seminario dica, 9 june · pdf fileseminario dica g. della vecchia compacted soils: what are...

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

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

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

Compaction plane:

• Optimum water content

• Dry side of optimum

• Wet side of optimum

Da Craig’s Soil Mechanics

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

Earth construction Compacted soils

• River embankments, flood defences and irrigation canal dikes

Jommi & Della Vecchia (2000)

• Earth dams

• Environmental barriers

Low hydraulic conductivity and diffusivity

• 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

Heater

Wetting

Heating

Engineered Barrier System for HLW disposal (FEBEX, Grimsel, Swiss Alps)

Sometimes compaction can be a problem!

Compacted soils are unsaturated soils

Solid grains Air

The behaviour of unsaturated soils is much more complicated than the behaviour

of saturated soils

Compacted soils are unsaturated soils 14

Complications:

• Three-phase medium: solid grains + water + air (+ air-water interfaces?)

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

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

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

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

Matric suction

Mostly, the dominant component for classical civil engineering problems is matric

suction

s = ua - uw

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

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

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

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

Alternative rapresentation of pore size distribution: Pore Size Density function

Pore size distribution f(r) Pore size density function PSD(r)

Assumption: pores as a bundle of cylindrical tubes

Washburn equation

s = 200 kPa

Sr = 0.8

Cumulative

function

R = 1.5 nm

F(R) = 0.8

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

An example: van Genuchten equation

van Genuchten equation: effects of parameter variation on the WRC and the

corresponding PSD

Parameter n

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)

1 10 100 1000 10000 100000 1000000

Boom clay (Della Vecchia

2009), e0=0.97, w=15%

Microstructure of compacted soils

Double porosity fabric: intra-aggregate (micropores) e inter-aggregate pores (macropores)

1 10 100 1000 10000 100000 1000000

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

Retention model for double porosity materials

Retention curve as superposition of two retention domains:

Water ratio ew = Vw/Vs

Seeming paradox:

0.0 0.2 0.4 0.6 0.8 1.0Water ratio, ew

Suction

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,

unimodal water retention curve as-compacted bimodal PSD

Confronto previsione modello – dati sperimentali:

Inada granite

Microfabric evolves along hydro-mechanical paths

As-compacted Boom clay,

w=15% (Della Vecchia

Constant volume

saturation

Saturated Boom clay

(Della Vecchia 2009)

PSD data from hydration tests:

- Transition from bimodal to unimodal PSD upon hydration

- WRC testing is an hydromechanical path itself

Romero, Della Vecchia, Jommi, 2011

Prediction: PSD evolution upon

hydration

Prediction: WRC evolution upon

hydration

Della Vecchia et al, 2015

Increasing ew

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

Drying curves for different constant

void ratios

Wetting curves for different constant

void ratios

Della Vecchia et al. (2015)

Imposed hydro-mechanical paths: cyclic wetting-drying at different stress levels

Test A Test B Test C

Imposed hydro-mechanical paths: cyclic wetting-drying at different stress levels

Test A Test B Test C

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

The role of chemistry

Heater

Wetting

Heating

What if the chemistry of

the wetting fluid is

different from the

chemistry of the pore

fluid?

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

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!

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)

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

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

Structure evolution upon chemical changes: MIP

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

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)

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

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

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

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

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

solute flux in macro-pores

Advection Diffusion

DM = effective

diffusion coefficient

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

upper reservoir:

measured c = c(t) bottom reservoir:

constant concentration

• Imposed: distilled water placed in the lower reservoir

• Measured: vertical displacements and concentration in the upper reservoir

0 50 100 150 200 250 300 350 400 450

Time elapsed from exposure to distilled water (days)

e NaCl upper reservoir

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

FEM simulation vs experimental results

Displacement Concentration

Della Vecchia & Musso (2016)

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

Acknowledgements 60

Cristina Jommi

Guido Musso

Enrique Romero

Anne Catherine Dieudonne

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.

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