geological models and parameters

8/12/2019 Geological Models and Parameters

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CIVL6079 Slope Engineering 2012/13 1st Semester Ryan Yan 1

Geological Models and ParametersGeological Models and Parameters


Ingredients

Geological information

Rock/soil types

Sand, silt, clay; weathered soil, rock; granite, volcanic tuff

Geological history

Past failures; past developments

Geological structure

Joints; faults

Hydrological information

Groundwater table; perched water table

Surface flow; sub-surface flow

Geographical setting

Existing underground structures, services

Material properties

Friction angle, shear strength, permeability, etc

Source: http://my.opera.com/clover1911/blog/index.dml/tag/cooking


2/20


Market

Existing maps, documents and records, aerial photos (desk study)

Field survey and mapping (e.g., joint survey, walk-over survey)

Ground investigation (e.g., boring, sample collection, trial pits)

Field tests (e.g., SPT, vane shear)

Laboratory tests (shear box, triaxial)

Source:


Soil Behavior

Shear strength

Mohr-Coulomb model perhaps is the most well-known one. Any others?

Shear stiffness

Describes the amount of deformation when the material is loaded.

Elastic v.s. plastic v.s. elastoplastic

Permeability

Governs the water flow response in the material

Compressibility

Also describes the amount of deformation when the material is loaded.

So whats the difference between shear stiffness, stiffness and

compressibility?

What makes soil so complicated ?

Source: www.ndmep.com


3/20


Shear Box Test

What have you observed ?


Coarse-grain Soils

n

n

n

v

peak

h

h

0

dilation

contraction

Dense sand

Dense sand

Loose sand

Loose sand

ultimate

n

slope = tan ult

ult


4/20


Fine-grained Soils

n

n

v

peak

h

h

0dilation

contraction

OC clay

OC clay

NC clay

NC clay

ultimate

c

OC clay

NC clay

n

n


Misunderstanding

n

c

Which one is the friction angle of the soil?

Does cohesion really exist?


5/20


Peak v.s. Residual Strength

h

Howcomewehaveapeakstrength?


True Bonding v.s. Pure Friction

N

F

F

hBond (eg. glue)


6/20


True Bonding v.s. Interlocking

F

h

N

F

bonding

N

Lift up

F

h


Triaxial Test

Most common test in geotechnical laboratory testing

Advantages over direct shear test

No predefined failure plane in triaxial tests. The effect of initial fabric/bedding

can be observed

Drainage can be properly controlled in triaxial tests

CD v.s. CU Test

What are they?

What do you measure?


7/20


Stress Variables

hh

v

v

h Direct shear box Triaxial

Since there is no shear stress acting on

the surface of the test specimen, the

applied stresses are principal stresses.

i.e.

v = 1h = 2 = 3

Mean (confining) stress:

Shear stress:

v p

q

3

2 31 p

31 q

u

How come we see s (s) and t in some papers?


Conventional Triaxial Tests

Three types

Consolidated Drained (CD)

Consolidated Undrained (CU)

Unconsolidated Undrained (UU)

33

1

1

p = (1+23)/3

q = (1-3)

p = 1/3q = 1

Therefore, q/p = 3

p

q


8/20


Critical State Stress Ratio

p

q

CSL

sin3

sin6

M

Can you derive ?

Is CSL the same as the Mohr Coulomb failure criterion ?


Determination of cu

Field vane shear

Hand-held shear device

Pocket penetrometer

Torvane apparatus

Unconfined compression test (UC test)

Unconsolidated undrained test (UU test)


9/20


Field Vane Shear Test

Peak cu

Residual cu


Hand-held Device

Torvane test (up to 200 kPa)

Penetrometer (up to 400 kPa)


10/20


Unconfined Compression (UC) Test

Action: vertical compression (displacement) vReaction: vertical force (Fv)v

v (Fv)

(Fv)

v

v = Fv / A

peak ultimate

Then what is a UU test ?


Unconfined Compression (UC) Test

0 80 120

cu (peak)cu (ultimate)

Mohr Circle Representation


11/20


Unconsol idated Undrained (UU) Test

v

v (Fv)

(Fv)

Suppose we have many identical soil specimens (same initial condition)

v

(Fv)

(Fv)

v

50

50

50

50

50

50

v

(Fv)

(Fv)

v

200

200

200

200

200

200

Confining pressure (no drain) Compression shear


Unconsol idated Undrained (UU) Test

0 120 170

cu (peak)

50 200 320

Looks like having u = 0

Remember: Same water content !

Quick Question: How about the effective stress ?


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Mohr-Coulomb Failure Criterion

' ' tan '

drained shear strength along the failure plane

' cohesion (apparant cohesion)

' - = effective normal stress acting on the failure plane

' drained angle of friction

f

f

c

c

u

Physical Meaning ???

Failure Criterion! What does it mean?

Any information about deformational response?

c


What is c ?

For unbonded soil, the c parameter is simply an artifact of the linear

approximation of true soil behavior due to stress-dependent

dilatancy. The applicability of the c parameter is therefore dependent

on the stress range from which this parameter is derived and the

stress level of the geotechnical problem.

'

c

The engineer willhave to make

judgment if the

c parameter is

justifiable.

For HK residual soils in natural state, there might be some bonding

between soil particles.


13/20


True Cohesion Slaking Test

compacted adding water 15 mins later

36


What is ?

The parameter should be treated as a model parameter for theMohr-Coulomb model. If physical meaning is to be added, two

separate contributions can be considered:

1. Intrinsic frictional resistance

2. Interlocking resistance

total can be treated at peak in the Mohr-Coulomb model. fcan betaken as the critical state friction angle, whilst inter is directlyrelated to dilation angle. There are many empirical equations for

estimating inter(hence total), for example, Bolton (1986).

For HK soils in natural state, the inter is very difficult to estimatesince it is dependent on the inherent structure of the soil

i.e.total = f+ inter

Bolton, M. D. (1986). The strength and dilatancy of sands.Gotechnique, 36(1), 65-78.


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Particle Characteristics

Cho, G.C., Doods, J., Santamarina, J.C. (2006). Particle shapeeffects on packing density, stiffness, and strength: natural and

crushed sands. J. Geotech. Geoenviron. Eng., 132(5), 591-602.

Altuhafi, F., Baudet, B.A. (2011). A hypothesis on the relativeroles of crushing and abrasion in the mechanical genesis of a

glacial sediment. Eng Geol. 120(1-4), 1-9.

Shape

v.s.

Surface


Summary on Shear Strength

Drained: c and Undrained: cu and u =0

How to use them ???

What is the corresponding stress-strain response ???

Only the yield point is described !!!!

Short-term v.s. long-term !!

Hey! Something wrong!?

Do you really understand ?

undrained shear strength

undrained angle of friction

u

u

c


15/20


16/20


Permeability Test

B. Falling head test (fine-grained soil)

H2

H1

H

L

Falling Head

Permeameter

Standpipe ofcross-sectionalarea a

Soil Sample

porous disk

During a time interval dt

The flow rate in the standpipe =

The flow rate in the soil sample =L

HAk

At time t = 0, H = H1At the end of the test, t = T ad H = H2

A

a

dt

dHa

L

H

a

kA

dt

dHFor continuity,

TH

H

dtaL

Ak

H

dH

0

2

1

2

1lnH

H

T

L

A

ak


Compaction Test

The characteristics of a soil under compaction is best reflected by Proctor

compaction test.

By compacting a soil at different water

contents with a constant compaction

effort (input energy), the relationship

between soil density and water content

can be found.


17/20


Unsaturated Soil Behaviour

Extended Mohr-Coulomb:

Matric suction

(ua-uw)

=(Sr)

[SWCC]

' tan ' tanf a a w bc u u u

Gan & Fredlund (1992)



Projections of the extended Mohr-Coulomb failure criterion:

if c = c + (uauw) tanb

= c + (-ua) tan

Gan & Fredlund (1992)

Apparent cohesion !!


18/20



Soil-water characteristic curve (SWCC):

idealised



Coefficient of permeability (k):


19/20


Pore Water Pressure

The distribution of pore water pressure directly affects slope

stability. The line joining the locations with zero pore water pressure

is termed water table. There are two types of water tables:

1. Main groundwater table

2. Perched water table

The pore water pressure distribution in slopes is very seldom

hydrostatic. This implies that even if the location of the water table

is known (or measured), the water pressure distribution has to be

calculated from flow calculations (e.g. using SEEP/w). The resulting

pore water pressure can be imported into slope stability calculations

(e.g. SLOPE/w). In many occasions, the gradient of the water levelis parallel to the topography of the slope, making adjustment of

piezometric data fairly straightforward.


Standard Penetration Test (SPT)


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Standard Penetration Test (SPT)

SPT-N value Description Relative density

50 Very dense 85 100%

SPT-N value Description Average su

150 kPa

And a lot of other empirical

correlations Can you name

some ?

Do they make sense to you ?

geological models and parameters

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