Soil Mechanics in Road Construction Cluj

8/13/2019 Soil Mechanics in Road Construction Cluj

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Geotechnical Group Graz

Soil Mechanics in

Road Construction

O. LeibnizInstitute for Soil Mechanics and Foundation Engineering


Graz University of Technology

Design and Construction

of Unbound Road Base Layers

Course Ongoing Aspects in Geotechnical Engineering

Universitatea Tehnica din Cluj-Napoca, Romnia

02. 03.06.2011


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Content

Investigations and tests to secure a highquality standard with regard to:

Some fundamentals about capillarity andpermeability

Observation of damages and someconsiderations about the causes

Assumptions:e.g. validity of Darcy`s law

From these knowledge:formulation of requirements

permeability and drainage capacity

of base layers

Conclusions, summary andprospect into the future


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Dealt Themes, e.g. are

Frost- / thaw damages in road construction

Capillary appearances - the phenomenon ofcapillarity in nature

In situ measurement of permeabilities

The reason for the existence of capillarity

Permeability and drainage capacityof base layers

Generals and fundamentals about the water



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Periodic System of The Elements

The electronegativity is the criterion

for the endeavour of an atom within a molecule,to which it belongs, to attract binding electrons.

HydrogenOxygen



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Oxygen is one of the elements with thegreatest elektronegativity.

For that the center of charge of the watermolecule shifts: It lays closer to the atom with the

greater elektronegativity !

For that the water molecule is a perma-nent dipole, comparable to a permanent magnet ...

Extraction

of The Table of The Periodic System


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Hydrogen bindingHydrogen binding

Hydrogen binding

... and hydrogen bindings arise:

For that water is liquid

and can play its great role aslife element.

Hydrogen Bindings in Liquid Water



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Highmelting-point

Greatest densityat + 4C

Volumeenlargement

during freezing

Comparison with other nonmetallic hydrides

Anomalics of The Water

Good solvent anddispersing agent,

etc.



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Basis for the existence of capillarity !!

Accumulation at Foreign Particles:

Hydrated Ions

Ion

Wasserdipol

- +

-

-

--++

++ ++

++- --

-+

+

+ +

++

+

+

-O Sauerstoffatom

+H Wasserstoffatom

water dipoles

ion

.Oxygen .Hydrogen


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Layers of Adsorbed Water

sSterns double layer

until 2.10-8 m off the grain

(one to two layers of molecules)hydrated water

bound with 400 bar

dDiffuse layer

2.10-8 to 5.10-7 m off the grain

hygroscopic bound water

bound with 400 bar to 50 bar

aOuter layer

5.10-7 to 1.10-5 m off the grain

adhesive water

bound with 50 to 0 bar

d a


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Capillary Menisci: Water Transport

Counteracting The Force of GravityDistribution

of the

size

of the

voids !!


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GWT

d

Z

hcap

tensile stress in the

ore water

capillary tuberamified

pore system

Z tensile force in the water

C capillary compressive force in the grain skele-

ton, surrounding the water filled void-tubes

Capillary Elevation and Its Consequences

C C


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cos4

w

scap

d

Th

The surface-tension Ts [kN m-1] of water with its specific

weight w [kN m-3] causes its rise in small tubes with the

diameter d [m] up to the capillary elevation hcap

[m].

....... capillary wetting angle(for glass it is ~ 0)

In the capillary tube the water has a tension Z, whichincreases linearly from the free ground water table to

the capillary meniscus up to w hcap [kN m-2].

Summary:




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The capillary elevation originates form the

equilibrium of the elevated column of the liquid with

the capillary force, respectively the surface tension:

The smaller the capillary tube, the higher thecapillary elevation.

The water column, being subjected to this tension,

causes additional compressive stress D in the grain

skeleton, defined as capillary pressure.

This additional capillary pressure increases the

adhesion between the grains.



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Capillary Block in Sealing Waste Deposits



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Never use fine material above the road baselayers to get a good formation level, it works

like a capillary layer collecting the water coming from

above throught older bituminous layers or cracksin it or from the side caused by bad surface and

underground drainage conditions.

Often also an uppermost crushed zone onto theupper road base is caused by rolling or traffic

after finishing the road base layer.Better to disregard a regulation course onto

the road base layers.

And furthermore such a material is actuallyfrost susceptible !!

Regulation Course for The Upper

Formation Level



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Illustration of a Damage



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Frost Damages in Spite of Not Frost

Susceptible Road Base

Necessity, to establish a good formation level,which is exact in longidudinal and cross

gradient, without a thin correction course

directly with the coarse material of the upper

road base layer (e.g. with a grading of 0/22,

0/25, 0/32 or 0/35), which shall be not frost

susceptible and enough water-permeable !



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Remove the uppermost crushed zone ontothe upper road base, caused by rolling or

traffic after finishing the road base layer, e.g.by a steel brush machine.

It is better, first to overdesign and surchargethe upper road base layer (5 -10 cm) and after-

wards to take away the excess overprofile with

the destroyed material by a grading machine.

Good Formation Level of The Road Base



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Origin and Rise of Ice Lenses

+

0- isothermic

border-line

Pore size distribution !!

New-fashioned, sophisticatedfrost heave tests


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The frost susceptibility of mineral material playsan essential role in the design of foundations

placed above the freezing front in frost suscept-ible soils.

Roads, airport runways, railways, buildings onspread foundations, buried pipelines, dams and

other structures may be subjected to frost heave

due to freezing of a frost-susceptible material,having access to water.

Frost Susceptibility - Objective



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Investigations to Determine The Frost

SusceptibilityThe risk of frost heaving may be defined from:

Correlations with soil classification properties(particle size distribution, height of capillary rise

and particularly the fines content and within that

the amount of frostactive clay minerals).

If the definition of frost susceptibility based onclassification properties does not clearly indicate

the absence of risk of frost heaving,laboratory tests should be run.



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Frost Susceptibility:

Laboratory Investigations

The frost susceptibility test in the laboratoryis a frost heave test.

Additional, to investigate the risk of thaw

weakening and to determine the loss of bearingcapacity, a CBR - test should be carried out

before and afterwards the freezing procedure.



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Road Base Layers

Above Frost Susceptible Subsoil

Road bases shall work as a capillary block and

shall have an adaquate design (sufficient

thickness) to compensate the differentialfrost heaves due to arising ice lenses.


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Frost-Damage Due to Water Saturation

in The Road base

Additional negative effects through snow clearingfollowed by deeper frost penetration and water

supply (wet road surface due to salt-spreading)


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Illustration of Frost-Damages (1)

(Especially Longitudinal Cracks)



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Thaw-Damage Due to

Insufficient Drainage Capacity

The only chance to get rid of the water, is by lon-

gitudinal drainage capacity and cross drain-

age arrangements out of the frozen zone !!

Sufficient drainage capacitywithin the base layers


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Seasonal Dependence of The Bearing

Capacity of Road Base Layers

In spite of non frost susceptible road base layersthere is nevertheless a decrease of the

bearing capacity in spring of about 30 % !!


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Illustration of Damages (1)



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Summary of The Damage Causes

Damage due to insufficient drainage capacityof the road base layers

Water supply, e.g. through: Bad quality of seams and/or cracks in thebituminous layer

Aging of the bituminous surface layers(porosity > 8 %)

Bad drainage conditions and water supplyfrom the side.

Snow clearing followed by deeper frostpenetration and additional water supply

(wet road surface due to salt-spreading),

etc.


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Demands (1)

If possible, the 0 - isothermic border line shouldcome to lie within the frost blanket course.

Sufficient longitudinal drainage capacity andcross drainage arrangements out of the frozen

zone, especially when the road base is embeddedin cohesive and impervious subsoil !!

But due to economical reasons it is often

not practicable ! At least adequate design (sufficient thick-ness) should compensate the differential

frost heaves due to arising ice lenses



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Demands (2)

Unbound road base layers shall have a certainwater permeability:

Last but not least: Construction of unbound baselayers avoiding any contamination with foreign

cohesive soil or enrichment of fine grains through

crushing to guarantee the permeability !

Previous investigation already during thequalification tests

Simulation of the quality tests (later duringsite construction) in the previous qualification

tests (e.g. compaction of the laboratory

samples according to modified proctor ordetermination of the LA - coefficient)

New device for in situ testing of permeability


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Permeability Testing of Road Base Layers

To obtain permeability values by in situ testingwas first necessary during construction of waste

deposits. For that the following explanations

are based on these experiences and also thetheoretical deduction and the development of a

measuring device.

Based on understandings from similar investigations

in constructing waste deposits:

Why that necessity ?


P bilit T ti f S li L


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Permeability Testing of Sealing Layers

Possibility to obtain

an undisturbed soil

sample



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Well Graded Mineralic Material

for Sealing Layers

silty clay

filler

bentonite

well graded

sealing material

gravel-sand

fuller parabola for d=20

fuller parabola for d=63

T ti S li L


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Testing Sealing Layers

of Well Graded Soil Material

So we have to carry out in situ permeability tests.

It is not possible to obtain undisturbed samples(e.g. by a piston sampler, forced into the soil by

dynamic impact) to determine the permeability-index k in the laboratory in a triaxial permeability

cell.

To calculate a permeability index k from themeasurements, it is necessary to develope atheoretical model.


V ti l I fl i t Th S il Sk l t


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Vertical Inflow into The Soil Skeleton

(Theoretical Approach)

In the following explanations it is assumed thatthe soil representing the halfspace consists of

three phases: the grains or solid components,

water and air in the voids.

We have to investigate vertical inflow into partiallysaturated soil from the surface of the halfspace

In traditional soil mechanics the velocity of thewater flowing in the soil is defined by Darcys law:


Darcys Law


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Darcys Law

ik

A

QDarcy

vDarcy Mean velocity of the flow of water in soil after

Darcy [m/s]

Q Volume of flow [m3/s]

A Sectional area [m2]

k Darcys coefficient of water permeability(permeability index) in saturated soils [m/s]

i Hydraulic gradient

Water Movement in Soils


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Water Movement in Soils

Mean distribution of velocity of the flow of water in soil

after Darcy (a), mean velocity (b) and real distribution

of velocity of water flowing through voids (c)

Visualisation in laboratory investigations ?

a) b) c)


Water Movement Between The Grains


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Water Movement Between The Grains

In reality the water is moving only in the voids.Therefore it is impossible to observe vDarcy in an

experiment. Instead of that one can observe an

inflow situation only determined by the mean

velocity va of water flowing through voids,

described as follows:

Here it is neglected that over the cross sectionof the voids the water flows with an unequal but

symmetrical distribution of velocity (remember

figure c).

nDarcy

a n .. Voids content [-]


Vertical Inflow into The Halfspace


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Vertical Inflow into The Halfspace

Schematic Sketch

h

z

dz

water table

surface

saturated

saturation front

soil layer

partially saturated

z

zhi

Vertical Inflow Saturation Front


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Vertical Inflow Saturation Front

It is assumed that the inflow into the soil forms asaturation front parallel to the surface.

In idealisation that implies that the diameters ofthe void channels of the soil are constant. Thisimagines, that many very small flow-tubes of the

same diameter stand next to one another.

The saturation front is the borderline betweenfully saturated and partially saturated soil.

The vertical advance (which distance in which time)of the saturation front downwards during the inflowof the water can be formulated mathematically as a

function of time as follows:



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Vertical Advance of The Saturation Front

dt

dza

n

ik

dt

dz

z

zhi

With Darcys law it results in

Our sketch has shown that the hydraulic gradient can

be calculated by



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dtdz

a nik

dtdz

zzhi

Inserting one equation into the next gives

z

zh

n

ka

dzzh

z

k

ndt

Inserting the differential expression, separating the

unknown parameters and setting the integral gives




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Considering the condition z ( t = 0 ) = 0,

C can be calculated and therefore the result is

With this equation one is able to determine the time

which the saturation front needs to proceed the

distance z into the soil-layer.

Czhhzk

nt )(ln

Solving the integral results in

[sec]ln

h

zhhz

k

nt


Examples


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p

for The Range of Inflow Distance

Sealing layer of a waste disposal:

Insertingn = 0,4; k = 1.10 - 9 m/s, distance z = 0,05 m, h = 2,0 m

gives an inflow-time of

about 4000 min or 2,8 days resp.

Road base layer:Inserting

n = 0,4; k = 1.10 - 6 m/s, z = 0,25 m and for h = 0,2 mgives an inflow-time of

about 35.000 sec or 10 hours resp.


Cross Section - Laboratory Model to Visua-


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y

lize The Inflow From an Insitu-Standpipe

Example of a Seepage Front with a


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Clayey Silt,

k = 1,2.10-8 m/s

Distance ofseepage front

after 140

minutes


Permeability Coefficient Relevant for

Sealing Layers of Waste Disposals




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a p e o a Seepage o t t a

Permeability Coefficient Relevant for

Road Base Layers

Sand,

k = 3,0.10-5 m/s

Distance of

seepage front

after 9 seconds


Example of a Seepage Front


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with a High Permeability Coefficient

Uniform grained

quartz-sandk = 5,0 . 10 3 m/s

Distance of

seepage frontafter 26 seconds


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Testing Permeabilities with a Standpipe


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g p p

for Waste Disposal Sites (2)

Constant pressure head

( for k < 1.10-7 m/s )

Testing Permeabilities Insitu with a

St d i f R d B L


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Standpipe for Road Base Layers

(Quality Control)

Falling pressure head

( for k > 1.10-7m/s )

Testing The Permeability

f R d B L M t i l D i Th


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of Road Base Layer Material During The

Qualification Test in The Laboratory

Standpipe,

as used for the

quality control testson site,

put on a Proctor- jar

with a diameter

of 250 mm


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Evaluation According to The


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Theory of Potential Flow

Theory of potential flow out of a source

Other influences on the movement of the water inthe soil skeleton, e.g. capillarity, suction stress or

water retention capacity, are hitherto neglected.

For the testing devices (stand pipes) with falling

pressure head Darcys coefficient of permeability

(permeability index) can be calculated as follows:


Potential Flow Out of a Spherical Source


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)2(

dr

dh

kik

24 r

dr

k

Qdh

0

2

0

4rh

r

dr

k

Qdh

0

1

4 rk

Qh

hr

Qk

04

04 rQhk

)1(4 2

r

Q

O

Q

Kugel

)3(

4 0

r

Qhkf

Potential function

Sphere

Equation to

D t i Th P bilit C ffi i t


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Determine The Permeability Coefficient

The volume of inflowing water can be determined

according to the velocity of the sinking water table in

the standpipe:

2

1

0

2

ln

4 h

h

tr

rk m

[m/s]

dtdhrAQ m

2

dt

dh

r

rhk

0

m

4

2

This expression is to be inserted in equation 3 of the

last picture:

remember:rm is the radius of the

measuring pipette

Through integration we get:

)3(4 0

r

Qhkf

Correction Coefficients


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Correction factor to consider the area of inflow inform of a disk instead of a sphere after een (1967):The radius of the inflow area ro must be divided

through 2,48. To consider the halfspace (inflow form the sur-face and not within the full space), the radius of

the inflow area ro must be multiplied with 0,55.

With the constant value of 4 that gives a combined

correction coefficient of 0,88 in the denominator.

The potential difference from the inflow area to theground water table, which also enlarges thepressure head at a very small account, shall be

neglected.



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Equation for The Device to Investigate

Road Base Layers


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Road Base Layers

The k10 values (related to a temperature of 10 C)

correspond with the following intervals of measure-

ment time t:

With the dimensions of the standpipe for testing

road base layers as mentioned above, it is:

tk

3

101035,1 [m/s]

... Temperature correction factor

k10 = 1.10-5 m/s 135 sec

k10 = 5.10-6 m/s 270 sec

k10 = 1.10-6 m/s 1350 sec

k10

= 5.10-7 m/s 2700 sec


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Conclusions, Summary and

Prospect into The Future (2)


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Capillarity, water retention capacity and matrixsuction stress, etc.

Permeability of road base layers when partiallysaturated and frozen

Furthermore there are still additional researchactivities necessary:

Beyond additional investigations and test series

to the influence of the permeability on the frostsusceptibility of road base layers, there is a crucial

need to investigate the influence of other soil

parameters. Special problems are e.g.:

Practical application of the testing

device for measuring permeabilities

of road base layers on site:

Prospect into The Future (2)


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THANK YOU

FOR

YOUR

ATTENTION

... and please are there any questions ?


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Soil Mechanics in Road Construction Cluj

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