Differential Settlement and its Importance onDifferential Settlement and its Importance on the Performance of Cover Systems at Radiological Waste Disposal Facilities

Robert C. Bachus, John F. Beech, and Leslie M. GriffinGeosyntec Consultants

Kennesaw GAKennesaw, GA

Workshop on Engineered Barrier Performance Related to Low-Level Radioactive Waste, Decommissioning, and Uranium Mill Tailings Facilities

R k ill MDRockville, MD4 August 2010

WorkshoppEngineered Barrier Performance Related to Low-Level Radioactive Waste, Decommissioning, and

U i Mill T ili F ilitiUranium Mill Tailings Facilities

Session 3Session 3Experience with Monitoring Devices and Systems

Used to Measure Performance

Presentationff SDifferential Settlement and its Importance on the

Performance of Cover Systems at Radiological Waste Disposal Facilitiesp

Differential Settlement and its Importance on the Performance of Cover Systems at Radiological e o a ce o Co e Syste s at ad o og ca

Waste Disposal Facilities

E i d C S t• Engineered Cover Systems– Components– Soils and Geosynthetic Material Characteristics

• Differential Settlement– Settlement Mechanisms– DOE-specific Issuesp

• Monitoring Devices and Systems– Types of Devices and Systems– Experience– Experience

• Recommendations for DOE Facilities– Monitoring and Reporting

R h bilit ti– Rehabilitation

Final Cover at the Fernald OSDFFinal Cover at the Fernald OSDF

Note: Soilcomponentscomponents comprise> 12 feet

Soil and Geosynthetic MaterialsLow Permeability Soil Components

• Performance ExpectationsPerformance Expectations– Hydraulic barrier– Support vegetation– Radiological barrier

• RealityP ti ft t ll d d i t ti– Properties are often controlled during construction

– Clays crack in extension at small strain– Desiccation cracking is de facto reality in clayDesiccation cracking is de facto reality in clay– Cover systems compress (not extend) upon settlement– Fine-grained soil erodes relatively easily– Soils (and wastes) are compressible

Waste CompressibilityWaste Compressibility• Mechanisms of waste compression that can lead p

to total and differential settlement– Mechanical Compression – compression of soil and

waste due to loadingg– Raveling – Internal erosion of soil due to water and/or

migration of soil due to large voids– Physico-chemical Changes – degradation of wastePhysico chemical Changes degradation of waste

and subsequent mass loss– Biomechanical Changes – biological decomposition of

wasteas e• Waste (and surrounding soil) compression leads

to total and differential settlement

Waste Settlement and Differential Settlement(admittedly worst case)(admittedly worst case)

– Settlement is not uniform because waste maySettlement is not uniform because waste may not be uniform

– Therefore, localized subsidence and ,differential settlement can occur

Waste may not be uniform

Often Includes (Temporarily) Stiff InclusionsOften Includes (Temporarily) Stiff Inclusions

DOE-Related IssuesDOE Related Issues

• Extreme conditions as shown on previousExtreme conditions as shown on previous slides may be just that…extreme examplesp– Old facilities – Uncontrolled trench disposal practices – Compromised covers and raveling conditions

• Can likely be controlled at new facilitiesy– Monitored waste placement– Waste placement plans

Concept of Stress RedistributionConcept of Stress Redistribution

Average Stress Actual Stressg

SOFT SOFTHARD HARD

Mother Nature does not like abrupt changes in material properties and will redistribute stress in direct response to materials stiffness

Cracks due to Differential SettlementCracks due to Differential Settlement

Waste Compressibility for DOEWaste Compressibility for DOE

• Mechanisms of waste compression thatMechanisms of waste compression that can lead to total and differential settlement– Mechanical compression – likely for DOEp y– Raveling – problematic for old DOE facilities– Physico-chemical changes – not likelyy g y– Biomechanical changes – not likely

• For DOE facilities, may have different , yconcerns and mechanisms when considering old versus new facilities

Performance Aspects of Cover System Components

Support Vegetation

PROTECTIVE SOIL LAYER

(24 INCH THICK)

Support Vegetation

Drainage

1 8 INC H SO IL B ARRI ERGEOC OMP OSIT ED RAIN AGE LAYE RPE GEO MEM BRAN E(40 MIL THIC K)

4 to 2 01

2 4 INC H PR OTE CTIV E SOI L LAY ER

12 INCH INT ERME DIAT E SO IL CO VER /

ST RUCT URA L FIL L

BARRIER

GEOCOMPOSITEDRAINAGE LAYER Low Permeability

Soil and Geomembrane

SOIL BARR

(18 INCH THICK)

PE GEOMEMBRANE(40 MIL THICK)

IATE SOIL COVER/

CH THICK)Isolation

R i f t

4 to 201 INTERMEDIATE S

STRUCTURAL FILL (12 INCH TH

Example

Reinforcement(optional)

20

ExampleUSEPA Subtitle D

Final Cover System

Properties and MeasurementsProperties and Measurements• How to Assess Propertiesp

– Laboratory tests– Performance monitoring and back calculation– Monitored large test fill– Monitored large test fill

• Candidate Field Monitoring Concepts/Devices– Visual inspection– Aerial survey– Settlement plates– Buried settlement platesBuried settlement plates– Hydraulic sensors– Instrumented geotextiles

Large-Scale Testing of WasteLarge Scale Testing of Waste

1500

500

1000

(psf

)

NORMAL STRESS = 1930 psfNORMAL STRESS = 1970 psf

NORMAL STRESS = 1980 psf

NORMAL STRESS = 1890 psf

-1500

-500

-1000

0

SH

EA

R S

TRE

SS

CYCLIC SIMPLE SHEAR TEST RESULTS FOR WASTE SAMPLE

-4 -3 -2 -1 0 1 2 3 4

SHEAR STRAIN (%)

Waste Compressibility

Primary Compression (C C )

p y(mechanical… but with time can be physico-chemical and biological)

Primary Compression (Cc, Cr)

C

nV

Cc

CrMaximum Preconsolidation

Stress = p’c

v

Secondary Compression (C)log n

vC

Initial&

PrimarySecondary

Log Time

C

Monitoring DevicesMonitoring Devices

• Visual inspectionVisual inspection– Probably best for DOE– Evidence of erosion and “bowl shapes”p

• Aerial survey– Excellent, except for resolution– Provides general assessment

• Settlement plates and surface monitoring points– Best, low-cost solution– Need to be where there is movement!

Riser Pipe and Surface Settlement PlPlate

Monitoring DevicesMonitoring Devices

• Buried settlement platesBuried settlement plates– Can help assess variations with depth– Help assess effects of ageHelp assess effects of age

• Hydraulic sensors– Good for automated monitoringGood for automated monitoring– Good for “profiling”

• Instrumented geotextilesInstrumented geotextiles– May be helpful in long-term study– Potential for automated profilingPotential for automated profiling

Instrumented Test Fill - MSWInstrumented Test Fill MSW

• Vertical and Lateral Expansion of South Shelby• Vertical and Lateral Expansion of South Shelby Landfill, Memphis, TN

• New Waste Placement Causes Considerable Settlements of OldSettlements of Old (Unlined) Waste

• Analogous to DOE placement of thick cover over existing waste

3D View of Test Fill3D View of Test Fill

20 foot platform

30-foot platform

10-foot platform

20-foot platform

View of Test FillView of Test Fill

Settlement Profiler SchematicSettlement Profiler Schematic

T t FillTest FillWater Reservoir

Settlement HubMeasurements Taken Every 1 foot

Settlement Hub

Settlement Profiler SystemSettlement Profiler System

Constant - Head Liquid Reservoir

Liquid- filled Tubing

Test Fill

a

ha hb

Settlement Calculation

Settlement = h = hb-ha

But we know from the pressure transducer that:

b

h

Electrical Pressure TransducerBut, we know from the pressure transducer that:

ub-ua = whb-wha

= w(hb-ha)

= w (h)

Electrical Pressure Transducer

Transducer Position Before Fill Placement (Output = ua = haw)

T d P iti Aft Fill Pl t (O t t h )h = (ub-ua)/w

Therefore, Transducer Position After Fill Placement (Output = ub = hbw)

Longitudinal Cross-SectionLongitudinal Cross Section

Test Fill Soil10’

20’30’

Existing Waste

Settlement Profile Pipe

Foundation Soils (thickness ~1000 ft)

Bedrock

Deep Settlement Plate

Surface Settlement Plate

(Not to Scale)

Surface Settlement PlateSurface Settlement Plate

40

60

et)

20

40

Hei

ght (

fe End of Construction

-20

00 20 40 60 80 100 120 140

t) x

10

-40

tlem

ent (

feet

-60Time (days)

Set

t

Surface Settlement Plates

D

Surface Settlement Plates

00 25 50 75 100 125 150

Days

1

2

3ft)

FillHeight~ 11 feet

3

4

5Set

tlem

ent (

f

~22 feet

5

6

7

S

~ 31 feet

8

Settlement Profile with DepthSettlement Profile with Depth

0.00 20 40 60 80 100 120 140

Days

0.5

1 0Deep

1.0

1.5

ettle

men

t (ft)

Medium

2.0

2.5

Se

Test Fill

Existing Waste Shallow

Surface

3.0

Surface

Settlement Profiler ResultsSettlement Profiler Results445

430

435

440

Ele

vatio

n (ft

) 8-Dec-0028-Nov-0017-Nov-0027-Oct-00

415

420

425

0 100 200 300 400 500 600 700

Fill

23-Oct-0019-Oct-00

5

6

7

r Pip

e (ft

)

19-Oct-00

Distance along Pipe (ft)

-1

0

1

23

4

0 100 200 300 400 500 600 700ve E

leva

tion

of P

rofil

er

23-Oct-0027-Oct-0017-Nov-00

28-Nov-008-Dec-00

-3

-2

-1

Distance along Pipe (ft)

Rel

ative

Settlement Profiler Results

5

7

vatio

n (ft

)

1

3

Rel

ativ

e E

lev

5

(ft)

-10 100 200 300 400 500 600 700

Distance from Eastern Hub (ft)

-1012345

190 195 200 205 210elat

ive

Ele

vatio

n

-3-2

Distance from Eastern Hub (ft)

Re

Implications to DOE SitesImplications to DOE Sites

• In the absence of raveling and theIn the absence of raveling and the introduction of water, differential settlement is NOT a major problemj p

• Most of the settlement is due to application of load

• Time-dependent settlements are small and relatively uniformy

• Monitoring requirements are relatively simplep

Liner System and Cover System (example for Subtitle D MSW)

PIPELINE TO LEACHATESTORAGE TANKOR TREATMENT PLANT

ACTIVEAREA

FINAL COVER SYSTEM

LANDFILL GAS WELL(TO FLARE OR TURBINE)INTERMEDIATE

COVER

OR TREATMENT PLANT

DAILY COVER

LEACHATE REMOVAL PUMP

SUMPLINER SYSTEM(COMPOSITE LINER OVERLAIN BYLEACHATE COLLECTION SYSTEM)

DAILY COVER

LEACHATE COLLECTION SYSTEM)

Leachate Generation Rates

300

350

400

WR

ATE

0

100

150

200

250

EAC

HAT

EFL

OW

MSW LANDFILL(PENNSYLVANIA)

Rainfall = 39 inches/years

RAINFALL

ACTIVEFILLING

COVER CELL CLOSED

0

50L

LEACHATETRANSFERPIPE

RAINFALLONTO ACTIVE AREA

LEACHATEGENERATION

LINER SYSTEMLEACHATE PUMPEDFROM SUMP

SUMP15

Generalized MSW Leachate Generalized MSW Leachate Generation Rates Generation Rates –– Rainfall FactorRainfall Factor

1000 – 2000 gpad(30 to 60% of rainfall)

300 – 600 gpad(10 to 20% of rainfall)

100 – 200 gpad(3 to 6% of rainfall)

20 – 40 gpad(0.6 to 1.2% of rainfall)

10 – 20 gpad(0.3 to 0.6% of rainfall)

Influence of Time after Closure on Influence of Time after Closure on L h t G ti R tL h t G ti R tLeachate Generation RateLeachate Generation Rate

HW LANDFILLS

RecommendationsRecommendations

• Monitor Performance of Existing FacilitiesMonitor Performance of Existing Facilities– Seems that “data” includes visual assessment – Limited quantitative dataq– Summary report of problems may exist

• Visual reportsS ttl t l t• Settlement plates

• Leachate generation rate

• Report FindingsReport Findings– Identify forum for presentation of monitoring results– Report performance of rehabilitation measures

Design Challenges and Solutions

Issue – Differential Settlements• Differential settlements along liner resulting from waste variability and local

ft h d t ( ll i id th “ t d f i t ”)soft or hard spots (e.g., collapsing void...the “rusted refrigerator”) may impair the liquid containment capability of the leachate collection system and/or cause localized settlements that result in the ponding of liquids or excessive liner system strain.

Solution• Assess actual site conditions to avoid “worst case” analyses. Incorporate

high stiffness geosynthetic reinforcement in the liner system or subbase g g y yand/or construct a foundation “buffer layer” between liner system and existing waste. Also, ground improvement techniques (e.g., deep dynamic compaction) can be used to minimize near-surface heterogeneities.

• Analytical procedures are available to select appropriate reinforcement• Analytical procedures are available to select appropriate reinforcement properties (e.g., strength, stiffness) or minimum thickness buffer layer.

Design Challenges and SolutionsDesign Challenges and Solutions“Rusted Refrigerator” Scenario

SURFACE BEFORE SETTLEMENT

SURFACE AFTER SETTLEMENT

WASTE

ZONE OF INFLUENCE

LOCALIZED COLLAPSE ORSETTLEMENT WITHIN WASTE

“SLOPE AREA”SLOPE AREA

Design Challenges and SolutionsDesign Challenges and SolutionsCompressible Void

FUTURE WASTE

LINER SYSTEMAFTER SETTLEMENT

COMPACTED CLAY

LOCALIZED LOW SPOT

COMPACTED CLAY

LOCALIZED LOW SPOT

EXISTING WASTE

COLLAPSIBLE VOID/HIGHLYCOMPRESSIBLE ZONE*

* DIFFERENTIAL SETTLEMENT MAY ALSO RESULT FROM HETEROGENEOUS NATURE OF WASTE

Design Challenges and SolutionsDesign Challenges and SolutionsGeosynthetic Reinforcement

LINER SYSTEM

GEOGRIDREINFORCEMENT

LAYERS

VOID

SOFT AND/OR COMPRESSIBLE/COLLAPSABLE

ZONE

ZONE OFINFLUENCE

EXISTING WASTE

ZONE

D i Ch ll d S l tiDesign Challenges and SolutionsBuffer Soil

LINER SYSTEM

ENGINEERED FILLFOUNDATION LAYERFOUNDATION LAYER

LIMITS OF DIFFERENTIALSETTLEMENT EFFECTS

WASTEVOID

Design Challenges and SolutionsDesign Challenges and SolutionsDeep Dynamic Compaction

EXISTING WASTESURFACE WASTE SYSTEM AFTER

DEEP DYNAMICCOMPACTION

SURFACE AFTER

EXISTING WASTE