soil structure and fabric - civil.emu.edu.trcivil.emu.edu.tr/courses/civl553/lec10 fabric...
TRANSCRIPT
5/22/2013
1
SOIL STRUCTURE AND FABRIC
The structure of a soil is taken to meanboth the geometric arrangement of theparticles or mineral grains as well as theinterparticle forces which may act betweenthem.
Soil fabric refers only to the geometricarrangement of particles (from Holtz andKovacs, 1981).
*Fabric and structure are used interchangeably sometimes.
5/22/2013
2
The interparticle forces (or surface forces)are relatively important for fine-grainedsoils at low confinement (low state ofstress).
Although the behavior of a coarse-grainedsoil can often be related to particle sizedistribution, the behavior of a fined-grainedsoil usually depends much more on:
geological history and
structure
than on particle size.
SOIL FABRIC AND STRUCTURE
Fabric is the arrangement of particles, particle group and pore spaces in a soil.
Structure is the combined effects of fabric, composition and interparticle forces.
5/22/2013
3
Microfabric → at least an optical
microscope is needed.
Macrofabric → stratification, fissuring, voids and
large scale inhomogeneties (by naked eye or a hand lense).
NET ENERGY AND FORCE OF INTERACTION
Dispersion or flocculation →
Fabric of soil →
Determines the engineering properties.
If repulsion → dispersion
If attraction → flocculation
5/22/2013
4
Very small particles – provide very large surface area.
Negatively charged surface –provide very active surface for chemical interaction.
(From Bennett and Hulbert, 1986)
DISPERSION AND FLOCCULATION OF CLAY
Colloidal clay
Clay is a colloid. Colloidal particles have special properties due to their very small size.
Firstly, their large surface area in relation to their mass makes them very reactive;
in clays, this reactivity is shown as an electrostatic attraction of cations.
5/22/2013
5
Secondly,
colloids can exist in water as either:
o suspensions (dispersed) or
o as gels (flocculated).
The tendency of a colloid to
o flocculate or
o disperse
depends on three things:
the nature of the colloidal particles;
the total salt concentration;
the nature of the adsorbed ions.
5/22/2013
6
The type and amount of different cations in a clay-water-electrolyte system have a major influence on double layer interaction.
Flocculation to describe particles that are connected
edge–to–edge or edge–to–face,
Aggregation to
describe particles that are connected
face–to–face.
TERMINOLOGY
Dispersed: No face-to-face association of clay particles
Aggregated: Face-to-face association (FF) of several clay particles.
Flocculated: Edge-to-Edge (EE) or edge-to-face (EF) association
Deflocculated: No association between aggregates or particles.
Face (F)
Edge (E)
Clay Particle
van Olphen, 1991 (from Mitchell, 1993)
5/22/2013
7
Flocculated fabric Dispersed fabric
Edge-to-face (EF):positively charged edges and negatively charged surfaces (more common)Edge-to-edge (EE)
The net interparticle force between surfaces is repulsive
Aggregated fabric
Face-to-Face (FF)
Shifted
Face-to-Face (FF)
CLAY FABRIC
Flocculated Aggregated
edge-to-face contactface-to-face contact
5/22/2013
8
ENVIRONMENT EFFECT ON CLAY FABRIC
Electrochemical environment i.e.:
pH,
acidity,
temperature,
cations present in the water
during the time of sedimentation influence clay fabric significantly.
5/22/2013
9
Flocculation is the first step in aggregate formation.Examples of flocculated and dispersed organic molecules.
Thickness of the diffuse double layer will depend on:
Concentration of soil solution:
High concentration of soil solution yields a thin DDL.
5/22/2013
10
Valence of exchange ions: Monovalent ions yield a thick DDL
Size of an ion (or hydration radius): Strongly hydrated ions yield a thick DDL.
Particles with thick DDL tend to
DISPERSE
Particles with thin DDL tend to
FLOCCULATE
Colloidal particles are either:
hydrophilic (water-loving) or
hydrophobic (water-hating).
Hydrophilic colloids form
stable suspensions and do not readily flocculate.
5/22/2013
11
Hydrophobic colloids form
unstable suspensions and flocculate easily.
The nature of the colloidal clay particle (hydrophobic) means that clay
will flocculate if allowed to.
This is good for soil structure!
The more concentrated the salts (electrolytes) in the soil solution,
the more likely it is that clay will flocculate.
This is the 'electrolyte effect'.
The salt is not necessarily common salt, sodium chloride.
Any soluble salt, such as gypsum, will have this effect.
5/22/2013
12
An 'electrolyte' is any salt.
It is not necessarily common salt (sodium chloride).
It could be any combination of cation and anion.
Salts in soil can come
from the weathering of soil minerals.
Weathering releases cations such as
sodium,
potassium,
calcium,
iron and magnesium.
Anions produced by weathering include:
sulphate,
chloride,
carbonate and phosphate.
5/22/2013
13
Calcium adsorbed onto the clay surface allows the clay to flocculate
when the total salt concentration is fairly low.
However,
Sodium adsorbed onto the clay surface
will not allow the clay to flocculate
until the total salt concentration is much higher.
Changes in the double layer thickness
modifies the soil properties like:
the shear strength,
compressibility and
plasticity.
5/22/2013
14
MODES OF PARTICLE ASSOCIATIONS IN CLAY SUSPENSION
1. Dispersed → no face to face association of clay particles.
2. Aggregated → face to face association of several clay particles.
3. Flocculated → edge to edge or edge to face association of particles or aggregates.
4. Deflocculated → no association of particles or aggregates.
PARTICLE ASSOCIATIONS
Dispersed and deflocculated Aggregated but deflocculated
Edge-to-face flocculated and aggregated
Edge-to-edge flocculated and aggregated
Edge-to-face and edge to edge flocculated and aggregated
Edge-to-edge flocculated but dispersed
Edge-to-face flocculated but dispersed
van Olphen, 1991
5/22/2013
15
FABRIC IN COHESIVE SOILS
Dispersed fabric: formed by settlement of individual clay particles. More or less parallel orientation.
Flocculant fabric: formed by settlement of flocs of clay particles.
Domain: aggregated or flocculated sub-microscopic units of clay particles.
Cluster: domains group to form clusters, can be seen under light microscope.
Peds: they are clusters group to form peds, can be seen without microscope.
DOMAIN CLUSTER PEDThe individual clay particles seem to always beaggregated or flocculated together in submicroscopicfabric units called domains.
Domains then in turn group together to form clusters,which are large enough to be seen with a visible lightmicroscope.
Clusters group together to form peds and even groupsof peds.
Peds can be seen without a microscope, and they andother macrostructural features such as joints andfissures constitute the macrofabric system.
5/22/2013
16
FABRIC OF NATURAL CLAY SOILS
Enlargement
Domains and clusters with micropores
1.Domain
2.Cluster
3.Ped
4.Silt grain
5.Micropore
6.Macropore
Yong and Sheeran (1973) (from Holtz and Kovacs, 1981)
Diagram of the fundamental particle units called domains that comprise the “building blocks” of clay fabric in sediments and rocks. (From Bennett et al., 1991)
5/22/2013
17
MICROFABRIC FEATURES IN NATURAL SOILS
1.Elementary particle arrangements, which consistof single forms of particle interaction at the level ofindividual clay, silt, or sand particles or interactionbetween small groups of clay platelets or clothedsilt and sand particles.
2.Particle assemblages, which are units of particleorganization having definable physical boundariesand a specific mechanical function. Particleassemblages consist of one or more forms ofelementary particle arrangements or smallerparticle assemblages.
3.Pore spaces within and between elementaryparticles arrangements and particle assemblages.
Collins and McGown, 1974 (from Holtz and Kovacs, 1981)
ELEMENTARY PARTICLES
Individual clay platelet interaction
Individual silt or sand particle interaction
Clay platelet group interaction
Clothed silt or sand particle interaction
Particle discernible
Collins and McGown, 1974 (from Holtz and Kovacs, 1981)
5/22/2013
18
PARTICLE ASSEMBLAGES
Collins and McGown, 1974 (from Holtz and Kovacs, 1981)
PARTICLE ASSOCIATIONS IN SOILS
Those main groupings can be identified:
1. Elementary particle arrangements, particle interaction of individual clay, silt or sand particles
2. Particle assemblages
3. Pore spaces
4. Intrapedal pores→ pore within the ped
5. Interpedal pores → pores between the ped
6. Transpedal pores → the pores that transverse the soil beyond the limits of a single ped.
Ped: it is an individual soil aggregate consisting of a cluster of primary particles and separated from adjoining peds by surfaces of weaknesses.
5/22/2013
19
PORE SPACE TYPES
Collins and McGown, 1974 (from Mitchell, 1993)
EARLY CONCEPTS OF CLAY FABRIC
Minerals of chemically sensitive clays:
in a flocculated system, “cardhouse structure”(flocculated ).
Lambe (1953), particle orientation in a dispersed system is a parallel arrangement (oriented fabric).
Mitchell (1956) pointed out important differences between dispersed and flocculated clays in relation to their geotechnical properties.
Cardhouse, of saltwater Cardhouse of freshwater
5/22/2013
20
Van olphen proposed various modes of particles association when clay particles flocculate: FF, EF, and EE.
EE and EF produce agglomerates (called “floc”).
FF association is termed “aggregation”.
Flocculation and aggregation have major effects on engineering properties.
Flocculation affects flow behavior.
It influences permeability,
the ease with which a liquid moves through the soil.
5/22/2013
21
Particles that are dispersed would have less permeability.
Flocculation also affects shear strength and compressibility.
Soils that have an edge-to-face contact of clay particles (flocculated) are much stronger than soils
with a parallel alignment (dispersed).
One effect of the double layer is to cause two
clay particles to repel each other when they approach so closely.
Repulsive forces caused by overlapping double layers have been used to describe the compression and swelling behavior of clays.
Dispersion phenomena is used to explain
erosion of clays and tunneling failures in dams.
5/22/2013
22
EROSION AND PIPING IN CLAYS
In the past, clay soils were considered to be highly resistant to erosion by flowing water;
however, in the recent years it is recognized that highly erodible clay soils exist in nature.
Some natural clay soils disperse or deflocculate in the presence of relatively pure water and are, therefore, highly susceptible to erosion and piping.
The importance of the subject in civil engineering practice was not recognized
until the early 1960's when research on piping failure in earth dams
due to dispersive clay behavior was initiated
in Australia because of many failures of small clay dams (Aitchison and Wood, 1965).
5/22/2013
23
The tendency for dispersive erosion in a given soil depends on variables such as:
• mineralogy and chemistry of the clay,
• dissolved salts in the water in soil pores
and in the eroding water.
Such clays are eroded rapidly by slow-moving water, even when compared to cohesionless fine sands and silts.
When dispersive clay soil is immersed in water, the clay fraction behaves like single-grained particles;
that is, the clay particles have a minimum of electrochemicalattraction and
fail to closely
adhere to, or bond with other soil particles.
5/22/2013
24
Thus, dispersive clay soil erodes in the presence of flowing water when
individual clay platelets are split off and carried away.
Such erosion may start in a drying crack, settlement crack, hydraulic fracture crack, or other channel of high permeability in a soil mass.
SUSCEPTIBILITY TO DISPERSION PIPING
One of the properties controlling the susceptibility to dispersion piping is
the percentage of adsorbed sodium cations within the clay particles relative to the quantities of
other polyvalent cations (calcium, magnesium, and potassium).
5/22/2013
25
A second factor controlling susceptibility of a clay mass to dispersion piping is the
total content of dissolved salts
in the reservoir or canal water.
The lower the content of dissolved salts in the reservoir or canal water,
the greater the susceptibility of sodium saturated clay to dispersion.
SWELL
Any change in the pore solution chemistry that
depresses or reduces the double layer leads to a
reduction in swell.
Calcium ions in the interlayer region
compress the double layer,
so the sheets are closer together and do not adsorb water and swell as easily.
5/22/2013
26
If DDL Thickness is small
swell is small.
With sodium ions,
the clay swells more easily.
Thus the clay mineralogy has a direct
effect on
its surface chemistry.
Through its effect on surface chemistry,
clay mineralogy controls
microstructure.
The result is the: engineering behavior of soil, its cohesive strength, flow behavior, permeability, and swelling potential.
5/22/2013
27
Dispersed fabrics are more common in clays deposited in fresh water,
while flocculated fabrics are typical of seawater deposition.
Remolding (disturbance) of soils alters flocculated fabrics
to dispersed fabrics.
Chemical factors favoring flocculation (favor structure):
High salt concentration Polyvalent cations Low pH
Chemical/physical factors favoring dispersion(unfavorable for structure)
Low salt concentration Sodium is dominant cation High pH Mechanical disturbance
5/22/2013
28
Saline water applied to soil will allow the clay to flocculate.
If the water is saline due to high levels of soluble calcium, the flocculation will persist.
If, however, the water is saline due to high levels of sodium,
the flocculation will last only as long as the soil solutionremains concentrated.
When rain washes excess salts from the soil,
the soil solution becomes dilute and the clay disperses.
5/22/2013
29
SALINE WATER
Saline water is a general term for water that contains a significant concentration of dissolved salts (NaCl).
According to United States Geological Surveythree categories of saline water:
Slightly saline water contains around 1,000 to 3,000 ppm,
Moderately saline water contains roughly 3,000 to 10,000 ppm.
Highly saline water has around 10,000 to 35,000 ppm of salt. Seawater has a salinity of roughly 35,000 ppm, equivalent to 35 g/L.
5/22/2013
30
Gypsum acts on clay in two ways.
Firstly, by raising the level of soluble salts in the soil solution,
gypsum allows the clay to flocculate even if the clay has a high percent of exchangeable sodium (this isthe electrolyte effect).
Secondly, soluble calcium in the gypsum replaces sodium on the cationexchange sites.
The calcium dominated clay will remain flocculated after the free sodium iswashed from the soil and the total salt concentration falls.
In practice, however, several follow-up applications of gypsum are necessary to maintain the electrolyte effect.
5/22/2013
31
PACKING IN COHESIONLESS SOILS
Dense packingLoose packing
Honeycombed fabric•Meta-stable structure
•Loose fabric
•Liquefaction
•Sand boil
Holtz and Kovacs, 1981
HONEYCOMEDRelatively fine sand and silt form small arches with chains of particles.
Such soils have large void ratio, e and they can carry ordinary static loads.
However under heavy loads or when subjected to dynamic loading, the fabric breaks down causing large settlements.
5/22/2013
32
PACKING -SAND BOIL
Loose sand
Kramer, 1996
THE RELATIVE DENSITY (DR)The relative density Dr is used to characterize the density of natural granular soil.
%100
%100ee
eeD
mindmaxd
mindd
d
maxd
minmax
maxr
(Lambe and Whitman, 1979)The relative density of a natural soil deposit very stronglyaffects its engineering behavior. Consequently, it isimportant to conduct laboratory tests on samples of thesand at the same relative density as in the field ( from Holtzand Kovacs, 1981). (compaction)
5/22/2013
33
THE RELATIVE DENSITY (DR) “The relative density (or void ratio)alone is not sufficient tocharacterize the engineeringproperties of granular soils” (Holtz and
Kovacs, 1981). Two soils with the samerelative density (or void ratio) maycontain very different pore sizes.That is, the pore size distributionprobably is a better parameter tocorrelate with the engineeringproperties (Santamarina et al., 2001).
2 1:Holtz and Kovacs, 1981
FABRIC IN COHESIONLESS SOILS
Single grained
Honey combed
Single grained: properties can be studied by uniformly sized spheres.
Type of packing
Coordination number
Porosity
(%)
Void ratio
Single cubic 6 47.64 0.91
Cubical tetrahedral
8 39.54 0.61
Teragonal &
Sphenoidal
10 30.19 0.43
Pyramidal 12 25.95 0.34
Tetrahedral 12 25.95 0.34