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Seminario DICA, 9 June 2016
Compacted soils: hydro-chemo-mechanical
issues
Gabriele Della Vecchia
G. Della Vecchia Seminario DICA
Compacted soils: what are they?
Compaction is the process of increasing the density of a soil by packing the
particles closer together with a reduction in the volume of air
No significant water volume variation is expected
G. Della Vecchia Seminario DICA
Compacted soils: what are they?
In situ compaction
In the construction of fills and
embankments, loose soil is
placed in layers ranging
between 75 and 450mm in
thickness, each layer being
compacted to a specified
standard by means of rollers,
vibrators or rammers.
In general: the higher the
degree of compaction, the
higher the shear strength and
the lower the compressibility of
the soil
G. Della Vecchia Seminario DICA
Compacted soils: what are they?
Compaction in the lab
Standard laboratory tests to assess the compaction properties of a soil
• Standard Proctor Test
• Static compaction
The results of laboratory compaction tests
are not directly applicable to field
compaction:
• Different compactive efforts
• Different way of applying loads
G. Della Vecchia Seminario DICA
Compacted soils: what are they?
Compaction plane:
• Optimum water content
• Dry side of optimum
• Wet side of optimum
Da Craig’s Soil Mechanics
G. Della Vecchia Seminario DICA
Compacted soils: where are they?
Engineered fill
Fill in which the soil has been selected, placed and compacted to an
appropriate specification with the object of achieving a particular engineering
performance, generally based on past experience.
The aim is to ensure that the resulting fill possesses properties that are
adequate for the function of the fill.
Generally, fluid transport and retention properties are the fundamental aspect,
but also the mechanical response can be important
G. Della Vecchia Seminario DICA
Compacted soils: where are they?
Earth construction Compacted soils
• River embankments, flood defences and irrigation canal dikes
G. Della Vecchia Seminario DICA
Compacted soils: where are they?
Earth construction Compacted soils
• River embankments, flood defences and irrigation canal dikes
Jommi & Della Vecchia (2000)
G. Della Vecchia Seminario DICA
Compacted soils: where are they?
Earth construction Compacted soils
• Earth dams
G. Della Vecchia Seminario DICA
Compacted soils: where are they?
Earth construction Compacted soils
• Environmental barriers
Low hydraulic conductivity and diffusivity
G. Della Vecchia Seminario DICA
Compacted soils: where are they?
Earth construction Compacted soils
• Environmental barriers: disposal of hazardeous waste in geological deep
formations
Grimsel Test Site (GTS)
1. Vessel
2. Nuclear waste
3. Host rock
4. Compacted clay
G. Della Vecchia Seminario DICA
Compacted soils: where are they?
Rock
Heater
Wetting
Heating
Engineered Barrier System for HLW disposal (FEBEX, Grimsel, Swiss Alps)
G. Della Vecchia Seminario DICA
Compacted soils: where are they?
Sometimes compaction can be a problem!
G. Della Vecchia Seminario DICA
Compacted soils are unsaturated soils
Solid grains Air
Water
The behaviour of unsaturated soils is much more complicated than the behaviour
of saturated soils
1rS
G. Della Vecchia Seminario DICA
Compacted soils are unsaturated soils 14
Complications:
• Three-phase medium: solid grains + water + air (+ air-water interfaces?)
G. Della Vecchia Seminario DICA
Compacted soils are unsaturated soils 15
Complications:
• Three-phase medium: solid grains + water + air (+ air-water interfaces?)
• Gas (e.g. Air) is compressible
• Terzaghi effective stress principle does not hold
G. Della Vecchia Seminario DICA
Suction: definition
The role of suction on the behaviour of unsaturated soil was already recognized
by the pioneeristic works of Croney (1952) and Bishop et al. (1959, 1963)
Suction = measure of the water retention capacity of a soil
Aitchinson et al. (1965) :
Total suction Ψ = energy density (per unit volume) of pure water in equilibrium
through a semi-permeable membrane (i.e. permeable just to water molecules)
with interstitial water
G. Della Vecchia Seminario DICA
Suction: definition
Total suction Ψ = energy density (per unit volume) of pure water in equilibrium
through a semi-permeable membrane (i.e. permeable just to water molecules)
with interstitial water
G. Della Vecchia Seminario DICA
Suction: components
Osmotic suction p (function of the chemical
potential of the system, independent from
water content)
s = matric suction = ua-uw (function of the
quantity of water in the pores and of the
geometry of the porous system)
The energetic component (NEGATIVE) which characterizes unsaturated soil
with respect to the saturated ones is given by total suction Y
Y = s + p
G. Della Vecchia Seminario DICA
Matric suction
Mostly, the dominant component for classical civil engineering problems is matric
suction
s = ua - uw
G. Della Vecchia Seminario DICA
Matric suction
The link between suction and pore geometry is give by the Laplace (Washburn) equation
s = suction (capillary pressure)
saw = interfacial tension between
water and air
q = contact angle between the
interface and the capillary tube
R = radius of the capillary tube
Ng & Menzies, 2007
G. Della Vecchia Seminario DICA
Matric suction
Air enters the pore if:
For each suction s = ua-uw , water will be just in the pores whose radius is lower than r
The link between suction and pore geometry is give by the Laplace (Washburn) equation
G. Della Vecchia Seminario DICA
The hydraulic behavior : water retention curve
Matric suction rules the quantity of water in the pores depending on its
porous network
Retention curve: link
between suction and
quantity of water in the
pores
G. Della Vecchia Seminario DICA
Link between water retention and pore size distribution
If the pores having radius lower than R are water filled:
f (r) = pore size distribution
F(R) = volumetric fraction of pores whose radius is ≤ R
G. Della Vecchia Seminario DICA
Link between water retention and pore size distribution
Alternative rapresentation of pore size distribution: Pore Size Density function
Pore size distribution f(r) Pore size density function PSD(r)
G. Della Vecchia Seminario DICA
Link between water retention and pore size distribution
Assumption: pores as a bundle of cylindrical tubes
Washburn equation
G. Della Vecchia Seminario DICA
Link between water retention and pore size distribution
WRC
s = 200 kPa
Sr = 0.8
Cumulative
function
R = 1.5 nm
F(R) = 0.8
G. Della Vecchia Seminario DICA
An example: van Genuchten equation
van Genuchten equation: effects of parameter variation on the WRC and the
corresponding PSD
Parameter a
Della Vecchia et al, IJNAMG, 2015
G. Della Vecchia Seminario DICA
An example: van Genuchten equation
van Genuchten equation: effects of parameter variation on the WRC and the
corresponding PSD
Parameter n
G. Della Vecchia Seminario DICA
Microstructure of compacted soils
Double porosity fabric: intra-aggregate (micropores) e inter-aggregate pores (macropores)
Sicilian scaly clay (Airò Farulla et
al. 2010), e0=0.58, w=15%
1 10 100 1000 10000 100000 1000000
diametro equivalente pori x (nm)
0
0.2
0.4
0.6
0.8
e in
truso
(-)
1 10 100 1000 10000 100000 1000000
diametro equivalente pori x (nm)
0
0.2
0.4
0.6
0.8
PS
D =
e
/lo
gx (
-)
Boom clay (Della Vecchia
2009), e0=0.97, w=15%
G. Della Vecchia Seminario DICA
Microstructure of compacted soils
Double porosity fabric: intra-aggregate (micropores) e inter-aggregate pores (macropores)
1 10 100 1000 10000 100000 1000000
diametro equivalente pori x (nm)
0
0.2
0.4
0.6
0.8
e in
truso
(-)
1 10 100 1000 10000 100000 1000000
diametro equivalente pori x (nm)
0
0.2
0.4
0.6
0.8
PS
D =
e
/lo
gx (
-)
Boom clay (Della Vecchia
2009), e0=0.97, w=15%
em = intra-aggregate void ratio (=Vvm/VS)
eM = inter-aggregate void ratio (=VvM/VS)
e = em + eM
G. Della Vecchia Seminario DICA
Retention model for double porosity materials
Retention curve as superposition of two retention domains:
Water ratio ew = Vw/Vs
G. Della Vecchia Seminario DICA
Retention model for double porosity materials
Seeming paradox:
0.0 0.2 0.4 0.6 0.8 1.0Water ratio, ew
0.01
0.1
1
10
100
Suction
(M
Pa)
Drying branches
axis translation (e=0.93)
axis translation (e=0.59)
vapour equilibrium (e=0.93)
vapour equilibrium (e=0.59)
SMI psychrometer (e=0.50 to 0.82)
WP4 psychrometer (e=0.93 to 0.99)
fitted curve e=0.93
fitted curve e=0.59
As compacted Boom clay (Della
Vecchia, 2009) WRC Boom clay (Romero et al,
2011)
unimodal water retention curve as-compacted bimodal PSD
G. Della Vecchia Seminario DICA
Confronto previsione modello – dati sperimentali:
Inada granite
Microfabric evolves along hydro-mechanical paths
As-compacted Boom clay,
w=15% (Della Vecchia
2009)
Constant volume
saturation
Saturated Boom clay
(Della Vecchia 2009)
G. Della Vecchia Seminario DICA
Retention model for double porosity materials
PSD data from hydration tests:
- Transition from bimodal to unimodal PSD upon hydration
- WRC testing is an hydromechanical path itself
G. Della Vecchia Seminario DICA
Retention model for double porosity materials
Romero, Della Vecchia, Jommi, 2011
G. Della Vecchia Seminario DICA
Retention model for double porosity materials
Prediction: PSD evolution upon
hydration
Prediction: WRC evolution upon
hydration
Della Vecchia et al, 2015
Increasing ew
G. Della Vecchia Seminario DICA
Retention model for double porosity materials
Calibration of the curve on drying/wetting branches for a single void ratio
(e=0.45)
Wetting parameters same as drying parameters, excluding a1m and a1
M
Drying curves for different constant
void ratios
Wetting curves for different constant
void ratios
Della Vecchia et al. (2015)
G. Della Vecchia Seminario DICA
Retention model for double porosity materials
Imposed hydro-mechanical paths: cyclic wetting-drying at different stress levels
Test A Test B Test C
Della Vecchia et al. (2015)
G. Della Vecchia Seminario DICA
Retention model for double porosity materials
Imposed hydro-mechanical paths: cyclic wetting-drying at different stress levels
Test A Test B Test C
Della Vecchia et al. (2015)
G. Della Vecchia Seminario DICA
Water retention curve: partial conclusions
• Conception of a framework for modelling WR behaviour of compacted
clays accounting for microfabric evolution
• Microstructural information directly embedded into WRC formulation
• WRC as an envelope of hydro-mechanical states rather than a characteristic
of the material
G. Della Vecchia Seminario DICA
The role of chemistry
Rock
Heater
Wetting
Heating
What if the chemistry of
the wetting fluid is
different from the
chemistry of the pore
fluid?
G. Della Vecchia Seminario DICA
The role of chemistry: the double-layer theory
Approach used to reproduce forces exchanged by particles in a system with uniform
pore size (clay suspensions)
Distance between particles is such that replusion
forces (of electrical nature) and attraction forces
come to an equilibrium.
The thickness of the double layer is smaller at higher concentrations of ions
prediction of volume reduction upon salinisation
G. Della Vecchia Seminario DICA
The role of chemistry: the double-layer theory
and its limits
Despite upon salinisation samples experiences global shrinkage...
...The hydraulic conductivity increases for increasing concentration of the pore fluid!
G. Della Vecchia Seminario DICA
Structure evolution upon chemical changes:
ESEM data on Febex bentonite
After static
compaction
After saturation with
distilled water
After saturation with a
0.5 M NaCl solution
Aggregate swelling upon saturation (reduction of voids between aggregates)
Extent of aggregate swelling influence by salinity of the pore fluid
Musso, Romero, Della Vecchia (2013)
G. Della Vecchia Seminario DICA
Structure evolution upon chemical changes:
double porosity framework
Transport: Barrenblatt, 1960; Warren & Root,1963; Gerke & Van Genuchten, 1993
Transport:
Water and solute fluxes occur through the
macroporosity and the microporosity
domain.
Exchange of mass can occur between the
two domains in virtue of potential
differences
G. Della Vecchia Seminario DICA
Structure evolution upon chemical changes:
double porosity framework
Mechanical:
The material (overall) and the aggregates
deform upon variation of chemical,
hydraulic and mechanical ‘loads’.
Transport parameters also evolve as a
consequence of fabric changes
Mechanical: Gens & Alonso,1992
G. Della Vecchia Seminario DICA
Structure evolution upon chemical changes: MIP
tests
Mercury Intrusion Porosimetry on freeze dried samples (Delage et al 1982)
Quantification of the evolution of the pore size distributions at different pore
fluid concentrations
All samples swelled
upon saturation
The amount of swelling
depends on the solution
used
Musso, Romero, Della Vecchia (2013)
G. Della Vecchia Seminario DICA
Evolution of the intra-aggregate porisity
Difference between cumulative intruded volume with a given solution and the one
obtained with distilled water (reference)
Macropores: pores with size
greater than 1000 nm
increase upon salinisation
Micropores: pores with size
smaller than 1000 nm
decrease upon salinisation
Musso, Della Vecchia, Romero, (2013)
G. Della Vecchia Seminario DICA
Evolution of the intra-aggregate porosity
Dependence of microstructural void ratio on osmotic suction
Osmotic suction p chosen
as stress variable
i = number of constituents
into which the molecule
separates upon dissolution
R = universal gas constant
T = absolute temperature
Musso, Romero, Della Vecchia (2013)
G. Della Vecchia Seminario DICA
Hydro-chemical transport equations
Need to take into account:
• Different transport properties in the two structural domains (micro and macro)
• Transient flux conditions
• Possible disequilibrium between the two structural levels: salt and water can be
exchanged between the two domains
Assumption: intra-aggregate and inter-aggregate domains are modelled as
homogeneous media with different hydraulic and solute transport properties,
superposed over the same volume
G. Della Vecchia Seminario DICA
Hydro-chemical transport equations:
water mass balance
Hp. No water flow occuring completely within the microporosity: mass of water
can move from the intra-aggregate pores only toward (or from) inter-aggregate
porosity
v = volumetric flow of water mass relative to solid skeleton
qwEX = water mass transfer term between micro and macro
G. Della Vecchia Seminario DICA
Hydro-chemical transport equations:
solute mass balance
Hp. No salt flux occuring completely within the microporosity: mass of solute can
move from the intra-aggregate pores only toward (or form) inter-aggregate
porosity
cm,cM = saline concentration in the micro- and macro-pores
j = total flux of solute mass
qsEX = solute mass transfer term between micro and macro
G. Della Vecchia Seminario DICA
Hydro-chemical transport equations:
water flow in macro-pores
Hp. Presence of hydraulic and osmotic potential gradients only
Direct flow
Coupled flow:
flow of water due to
differences in concentration
Kp = osmotic permeability
w = osmotic efficiency = f (eM) (Bresler 1973,
Musso et al 2013): bentonite as a semi-
permeable membrane, through which water
can pass but solute (sale) cannot.
w=0 no membrane behaviour
w=1 perfect membrane behaviour
G. Della Vecchia Seminario DICA
Hydro-chemical transport equations:
solute flux in macro-pores
Advection Diffusion
DM = effective
diffusion coefficient
G. Della Vecchia Seminario DICA
Application of the framework:
salt diffusion test in oedometer
top reservoir:
c (t=0) = 5.5 M
bottom
reservoir:
c (t=0) =5.5 M
Preparation:
• statically compacted specimen (w=12%)
• loaded up to 200 kPa
• saturated with saline water (NaCl 5.5 molar)
Modified oedometer cell:
• Each porous stone connected with a reservoir which can host small
quantities of solute
G. Della Vecchia Seminario DICA
Application of the framework:
salt diffusion test in oedometer
upper reservoir:
measured c = c(t) bottom reservoir:
constant concentration
c=0
Test:
• Imposed: distilled water placed in the lower reservoir
• Measured: vertical displacements and concentration in the upper reservoir
G. Della Vecchia Seminario DICA
Application of the framework:
salt diffusion test in oedometer
0
50
100
150
200
250
300
350
0,7
0,74
0,78
0,82
0,86
0,9
0 50 100 150 200 250 300 350 400 450
vo
id r
ati
o, e
(-)
Time elapsed from exposure to distilled water (days)
e NaCl upper reservoir
Mo
lar
co
ncen
trati
on
c (m
ol l-
1)
(g/l
)
As a consequence of induced water and salt flux:
• Salt concentration reduction in the upper reservoir
• Chemo-mechanical swelling
Process reversal: back
to initial conditions
G. Della Vecchia Seminario DICA
Application of the framework:
FEM simulation vs experimental results
58
Displacement Concentration
Musso, Romero, Della Vecchia (2013)
Della Vecchia & Musso (2016)
G. Della Vecchia Seminario DICA
Conclusions 59
• Significant influence of chemical composition of the pore fluid on mechanical
and hydraulic behaviour of active clayey soils
• Need for a double porosity framework: double structure and its evolution
documented through MIP and ESEM
• Characterization:
micro through MIP results
macro through oedometer tests (swelling + mech loading)
• Compacted soil multiphysical coupling
multiscale coupling
microstructure evolution cannot be neglected
G. Della Vecchia Seminario DICA
Acknowledgements 60
Cristina Jommi
Guido Musso
Enrique Romero
Anne Catherine Dieudonne
G. Della Vecchia Seminario DICA
References 61
• Romero, E., G. Della Vecchia, C. Jommi, 2011. An insight into the water retention properties of compacted clayey
soils. Géotechnique 61 (4), 313-328.
• Della Vecchia, G., C. Jommi, E. Romero, 2013. A fully coupled elastic-plastic hydro-mechanical model for
compacted soils accounting for clay activity. International Journal for Numerical and Analytical Methods in
Geomechanics 37 (5)
• Musso, G., E. Romero, G. Della Vecchia, 2013. Double structure effects on the chemo-hydro-mechanical behaviour
of compacted active clay. Géotechnique 63 (3), 206-220
• Cattaneo, F., G. Della Vecchia, C. Jommi, 2014. Evaluation of numerical stress-point algorithms on elastic-plastic
models for unsaturated soils with hardening dependent on the degree of saturation. Computers and Geotechnics 55,
404-415
• Della Vecchia, G., A.C. Dieudonne, C. Jommi, R. Charlier, 2015. Accounting for evolving pore size distribution in
water retention models for compacted clays. International Journal for Numerical and Analytical Methods in
Geomechanics 39 (7), 702-723
• Della Vecchia, G., G.Musso, 2016. Some remarks on single and double porosity modelling of coupled chemo-hydro-
mechanical processes in clays. Accepted for publication in Soils and Foundations, in print.
• Dieudonnè, A.C., G. Della Vecchia, R. Charlier, 2016. A water retention model for compacted bentonites, Submitted
to Canadian Geotechnical Journal, under review.