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The University of NottinghamMalaysia Campus
DEPARTMENT OF CIVIL ENGINEERING
A LEVEL 2 MODULE, SPRING SEMESTER 2009-2010
GEOTECHNICS 2
Time allowed TWO Hours
Candidates may complete the front cover of their answer book and
sign their desk card butmust NOT write anything else until the
start of the examination period is announced
Answer ALL questions
This module has 20% coursework assessment
All questions and parts of questions carry marks as indicated in
brackets.
Only self-contained calculators with a single-line or dual line
display are permitted in thisexamination
Dictionaries are not allowed with one exception. Those whose
first language is not Englishmay use a standard translation
dictionary to translate between that language and English
provided that neither language is the subject of this
examination. Subject specific translationdictionaries are not
permitted.
No electronic devices capable of storing and retrieving text,
including electronic dictionaries,may be used.
DO NOT turn examination paper over until instructed to do so
ADDITIONAL MATERIAL: Data sheet (2 sides)
2mm graph paper (3 sheets)
INFORMATION FOR INVIGILATORS: Please collect the question sheets
from studentsat the end of the examination.
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1. (a) Triaxial compression tests on three specimens of a soil
were performed. Each
test was carried out until the specimen experienced shear
failure. The test dataare tabulated as follows:
SpecimenNo.
Minor Principal Stress, 3
Confining pressure(kPa)
Deviator Stress at Failure
q(kPa)
1 69 276
2 146 328
3 207 359
i) Sketch the Mohrs circle for the three tests.[4 Marks]
ii) Determine the soils cohesion and angle of internal
friction.[4 Marks]
iii) Using the test result of specimen No.1 plotted on Mohrs
circle, find theangle of the failure plane (), shear () and normal
(n) stresses acting on
the failure plane.[5 Marks]
iv) Determine the value of (), () and (n) on the failure plane
for testspecimen No.1 by computation.
[6 Marks]
(b) A sample of sand with 'ult = 30 is tested in the triaxial
apparatus. The sampleis saturated with water and the cell pressure
is increased to 200 kN/m2. At thispoint the test specimen has a
diameter of 38 mm and is 76 mm high. Thedrainage tap is then closed
and the axial load is increased, giving an ultimate
load of 318 N at an axial strain of 7.0 %. Deformation of the
sample isapproximately uniform.
i) Calculate the pore pressure on the failure plane[6 Marks]
ii) Sketch Mohrs circle of total and effective stresses at the
ultimatecondition.
[2 Marks]
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2. A proposed L-shaped reinforced concrete retaining wall is
shown in Figure Q.2 withrelevant material properties. The unit
weight of the concrete is c=24 kN/m
3. The
coefficient of base friction is estimated to be 0.48 and
allowable soil pressure for thefoundation soil is 190kN/m2.
(a) Assuming that the wall has a smooth back and that g=10 m/s2,
sketch the
earth pressure diagram for the retained soil and label all
relevant values. Youmay assume the soil and wall to be
free-draining (so that there is no porewater).
[6 Marks]
(b) Determine the safety factor against overturning about the
left toe (ignore thepassive pressure in front of the wall).
[5 Marks]
(c) Determine the factor of safety against sliding.[4 Marks]
(d) Determine the maximum and minimum base pressures.[6
Marks]
(e) The drains in the wall become blocked so that the water
rises to 1.5m above thebase of the wall in the retained soil.
Assuming that the unit weight of Soil 2 is
the same above and below the groundwater table, sketch the new
earthpressure, labelling relevant values.
[6 Marks]
Fig. Q.2
2.5m
0.5m
3.5m
3.0m
0.5m
Soil 1:dry = 16 kN/m
3
= 30
Soil 2:dry = 17 kN/m
3sat = 20 kN/m
3
= 35
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3. A cutting 8m deep is to be excavated in a saturated clay with
angle 1 (vertical):1 (horizontal). Figure Q.3 shows relevant
information for a circular and wedge
slip. The unit weight of the clay is 20kN/m3 and the design
shear strengthparameters are cu=45 kN/m
2 and u=0.
(a) Assuming an undrained behaviour, determine the arc length
ABC and the lengthof chord AC and calculate the FoS against
instability for each mechanism.
[9 Marks]
(b) A hard stratum underlines the clay at a depth of 12m below
the ground level.Using Taylors stability coefficients, determine
the FoS against instability.
[3 Marks]
(c) What is the factor of safety for the rotational mechanism if
allowance is made forthe development of tension crack?
[4 Marks]
Wedge slip: Area of ACD = 28 m3/mCircular slip: Area of ABCD =
70 m3/m
Fig. Q.3
4. A strip footing is to be designed to carry a load of 500 kN/m
at a depth of 1.1min a gravely sand. The appropriate shear strength
parameters are c=0 and=30.
(a) Determine the width of the footing if a safety factor of 2.5
against shearfailure is specified and assuming that the water table
may rise tofoundation level. Above the water table the unit weight
of the sand is 17kN/m3 and below the water table sat= 20 kN/m
3.
[8 Marks]
(b) Determine the ultimate bearing capacity if the water table
is at thesurface.
[2 Marks]
4.50 m
3.50 m
W
C
B
D
A
O
8.0 m
89.5 r =12.10 m
45
Ground level
