2 environmental geotechnics

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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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Remove and treat

In‐situ treatment


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


Free water

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• 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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• 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

2 environmental geotechnics

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