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E i l G h i
Marolo C. Alfaro, Ph.D., P.Eng.Department of Civil EngineeringUniversity of Manitoba
Environmental Geotechnics
University of Manitoba
Sponsors
f d h lDepartment of Science and Technology
Republic of the Philippines
Department of Civil EngineeringDepartment of Civil Engineering
University of Manitoba, Canada
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Host institutions
Philippine Institute of Civil Engineers
Association of Structural Engineers of the Philippines
Angeles University Foundations, Angeles City
De La Salle University, Metro Manila
University of the Philippines, Metro Manila
University of San Jose Recoletos, Cebu City
University of Mindanao, Davao City
University of Southeastern Philippines, Davao City
Definition• A sub‐discipline within geotechnical engineering which is the application of geotechnical principles, processes and
Environmental geotechnics
pp g p p ptechniques in situations where there is a major environmental component.
Scope• Waste disposal and safe containment of waste• Isolation of contaminated sites• Remediation of contaminated ground and derelict lands
→ Soil properties and their behavior over a range of conditions are of major importance.
→ It is also important to understand how environmental processes influence soil properties and their behavior.
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Waste disposal and safe containment of waste
Municipal waste landfills
Isolation of contaminated sites
Hazco
Petro Canada
Compacted clay liners for containment can be affected by chemical waste
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Remediation of contaminated ground
Nuclear waste repositories ‐ AECL
Isolation of contaminated sites
Permeable reactive barrier
Engineered clay barrier
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Remediation of contaminated ground
Remove and treat
In‐situ treatment
Remediation of contaminated ground
Ground contamination from gasoline spillage
Biological remediation
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Physico‐chemical effects on soil properties
• Attention is focused on the solid phase in most
geotechnical studies.
• However, properties of pore fluid (pore water) and
the influences of system chemistry must be taken into
account.
• Fine‐grained soils (e.g. clays) are more sensitive to
environment
• Interactions of the pore fluid and solid phases of a soil
are important in the overall physical and chemical
behavior
Geotechnical engineers discover chemistry
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Elements of earth
8‐35 km crust% by weight in crusty g
O = 49.2Si = 25.7Al = 7.5Fe = 4.7Ca = 3.4Na = 2.6K = 2.4Mg = 1 9
82.4%
Mg = 1.9other = 2.6
Clay minerals
CLAYS are composed of clay minerals.
Clay minerals are made of two distinct structural units:
1) Silicon tetrahedron 2) Aluminum Octahedron
Silica (+charges) surrounded by four oxygen (‐charged)
Aluminum (+charges) surrounded by six oxygen (‐charged)
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Clay minerals
Joined by strong H‐bond Joined by K+ ions Joined by weak van der Waals Joined by strong H bond∴ No easy separation
Joined by K ionsK+ fit into the hexagonal
holes in Si‐sheet
ybond
∴ Easily separated by water
• Monmorillonite structure swells on contact with water. Often called expansive clays.
• Bentonite clay belongs to montmorillonite family. Used as drilling mud, in slurry walls,
and in stopping leaks.
Soil fabric of clays
Close up view photo of clay particles from SEM
Clay particle:Plate‐like or flaky shape
Flocculated Structure
• Flocculated structure has edge to face contacts of clay particles
Di d h f f Flocculated Structure
Dispersed Structure
• Dispersed structure has face to facecontacts of clay particles
• Electrochemical environment during time sedimentation influences clay fabric
• Clay particles tend to align perpendicular to load applied on them
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Soil fabric of clays and sands
Close up view photos of clay particles from
Flocculated Structure
SEM
Loose StateFlocculated Structure
Dispersed StructureDense State
Isomorphous substitution
• Substitution of Si4+ and Al3+ by other lower valence
(e.g., Mg2+) cations
• Results in charge imbalance (net negative)
+ + ++positively charged edges
• Results in charge imbalance (net negative)
• Cations ‐ positively charged ions; anions ‐ negatively charged
• The replacement power is greater for higher valence and larger cations.
Al3+ > Ca2+ > Mg2+ >> NH4+ > K+ > H+ > Na+ > Li+
++
+ +
+
+
__ _
_ _
_
_
___
_
_
_
_
_
_
_
_
_
__
__
negatively charged faces
Clay particle with net negative charge
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Cation concentration in water
• Negative surface charges attract cations and positively charge water molecules
• Cation concentration is high at the clay surface and decreases with distance
Diffuse double layer
Free water
Adsorbed water
• Thin layer of water, called adsorbed water, is bonded to the negativelycharged surface
• The adsorbed water is more viscous than free water
Diffuse double layer
Free water
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Diffuse double layer
• Negatively charged clay surface and the positively charged cations near the particle form two distinct layers, known as diffuse double layer (DDL)
• The thickness of DDL depends on pore fluid chemistry and temperature,The thickness of DDL depends on pore fluid chemistry and temperature,
etc.
Double layer interactions
R‘Effective stress’ contains b th ‘ t t’ d ‘ l t
Practical Implications: • Changes in pore fluid chemistry change DDL thickness (extent of potential fields)
A
both ‘contact’ and ‘electro‐chemical’ unit forces:
{σ′} = {σ* + |R‐A|}
potential fields)
• When potential fields interact, changes in DDL thickness result inchanges in repulsion between two particles
• Changes in repulsive force, R affects:deformation, strength, and hydraulic conductivity
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Double layer interactions
R‘Effective stress’ contains both ‘contact’ and ‘electro‐
Thick DDL:
• High repulsion (R↑)• Swelling of expansive clays or increase swelling pressure (confined)
A
both contact and electrochemical’ unit forces:
{σ′} = {σ* + |R‐A|}
g p y g p ( )
• Soil strength is decreased
Thin DDL: • Low repulsion (R ↓)• Expansive clays consolidate (if drained condition)
• Soil strength is increased
Example 1: Leaching of bonds on clay foundations
Seven Sisters generating Station, Manitoba, Canada
Garinger, B., Alfaro, M.C., Graham, J., Dubois, D. and Man, A. (2004). Instability of dykes at Seven Sisters generating station. Canadian Geotechnical Journal 41:5, 959‐971.
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Typical dyke section
4.3 m
Water~ 8 m
~ 6 m
Impervious Clay CoreRip‐Rap Shell
Upper Foundation
Lower Foundation
Problem
• Dike instability has occurred irregularly at Seven Sisters for 50 years
• It is unclear why some sections have become unstable while others have remained stable
1.011
(m)
273
275
277
Distance (m)0 1 2 3 4 5 6 7 8 910 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64
Ele
vatio
n (
263
265
267
269
271
273
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Triaxial test results
SS‐036 – unstable
SS‐040 – stable
Triaxial test results
SS‐036 ‐ unstable
SS‐040 ‐ stable
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Stability analysis results
0.974
tion
(m)
269
271
273
275
277
Distance (m)012345678910 12141618 202224 262830 323436 384042 44464850 525456 58606264
Ele
vat
263
265
267
269
(a) Section SS‐036 (unstable section)
1.011
) 275
277
Case Foundation Strength Unstable Stable
1 Critical State 1.15 1.21
2 Residual 0 97 1 01
(b) Section SS‐040 (stable section)
Distance (m)012345678910 1214 161820 222426 28303234 363840 42444648 505254 565860 6264
Elev
atio
n (m
)
263
265
267
269
271
273
275 2 Residual 0.97 1.01
Pore fluid chemistry analysis
Analysis done at three sections:• Background section• Unstable section beneath dike
Removal of CaSO4
by leaching• Unstable section beneath dike• Stable section beneath dike
Tested for concentration of Na+, Ca2+, Mg2+, Cl‐, SO4
2‐, and bicarbonate
Background
Soil
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Pore fluid chemistry analysis result
Pore fluid chemistry analysis result
Location Ca2+
(mg/L)SO4
2-
(mg/L)EC
(μS/cm)Na/Ca
(mg/L) (mg/L) (μS/cm)
Background: 125-680 290-1250 1460-4160 0.27-0.51
Stable: 28-220 51-672 546-3520 0.34-0.95
Unstable: 30-172 81-324 772-1650 0.68-2.10
Red => out of background range
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• Significant differences found between background section and sections beneath the dike
Pore fluid chemistry analysis result summary
• Beneath dikes there is a loss of dissolved ions and increase in Na/Ca ratio
• Ca2+ and SO42‐ depletion probably as a result of gypsum
dissolution
Soil structure after deposition
Edge to face flocculated aggregates with cementation at contacts
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Pore fluid chemistry changes
• Leaching of CaSO4 increased Na/Ca ratio (i.e. decreased valence), which increases interparticle repulsion (R↑, σ’↓)
• Less cementation at contacts with increased degree of leaching
• Quasi‐stable edge to face flocculated structure maintained until destroyed by straining (increased strain softening)
Concluding remarks
• The thickness of adsorbed water is much greater than clay particles, and therefore has significant implicationsto clay behaviour.
• Clays, particularly those with montmorillonite minerals,rely heavily on the DDL for a portion of load carrying capacity
• Changes in pore fluid chemistry (and temperature) affects:
→ Compressibility
→ Strength
→ Hydraulic conductivity
• Design considerations of foundations and earth structures
→Mechanical load/resistance
→ Environmental load/resistance
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Example 2: Clean‐up of contaminated ground
Oil Storage Facility, Alberta, Canada
Wong, R.C.K. and Alfaro, M.C. (2001). Fracturing in low‐permeability soils for remediation of contaminated ground. Canadian Geotechnical Journal 38:2, 306‐327.
Alfaro, M.C. and Wong, R.C.K. (2001). Laboratory Studies on Fracturing of Low‐Permeability Soils. Canadian Geotechnical Journal 38:2, 303‐315.
Alfaro, M.C. and Wong, R.C.K. (2003). Correlation between Air Permeability and Biodegradation in Hydrocarbon‐Contaminated Soil Columns by X‐Ray Computerized Tomography (CT). Proc. of the International Workshop on X‐Ray CT for Geomaterials, Kumamoto, Japan, 165‐171.
Background
• Large costs of removal and off‐site treatment
• Site methods available: bioventing, vapor extraction, bioremediation, soil flushing, and pump and treat
• They involved promoting liquid or vapor flow through the contaminated site
• Limiting factor is soil permeability
• Hydraulic fracturing used to enhance in‐situ remediation in low‐permeable soils such as clays and siltslow permeable soils such as clays and silts
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Fracture mechanism
A A
Section A‐AFracture sample
Laboratory fracturing tool
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Fracture geometry
σ’v > σ’h → Ko = σ’h/σ’v < 1.0
Fracture geometry
σ’v > σ’h → Ko = σ’h/σ’v < 1.0
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Fracture geometry
OCR = 3 → Ko = σ’h/σ’v = 0.8 OCR = 6 → Ko = σ’h/σ’v = 1.2
OCR = 8 → Ko = σ’h/σ’v = 1.4
Fracture geometry
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Bioventing test setup
• Determine the rate of biodegradation of hydrocarbon in the soilOff l f h l• Off‐gas samples for each column were analyzed using gas chromatograph (GC) in order to determine CO2
production rates• Nutrients were also injected• After a series of calculations, this data was converted into air permeability
Air permeability and biodegradation results
• No clear correlation exists between the biodegradation ratebiodegradation rate and air permeability
• To assist in interpreting the results of air permeability and biodegradation measurements, it wasmeasurements, it was decided to use CT scan data
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Air permeability and biodegradation results
Air permeability and biodegradation results
• The CT images of two of the soil columns show that there could be channeling inside the columns due to cracks.
• However the presence of fractures alone did not necessarilyHowever, the presence of fractures alone did not necessarily resulted to higher air permeability.
• Higher percentage of pore volume above the average pore volume of the entire column resulted in higher permeability.
• It was also found that the more uniform soil columns have higher permeability compared to the less uniform soil columns.
• The amount of bioremediation is inversely proportional to the air permeability, such that low biodegradation in the hydrocarbon‐contaminated soil columns is associated with high air permeability.
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Field fracturing
Mapping of fractures by excavation
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Mapping of fractures
Mapping of fractures by tiltmeters
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Concluding remarks
• Initial fracture slots did not necessarily define orientationof propagating fracture
• Fracture orientation is perpendicular to the minimumFracture orientation is perpendicular to the minimumcompressive stress (importance of determining OCR)
• Smaller contrast of major and minor principal stresses
favors multiple deviated fractures, larger favors distinct
fractures
• Field fractures were found to be nearly horizontal indicating
overconsolidated subsoil conditions
• Tiltmeter data analysis of the orientation and extent of
fractures seemed to conform closely with actual fracture
placements
Thank you