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

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

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

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

Pa acts at H/3 unless…

Example AnswerPa=117.5Ka=1=0.333Ka2=0.271

Solve for sliding and Pa

c=25=16

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?

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

Passive Aggressive

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

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

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

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?

Crap, don’t I need a bad ass table for this question

Yes you do

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

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

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?

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

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

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?

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

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

Soil Strength

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’

t vs s space the final frontier

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?

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

Consolidation

• Spring analogy• Time rate of

consolidation• Differential settlement• Excess pwp• OCR

Weight and Volume

• Redo the buoyancy question from your first assignment.– Dry density– Saturation– Void ratio– Water content

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

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

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

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?

• Assumptions

• What would you look for?

• Calculations?