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354 Final Exam Tutorial Topics
• 353 review– Consolidation
– Settlement (differential)
– Weight volume relationships
– Excess Pore pressure
– Effective/total stress
• Soil Strength– Peak ultimate and residual
– t vs s and Mohr Coulomb
– Undrained vs drained
– Skempton’s A & B
– Loading and unloading
• Slopes– Wedges and circular failures– Mitchell charts (Ru)– Simple derivations
• Bearing Capacity– Terzaghi BC equation– Rafts and footings– Undrained and drained– Settlement: elastic - consol– Rocks, and N values– Eccentricity
• Retaining Walls– Earth pressures– Rankine and Coulomb– Factors of safety
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Retaining Walls - Rankine
• Assumptions
1. Vertical wall
2. No vertical wall friction
3. Failure planes @…
Active
Passive
• Deviate from Ko
• **
• Active state soils push wall back
• Wedge is steeper than passive case
245
245
voh K
245tan2
aK
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Rankine• Inclined deviates by:
22
22
coscoscos
coscoscoscos
aK
22 2
22
cKc
KHP a
aa
therefore
However must still project by
cos horizontal sin vertical
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Rankine Pa
• First term is soil pressure• Second term removes cohesion• Third term decreases cohesion to
account for tension cracks
22 2
22
cKc
KHP a
aa
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Pa acts at H/3 unless…
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Example AnswerPa=117.5Ka=1=0.333Ka2=0.271
Solve for sliding and Pa
c=25=16
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Cohesion
• What do you do when it is given– Can you trust it– Should it be there
• Water is bad, can appear even in free-draining materials. Freezing? Uplift pressure?
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Active vs Passive• According to C.F.M.:
– Two times the strain required for Ka is needed for full development of Kp
• @ Ka use 0.5 Kp
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Passive Aggressive
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Slopes and Sliding
• Slopes was a big section, but we didn’t have time to quiz you on a lot of it because of time constraints, but on an exam there is lots of time….
• Sliding blocks
• Wedges
• Bishops long hand
• Mitchell charts
• Free body diagrams
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Wedgies
Force balance
driving resisting
WeightWater?
FrictionCohesionWater?
sin
tan'
W
NLcFs
You need to be able to understand the force systems from a first principles standpoint
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Bishop
• Produces a factor of safety on a given slip circle
• No inter-slice forces
• No inter-slice shear
• Factor of safety for all slices is the same
• Assumes soil is a rigid plastic
• Simplified analysis but generally good for Fs>1.2
• The circle with the the lowest Fs is the critical circle
• When the slope angle exceeds 53 degrees the critical circle will pass through the toe
• Must analyze on a slice by slice basis
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Bishop
sin
tan)1(
dW
m
RdWcdx
F
u
s
sliceslice
wwu H
HR
assumedFsm
sintan
cos
Can you use this method on a wedge?What is angle Why does factor of safety appear twice?Is a drawn slip circle necessarily the lowest factor of safety circle?Where is ground surface?
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Crap, don’t I need a bad ass table for this question
Yes you do
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Mitchell Charts
• Design charts solved for lowest factor of safety circle
• Three charts Ru=0.0, 0.3, and 0.6
• Calc and use average Ru for slope and linear interpolate factor of safety between charts
• Remember using it backwards?
)cot()tan(
'
hc
hFs
c
'
F
)tan(Use To get
Ru 0 0.3
Fs
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Slippery slopes
• What about water at the toe
• Excavated slopes?• To dewater or not to
dewater that is the question
• Rapid drawdown
• Tension cracks and slopes– Not out of the question
– There are even allowances on Mitchell charts for this
• Excavated slopes and stress path
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Footings, I got your footing right here buddy
qfqa
max
2
'''
iSBNiSNiSNcq qqqocccf
a
fs q
qF
of
a F
qq '
Take note of the omission of D. Oversight, I think not.Assumptions?What is the physical meaning of each term?
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Have I ever told you I hate footings
• Don’t forget about inclined loading
• Settlement– Immediate drained
– Immediate undrained
– Consolidation settlement
– Layer model with 10
• Stress distributions and settlement
• Fadum is Dr. Knight’s Favourite
• Could also use table from text if provided
• Don’t forget RQD but don’t obsess either
• N values re: qf
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Like I need to talk to you about stress.Critical depth = 2B
What about Fadum with a point of interest outside of the footing
I see another stress distribution approximation for point loadds
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Eccentricity! Doesn’t that have something to do with Liberace
B
e
A
WPq
fmim
61
B
e
A
WPq
f 61max P
Me
eBB 2'
eLL 2'
• Use qf from bearing capacity and it terns out that using B’ and L’ in the shape factors is more work than it is worth
• If e>B/6 then must use B’ and L’ in qmax & qmin
• What about strip footings and retaining walls?
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Soil Strength
M-C Failure Envelope
Sample will fail at intersection with envelope
’
(Failure)
’n
c’
(Failure)
Test 1 Failure Circle
failure (1)
failure (2)
Test 2 Failure Circle
NOTE: all stresses are effective
Each test is performed at a set void ratio
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Skempton
• Developed “A” parameter for indication of the denseness of a sample
• Develpoed “B” for a measure of satuation
interest ofpoint at stress deviatoric)(
interest ofpoint at pressure pore
pressure pore initail
)(
:
)(
31
31
i
i
o
oii
i
i
where
A
3
B
What about negative values of Af
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Soil Strength
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Soil Strength
Stress Path of Sample
1
2
3
’
t = d' + S tan '
s = (1' + 3')/2Centre of Circle
d = c' cos 'tan = sin '
S
(1 - 3)/2 = tRadius of Circle
d
s
t
sin’ = tan’
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t vs s space the final frontier
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T & S is that anything like S&M?
• Things to note– Ko line, how do you get it– Loading 1:1 to the right– Unloading 1:1 to the left– Time rate of dissipation of pwp– Stress path line– Better or worse than M-C?
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Effective vs Total and Drained vs Undrained
• Pore pressure is the difference– Effective the what the
soil “feels”
• Can effective exceed total?
• What does Drained mean.
• What are the implications of undrained?
• Bearing capacity and slopes
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Consolidation
• Spring analogy• Time rate of
consolidation• Differential settlement• Excess pwp• OCR
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Weight and Volume
• Redo the buoyancy question from your first assignment.– Dry density– Saturation– Void ratio– Water content
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Jojo’s Network
• Bearing capacity– Raft or footing
– Fs against load, against settlement
– Inclined load, eccentric load
– Two layer settlement
• Retaining walls– Rankine
– Two layer
– Inclined
– Passive resistance
– Water
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Jojo’s Network
• Stress– Calc and plot stress paths
– Convert M-C data to t&s
– Peak vs ultimate and c
– Dilatency
–
• Slopes– Be prepared for a slice
analysis
– Draw force diagrams
– Water effects
– Excavations
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Psychic Friends Network
• Redo the buoyancy question from your first assignment.– Dry density
– Saturation
– Void ratio
– Water content
• Theory– Assumptions
– Coulomb
– Af
– Total vs Effective
– Consolidation
– Mitchell vs Bishop
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Thought game
• In central America several ancient structures that rivaled the pyramids existed.
• Some were destroyed some remain
• How could you use geotech to determine if you were on the site of a large temple that had been destroyed
• The panama government has a site that may be of significance to archeologists, but a mining company wants access to subsurface ore deposits. If mining begins al archeological evidence will be destroyed.
• Can you help determine if the site is important?
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• Assumptions
• What would you look for?
• Calculations?