete.

(d) A point in an elastic soil mass experiences total stress changes xx = 50 kPa, yy = 30 kPa and a
resulting strain zz = 0.001. Calculate the total stress change zz given the soil response is
undrained and the elastic properties are E = 5 MPa,  = 0.25.

(e) A consolidated undrained triaxial test is performed on a stiff over-consolidated clay specimen. If
the cell pressure is 150 kPa, the deviator stress at failure is 348 kPa and the pore pressure at
failure is -70 kPa determine the undrained strength cu and the effective friction angle ′ (assume
c′ = 0).

(f) Calculate the ultimate settlement of a 3 m thick silty clay layer, with properties Cc = 0.65, Cr =
0.08, subjected to a uniform increase in vertical stress of 40 kPa. The initial state of the clay is
given by an effective vertical stress of 15 kPa, a void ratio of 0.8 and an over-consolidation ratio,
OCR of 4.


Stiff clay
Permeable Rock
5 m
A
10 m
1 m
100 kPa
Page 3 of 7

(g) A 3m high retaining wall is being used to provide horizontal support. For the failure mechanism
shown in Figure 2 determine the maximum force F that can be resisted by the soil. The dry
retained soil has d = 18 kN/m3 and strength parameters c′ = 0, ′ = 30o. Assume the strength
parameters for shearing between the soil and the wall are the same, that is c′w = 0,  ′w = 30o.










Figure 2

(h) Figure 3 shows a flow net for water flowing underneath an impermeable structure. Determine
the pore water pressure at point X.


Water
Water 2 m
X
4 m
12 m
6 m
4 m
9.5m
Thursday Tute
FLOW NET 70
o
Assumed Failure Plane
F Dry Soil
Figure 3
Page 4 of 7

Question 2
Figure 4 shows a section through a site where a road embankment, over 100 metres long, is constructed
leading to a bridge river crossing. The embankment is 3 m high, 15 m wide and is constructed of well
compacted fill with a bulk unit weight of 22 kN/m3.
The soil profile at the site consists of 2.5 m of dense silt overlying a 10 m thick layer of soft clay over an
impermeable rock layer. Site investigations have indicated the clay has an over-consolidation ratio that
decreases from 1.8 at the top of the clay layer to 1.2 at the base. The clay properties are: void ratio e =
1.50; Specific gravity Gs = 2.6; Compression index Cc = 0.85; Re-compression Index Cr = 0.09 and
coefficient of consolidation cv = 4 m2/yr. The water table is 2.5 m below the surface, level with the top of
the clay and the silt has bulk unit weight bulk = 18 kN/m3.
a. Determine the settlements, assuming 1-D compression, under the centre of the embankment at
points A (at the end of the embankment next to the river) and B (20 m back from A). Use a single
soil layer and assume the settlement of the silt is insignificant.
b. At the time the road is opened the excess pore pressure measured at the centre of the clay layer
at B is 15 kPa. Determine the degree of consolidation and hence how much settlement is still to
occur at point B.
c. If the gradient of the road (s/L) exceeds 0.001 between A and B then pavement relevelling will
be required. Determine at what time after road opening this will occur. Assume that the degrees
of consolidation at A and B are identical.
d. Discuss the accuracy of the settlement and time calculations in parts a to c.










Figure 4

A B
Embankment
Silt
Bridge
20 m
>100 m
River Clay
Impermeable Rock
Page 5 of 7

Question 3
A large water tank, 10 m in diameter, is to be constructed on a site where 3.5 m of stiff clay overlie
impermeable rock. A thin layer of gravel is placed between the tank and the clay to level the site and
allow drainage.
To estimate the elastic properties of the stiff clay a cuboidal sample was set up in a test apparatus and
allowed to drain and reach equilibrium with effective stresses ´xx = ´yy = ´zz = 100 kPa, and pore
pressure u = 0 kPa. The sample was then subjected to increases in total stress, xx = 40 kPa, yy = 20
kPa, while maintaining zero strain in the z direction. Drainage was then permitted keeping the
stresses xx, yy, constant and maintaining zero strain in the z direction. After reaching equilibrium, with
u = 0, a volume strain of 0.00156 was measured and the stress in the z direction was zz = 118 kPa.
a. Determine the elastic properties of the clay E′ and ′.
b. Determine the volume of water that can be held in the tank if the settlement under the centre
cannot exceed 10 mm. Assume that the tank base is rigid.
c. As part of a proof test the water tank is filled to the maximum level determined in part (a). After
3 months the settlement is 5mm and only increasing relatively slowly, and at this time water is
removed from the tank. The operators (not understanding soil mechanics) decide that more
water can be stored and decide to increase the maximum water height by 50% and refill the
water tank to this level. Estimate the coefficient of consolidation, cv and hence determine at
what time the settlement will exceed the 10 mm limit.
d. Discuss the applicability of elastic analysis to the prediction of soil settlements.

Page 6 of 7


Question 4
Figure 5 shows a section through a development site where a deep excavation in silty sand has been
constructed near an existing building. Due to difficulties during construction the basement walls and
base are cracked and provide no restriction to flow of water and pumps have to be provided to keep the
excavation dry. The silty sand has uniform and isotropic permeability, k = 1 × 10-6 m/s. Due to the
presence of a nearby river the water table in the ground is maintained 1 m below the ground surface
and is unaffected by the flow.
a. Draw a flow net for the situation shown in Figure 5 marking clearly the boundary conditions on
your drawing.
b. Determine the required capacity of the pumps.
c. Determine the pore pressures and hence the change of effective stress at A and B due to the
water flow.
d. Determine the tilt of the building caused by water flow if the coefficient of compressibility, mv =
0.000025 m2/kN for the silty sand.
e. Suggest 2 methods available to the owners of the existing building to prevent damage to their
building that do not involve fixing the leaking basement.



Figure 5 (Drawn to scale, x= 12 m)
Ground
Water Level
Impermeable Rock
Silty Sand
Existing
Building
A
B
x
x
x
x
Page 7 of 7

Question 5
Part 1
Using sketches and by reference to the critical state line concept, or otherwise:
a. Explain why some particulate soils (sands and gravels) are referred to as cohesionless, whereas
others (clays and silts) are referred to as cohesive.
b. Explain why this distinction between cohesive and cohesionless is unhelpful when considering
the stress-strain-strength behaviour of sands and clays.
Part 2
Three identical normally consolidated specimens of clay are set up in shear box apparatus with initial
state  = ′ = 100 kPa,  = 0, e = 1.0.
Specimen 1 is sheared to failure keeping ′ constant, and at ultimate failure  = 50 kPa.
Specimen 2 is one dimensionally compressed to a state with ′ = 200 kPa,  = 0, e = 0.8. It is then
sheared to failure rapidly so that no drainage occurs. At ultimate failure  = 50 kPa.
a. Determine the critical state line (CSL) parameters ′cs,   Note that the CSL is described by the
equations  = ′ tan ′cs and e =  −  ln (′) and that the CSL and normal 1-D compression lines
have the same gradient.
b. Determine the ultimate shear stress and pore pressure of Specimen 3 if it is sheared undrained
and the total stress is maintained constant,  = 100 kPa

END OF EXAMINATION

欢迎咨询51作业君