geo-e2010 advanced soil mechanics d

03 March 2021

Geo-E2010 Advanced Soil Mechanics D

Wojciech Sołowski

About the course

Department of Civil Engineering

2

1. Lectures : 1/2 of the final grade

1. Either graded based on partial exams, or based on online

quizzes and essays

2. Q&A sessions – we can set dates if we decide to use

partial exam for assessment

2. Laboratory : 1/6 of the final grade

3. Exercises: 1/6 of the final grade (attendance compulsory)

4. Design project: 1/6 of the final grade5. Tutorials: aim of those is to help you with the more challenging

lecture materials (mainly constitutive modelling)

6. Each part (lectures, laboratory, exercises, project) must be passed

Advanced Soil Mechanics W. Sołowski

Final % calculated as: (3*lectures+lab+ex+project)/6grades: 1: 50-60%, 2: 60-70%, 3: 70-80%, 4:80-90%, 5: 90-100%

Extra materials / journal papers are mainly intended for the PhD students

About the course


3

Lectures: 50% of the final mark

Option 1: grade based on 3 partial exams during the lectures

Completing non-obligatory MyCourses quizzes will give you some %

towards passing the exam.

- test dates are in the timetable, but they can be moved if needed

Option 2: grade based on quizzes in MyCourses and essays

- essay typically around 2000-2500 words (5-7 pages)

- will be checked with Turn it In system for originality


Any comments ?

About the course


4

Lectures: 50% of the final mark

If you choose essay, typical subject will be similarly structure as the

one from Finite Element Method course:


Any comments ?

Essay subject: Finite Element Method algorithm for nonlinear materialsThe essay should be 4-7 pages long, Arial 12 or similar font, figures included in the page count.Essential parts of the essay:1.Introduction (what is Finite Element method, what kind of problems we can solve with FEM, what is nonlinear material, approximately 0,5-1 page)2.Finite Element algorithm for nonlinear materials – points and short description, possibly few equations, explanation where in the algorithm the Newton Raphson method is used and reference to the next section (1-2 pages)3.Explanation of Newton – Raphson method – in depth explanation of subsequent actions taken and how those are performed, some equations, figure (1-2 pages)4.Possible errors in non-linear analysis on an example. Discuss errors in the analysis related to calculation of settlements of a foundation and limit load of a foundation (alternative – settlements of embankment and limit load leading to loss of stability of an embankment) (1.5 – 3 pages).5.Conclusions (short)If needed, you can include references, those do not count towards page limit

About the course - communication with teachers


5

Forums:

- direct communication channel with the teachers

- dedicated forums for different parts of the course

- forums should be used for almost all the out-of-hours

questions you may have about course material, exercises and

lectures

Classes:

The main role of all the classes is a direct communication and

discussion. You are encouraged to ask questions!

- lectures, exercises and tutorials exist mainly so you can ask

questions - otherwise we can just distribute the materials to you!

- only sensitive questions (e.g. revealing personal information,

such as e.g. health related issues) should be asked over email.


About the course: feedback channels


6

How would you like to have feedback organized?

Suggestion:

- let’s choose representatives who would give me

regular anonymous feedback from the whole group

- especially important if something needs to be

changed quickly

- email would work great for that – I will reply to the

whole group on the forum, possibly discuss the issue(s) during

lectures

- feedback representatives will be invited for a chat over a coffee

around the end of the course (covid situation permitting)


Workload research


7

If you want, we can continue with the workload research.

Workload Google spreadsheet

- anonymous (select nickname , decoding list in a sealed envelope)

- please indicate ONLY work outside of class (classes are counted

separately)

- the aim is to assess self-work time, as well as schedule the

courses so the workload is divided more evenly through the

course

- students well-being is at the centre of Aalto University


Course books


8

Unfortunately, no single book covers everything we talk about, but Mitchell &

Soga ‘Fundamentals of soil behaviour’ is a good start

while book like ‘Soil behaviour and critical state soil mechanics’ by David

Muir Wood covers the more advanced parts of the course.

Both books contain material the course is not covering, and are suggested

extra reading for doctoral level students. The books are not necessary to

pass the exams – the lectures will give the information needed.

Finally, the single book which almost cover the whole course is

‘Geotechnical engineering : unsaturated and saturated soils’ by J-L Briaud,

but even ‘Smith's elements of soil mechanics’ by I. Smith will be useful,

especially for the applied/ design subjects.

More books in the materials section of MyCourses


https://primo.aalto.fi/discovery/fulldisplay?docid=alma999348592406526&context=L&vid=358AALTO_INST:VU1&lang=fi&search_scope=MyInst_and_CI&adaptor=Local%20Search%20Engine&tab=Everything&query=any,contains,mitchell%20soga

https://primo.aalto.fi/discovery/fulldisplay?docid=alma999353904706526&context=L&vid=358AALTO_INST:VU1&lang=fi&search_scope=MyInst_and_CI&adaptor=Local%20Search%20Engine&tab=Everything&query=any,contains,Geotechnical%20engineering%C2%A0:%20unsaturated%20and%20saturated%20soils&offset=0

https://primo.aalto.fi/discovery/fulldisplay?docid=alma996863684406526&context=L&vid=358AALTO_INST:VU1&lang=fi&search_scope=MyInst_and_CI&adaptor=Local%20Search%20Engine&tab=Everything&query=any,contains,Smith%27s%20elements%20of%20soil%20mechanics&offset=0

https://mycourses.aalto.fi/course/view.php?id=28954&section=2


9

SOIL MICROSTRUCTURE



10


So... todayClay:

- microstructure

- clay behaviour and how it is influenced by microstructure

- influence of chemical additives in pore fluid on microstructure

(in this lecture – only NaCl)

- sensitive clays

- influence of unsaturation

- anisotropy & cyclic loading

- small strain


11


To learn:

Clay:

- what is clay microstructure?

- how it qualitatively changes during loading?

- how clay behaves under loading?

- how chemicals influence microstructure?

- what is the microstructure of sensitive clays?

- what is the influence of unsaturation on clay behaviour and

clay microstructure?

- how clay behaves when anisotropic?

- how clay behaves under cyclic loading?

- what is clay behaviour at small vs large strain?


12

Advanced Soil Mechanics. W. Sołowski

Clay microstructure

Mitchell & Soga 2005


13


Clay microstructure

Van Olphen 1977


14


Clay microstructure: NaCl influence

Dolinar & Tranuner 2007


15


Clay behaviour



16


Clay behaviour


1. Under a given effective consolidation

pressure, a soil with a flocculated fabric is less

dense than the same soil with a deflocculated

structure.

2. At the same void ratio, a flocculated soil

with randomly oriented particles and particle

groups is more rigid than a deflocculated soil.

3. Once the maximum precompression stress

has been reached, a further increment of

pressure causes a greater change in fabric of a

flocculated soil structure than in a

deflocculated soil structure.

4. The average pore diameter and range of

pore sizes is smaller in deflocculated and/or

destructured soils than in flocculated and/or

undisturbed soils.


17


Clay behaviour


5. Shear displacements usually

orient platy particles

and particle groups with their

long axes parallel to the

direction of shear.

6. Anisotropic consolidation

stresses tend to align platy

particles and particle groups

with their long axes in the major

principal plane.


18


Clay behaviour


Glacial till

7. Stresses are usually not distributed

equally among all particles and particle

groups. Some particles and particle groups

may be essentially stress free as a result of

arching by surrounding

fabric elements

8. Two samples of a soil without

cementation can have a different structure

at the same void ratio - effective stress

coordinates if they have different stress

histories. The stress–deformation properties

of the two samples will differ.

The overconsolidation ratio (OCR), defined

as theratio of the maximum past

consolidation effective

stress to the present overburden effective

stress is a good measure of stress history.


19


Clay behaviour


9. Volume change tendencies

determine pore pressure

development during

undrained deformation.

10. Changes in structure of

a saturated soil at constant

volume are accompanied

by changes in effective

stress. These effective

stress changes are

immediate.

11. Changes in structure of

a saturated soil at constant

effective stress are

accompanied by changes

in void ratio. The change in

void ratio is not immediate

but depends on the time

for water to drain from or

enter the soil.


20


Clay behaviour

Calculate ultimate e and p’ when quick / slow shearing starts at point A / B

ln p’

e

slope l=0.3

2.0

p’=10 kPa

A(1.3,30)

B(1.8,300)

e = 2.0 – lln(p’/10)


21


Clay behaviour: slow (drained) shear


ln p’

e

slope l=0.3

2.0

p’=10 kPa

A(1.3,30)

B(1.8,300)

e = 2.0 – lln(p’/10)


22


Clay behaviour: quick shear


ln p’

e

slope l=0.3

2.0

p’=10 kPa

A(1.3,30)

B(1.8,300)

e = 2.0 – lln(p’/10)

23

Quick clays

After Mitchell & Soga 2005, clay

sensitive to remoulding

Leda clay, Tovey 1971


24


Sensitive clay behaviour – salt content

Bjerrum 1954


25


Sensitive clay: time

Bjerrum 1954Lessard, 1978


26


Unsaturated soils

Soil grains tend to

get grouped

together because

of the capillary

pull forces:

Lourenço 2008


27


Let’s look into more details

Soil:

- Soil skeleton

- Bulk water

- Meniscus water

- Air

(+ in fine grained soil, we

have absorbed layers of

water associated with

the clay platelets)

Wheeler & Karube 1995


28


Real soil particles are not spheres…

High suction (Monroy 2005)

… but still aggregate together when dried


29


Real soil particles are not spheres…

High suction (Monroy 2005) Suction 40 kPa (Monroy 2005)

… but still aggregate together when dried


30


However, when fully wetted…

reconsitituted soil (Monroy 2005),

(sample mta-10)

fully hydrated soil (Monroy 2005)

(sample mta-2)

… the aggregates tend to disappear (until drying)


31


Macro: higher suction, stronger soil…

silt, Vasallo Mancuso Vinale 2007


32


Microstructural changes during collapse

- dried, clay particles create

aggregates.

- aggregates fully wet inside,

- significant strength increase

at high suctions

- soil can sustain higher load.

- when wetted suction

decreases, the aggregates

become weaker

- soil structure start filling the

intra-aggregate voids…

… and we have collapse Figures © Monroy 2005


33


Link between suction and water content…

Tarantino 2007


34


Why hysteresis?

Gallipoli 2006

- bottleneck effect: while

drying reaching some

bigger pores may require

drying small bottleneck pore

- while wetting, the bigger

pores generally will fill first

(though some air bubbles

may left in the soil

- Drying and wetting does

change soil microstructure,

therefore the wetting –

drying paths will also differ

- There is significant

influence of stress level on

water retention behaviour

(hydro-mechanical coupling)

Winter times – freeze thaw cycles!

- freezing is complex!

- water – ice interface similar to

water – air interface

- especially important in

permafrost areas

- some water does not freeze

Williams 1964 (after Gens 2010)

Gens 2010


36


Anisotropy…

Now it is becoming well

agreed that unsaturated

soils are mostly

anisotropic…

Especially when samples

prepared with one-

dimensional compaction

when one direction is clearly

privileged

Cui & Delage 1996


37


Simplified summary: clayHow clay microstructure qualitatively changes during loading?

- increase of mean effective stress leads to denser

microstructure

How clay behave under loading?

- slides 13-17

How NaCl influences microstructure and soil behaviour?

- the clay platelets tend to come together and form

aggregates more easily (slide 12)

What is the microstructure of sensitive clays?

- it is very loose and unstable, created during sedimentation,

often held together by some forces related to salt content (slides 18-20)


38


Simplified summary: clayWhat is the influence of unsaturation on clay behaviour and

clay microstructure?

- the more unsaturated the clay is, the stronger it is

- microstructure becomes denser and stronger, see 21 - 29

How clay behaves when anisotropic?

It is elastic for longer when the stress state corresponds to the

anisotropic stress in situ. When sheared / stress changes, the

anisotropy of the material evolve. More in lecture 12.

How clay behaves under cyclic loading?

- the deformations increase the more cycles there are

- the larger the stress in the cycles, the quicker the

deformations increase, till failure

What is clay behaviour at small vs large strain?

- at small strain clay is significantly stiffer and deform less than

at larger strains


39


Questions?Underwater landslides tend to be the biggest landslides

observed, often taking kilometers in length and outreach.

Discuss the processes in the landslide, in particular,

generation of pore pressure. Are those processes likely to

lead to larger landslides?

Dynamic compaction soil improvement method is relying on

compaction due to repeatably falling weight on soil. Discuss

the processes in clay and explain why dynamic compation is

usually not effective for clay materials.

In winter we can easily walk on any soil. However, when the

clay thaws, it turns into a very soft mud and we avoid

stepping on it. Explain why.

03 March 2021

Geo-E2010 Advanced Soil Mechanics L

Wojciech Sołowski

Lecture 2. Sand behaviour


42


So... todaySand:

- microstructure & forces in granular materials

- unsaturated soils

- role of particle shape

- influence of silty / clay additions

- density and shear behaviour

- small strain

- liquefaction

- cyclic loading

- introduction to critical state soli mechanics


43


To learn:

Sand:

- what is sand microstructure like?

- what is the influence of unsaturation?

- what is the influence of silty / clay additions on sand

behaviour?

- how sand behaves when the strain is small

- how it behaves when the strains are larger?

- when sand can liquefy? Why?

- how granular materials behave under cyclic loading?

- what is the critical state?

- how the soil behaves during rapid shearing?

- how it behaves at slow (drained) shearing?

- how the density of the soil influences that behaviour

Department of Civil and Environmental Engineering

44

W. Sołowski

Soil microstructure


45

W. Sołowski

Soil microstructure: skeletal forces

Santamarina 2005

soil:

coarse

(e.g. sand)

stress concentration

will lead to accelerated

point erosion / cracks,

erosion will lead to

particle rearrangement

and creep behaviour

(that is increase of

deformations without

changes in load)

- issue in railways


46

W. Sołowski

Soil microstructure: skeletal forces


soil: fine

(e.g. clay)

Santamarina 2005

...can be very

important...


47

W. Sołowski

soil

If we have air – soil is unsaturated

(partially saturated)

- leads to a more complex theory

- currently unfrequently used in practice

We usually assume that soil is fully saturated or dry – it is

a generally safe assumption leading to much simplification

Soil microstructure


48


Let’s look into more details

Soil:

- Soil skeleton

- Bulk water

- Meniscus water

- Air

(+ in fine grained soil,

we have absorbed

layers of water

associated with the

clay platelets)

Wheeler & Karube 1995


49


Unsaturated soils

Soil grains tend to

get grouped

together because

of the capillary

pull forces:

Lourenço 2008


50


Unsaturated soils

Soil grains tend to

get grouped

together because

of the capillary

pull forces:

Winter times – freeze thaw cycles!

- freezing is complex!

- water – ice interface similar to

water – air interface

- especially important in

permafrost areas

- some water does not freeze

Williams 1964 (after Gens 2010)

Gens 2010


52


Summary (1)

1. Grains in granular material are big, compared to e.g. clays

2. Force chains are present, inter-granular stress much higher than

average stress. Skeletal and self-weight dominant.

• may lead to damage, cracking, breakage at the contact

3. The electrical forces are less important in granular materials, may

be dominant in clays.

4. Capillary forces may be somewhat important in finer materials

5. Capillary forces – perhaps surprisingly – play a significant role

during soil freezing

Role of shape


54


Granular material: role of particle shape

Extremes:

- glass bead

- glass cube with puzzle-like locks

sands / gravels usually

somewhere in between


55


Granular material: role of particle shape

Round particles – lower shear

strength, difficult to construct

anything

Edgy particles – higher shear

strength, can be used in

construction

Friction angle gives macroscopically – among others

– information on particle shape.

More edgy particles, should lead to a higher friction

angle, rounder particles – to a lower friction angle and

lower strength

Cheng NS. (2018)


56


Role of particle shape

Wei & Yang 2014


57



Wei & Yang 2014

clearly particle

shape is

important


58


Microstructure influence

Wei & Yang 2014


59

Advanced Soil Mechanics. W. SołowskiWei & Yang 2014


clearly particle

shape is

important


60


Summary (2)

1. Shape of the particles is very important:

• Round particles lead to much lower strength

• Edgy particles generally lead to higher strength

2. Even small amount of round particles affect

properties of sand immensely

3. Even relatively small amount of fine particles may

have significant effect on the sand behaviour and

strength.

Role of amount of strain and density


62


Density influence

Peak and residual failure stresses

Same ultimate strength,

but different peak strength

and volume change

No volume change after

large shearing


63


Microstructure influence

Peak and residual failure stresses


64


Small strain stiffness



65



Data for sand, Oztoprak & Bolton 2013


66



Wichtmann et al. 2011


67


Anisotropy

Oda 1972

Effect of initial fabric

anisotropy on stress–

strain and volume

change behaviour of

Toyoura sand.


68


Anisotropy

Park and Tatsuoka,1994


69


Summary (3) 1. Shearing at given confining pressure leads to the same ultimate

strength if volume change is allowed (drained, no pore pressure

build up)

2. That ultimate strength is reached at the same volume

3. Hence, for given volume of granular material (isotropic), we

have single ultimate strength

4. If we start shearing at denser state, we will dilate while reaching

that ultimate strength. For a while, our shear strength may be

higher than the ultimate strength

5. If we start shearing at loose state, we will compact while

reaching the ultimate strength. The shear strength will slowly

approach the ultimate strength

6. At small strain, the behaviour of the material is very different

and much stiffer.

7. Anisotropy is usually neglected, but has significant (30%?)

influence on sand behaviour, important e.g. in slope stability.

Solid – liquid transition and cyclic loading


71


Solid – liquid transition

Once the specific volume is high enough, the soil particles

loose contact:

→ soil behaves like a gas (if dry) or liquid (if wet)

However, the soil particles will loose any friction between

them when the pore pressures are high enough (so the

effective stress is zero or less).

→ soil behaves like a liquid


72


Sand liquefactionRapid shearing

Sand wants to densify, but

constant volume leads to

particles loosing contact…

Build-up of pore pressures

Reduction of effective stress

and shear strength

Liquefaction


73


Sand liquefaction


74


Sand: cyclic loading

Wichtmann et al. 2011

z = qampl /p0


75


Sand: cyclic loading


76


Clay: cyclic loadingWichtmann et al. 2013


77


Summary (4)

1. Liquefaction is a result of loose sand shearing. The

sand is sheared at constant volume, and there is not

time for it to contact, hence the pore pressure is

generated to reach critical state. This reduces

effective stress and shear strength.

2. Cyclic loading may lead to:

Damage, cracking, breakage at the contact,

• Leading to changes in grain size distribution and

grain shapes,

• Leading to reduction of strength

• Short term, it may lead to liquefaction of loose sand


78


Simplified summary: sandWhat is sand microstructure like?

Can be loose / dense; force chains exist

What is the influence of silty / clay additions on sand behaviour?

Additions significantly reduce sand strength

How sand behaves when the strain is small?

Sand is much stiffer, less deformations. Perhaps little / no movement of

particles

How it behaves when the strains are larger? That is what we usually

test… You need to know the difference between dense and loose sand.

When sand can liquefy? Why?

Sand liquefies usually under rapid cyclic loading. Only loose sand liquefies.

Liquefaction is due to undrained sand densification

How granular materials behave under cyclic loading?

The larger the amplitude of load, the quicker they loose strength. With very

low amplitudes, grain wear (grain becomes rounder) and grain cracking

matter a lot (especially train lines)

Thank you

geo-e2010 advanced soil mechanics d

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