Soil Mechanics in Road Construction Cluj
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Transcript of 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
Geotechnical Group Graz
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:
Capillary Elevation and Its Consequences
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Capillary Elevation and Its Consequences
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 ?
Geotechnical Group Graz
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.
Geotechnical Group Graz
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:
Geotechnical Group Graz
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)
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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 [-]
Geotechnical Group Graz
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
Vertical Advance of The Saturation Front
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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
Vertical Advance of The Saturation Front
Vertical Advance of The Saturation Front
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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
Vertical Advance of The Saturation Front
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.
Geotechnical Group Graz
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
Example of a Seepage Front with a
Permeability Coefficient Relevant for
Sealing Layers of Waste Disposals
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Example of a Seepage Front with a
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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
Geotechnical Group Graz
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:
Geotechnical Group Graz
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 ?
Geotechnical Group Graz