Soil Testing and Soil Testing and Analysis for Analysis for

Waste Disposal Waste Disposal FacilitiesFacilities

A Presentation to the

November 17, 2005By

Dr. Andrew G. Heydinger Department of Civil Engineering

2

IntroductionIntroduction

BSCE University of CincinnatiMSCE University of PittsburghPh.D University of Houston

Experience as a consultant and with the U.S. Army Corps of Engineers and23 years at the University of Toledo

I consider myself a ‘Soils Mechanician.’

3

Purpose of PresentationPurpose of Presentation

• Discuss fundamental concepts pertaining to soil properties and behavior.

• Discuss details of soil testing and analysis of results.• Discuss geotechnical analysis for

waste management facilities.

4

Some Fundamental Some Fundamental ConceptsConcepts

5

The effective Stress The effective Stress ConceptConcept

Effective soil stress is defined as

' = – uw

where

' = effective stress

= total stress

uw = pore water pressure.

6

What Effective Stress Isn’tWhat Effective Stress Isn’t

Effective soil stress is not the

actual stress acting at the areas of

contact between soil particles.

7

What Effective Stress IsWhat Effective Stress Is

Effective soil stress corresponds to

the stress transmitted through the soil

mineral skeleton.

8

Why Effective Stress?Why Effective Stress?

Effective soil stress is a stress state

variable that is useful to characterize

behavior occurring in saturated soils

including volume change, permeability

and shear strength.

9

Pore Water PressurePore Water Pressure• Hydrostatic or geostatic pore water

pressure is the pore water pressure in soil due to geologic conditions.

• Excess pore water pressure is the pore water pressure that results when soil is loaded.

• Back pressure is the pore water pressure applied directly to soil specimens during laboratory testing.

10

Consolidation of Saturated SoilConsolidation of Saturated Soil

• When saturated soils are loaded, they develop excess pore water pressures that dissipate over time.

• As water flows from the soil the excess pore water pressures dissipate resulting in settlement.

• This process is referred to as primary consolidation.

11

Consolidation Stresses Consolidation Stresses

'vo = effective vertical overburden stress

'vc = maximum past consolidation stress in geologic history

'v = increase in vertical effective stress due to

loading

'vo + 'v = effective consolidation stress at end of

primary consolidation

OCR = 'vc / 'vo = overconsolidation ratio

12

Settlement and Settlement and Settlement RateSettlement Rate

• Results from plots of void ratio vs. log of effective stress and the

log of time are used to compute primary and secondary consolidation settlement.

• Results from plots of deformation vs. time are used to compute

consolidation rate.

13

Primary Consolidation SettlementPrimary Consolidation Settlement

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.1 1 10 100

Vertical Effective Stress, 'v

Vo

id R

ati

o,

e Cr

Cc

'vo'vc

'vo+'v

e

e

'

''

o

loglog1

1

vc

vcr

o

op

oo

vo

COCRCe

HS

He

eHS

14

Secondary Consolidation Secondary Consolidation SettlementSettlement

Slope = C

Log t

Secondary Consolidation

Primary Consolidation

Vo

id R

atio

, e

ep

et

tp t

pp

os t

tC

e

HS log

)1(

Figure 2 – Secondary Consolidation

15

Square Root of Time Square Root of Time MethodMethod

16

Log of Time MethodLog of Time Method0.0050

0.0150

0.0250

0.0350

0.0450

0.10 1.00 10.00 100.00 1000.00

Log of Time (min)

Def

orm

atio

n,

d (

cm)

d0

50

250197.0

t

Hc Dv

d100

t50

d50

17

ConsolidationConsolidation Rate Rate

• Dimensionless Time Factor, T

• Average Percent Consolidation, U0

20

40

60

80

100

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2

Tv

U (

%)

250D

vv H

tcT

Time for U% Consolidation, t

v

drv

c

HTt

2

18

Consolidation TheoryConsolidation Theory

• One assumption is that consolidation is one-dimensional.

• Therefore, consolidation settlement is computed assuming vertical

strain.

• The solution for consolidation rate is derived assuming vertical

porewater flow.

19

Total HeadTotal Head

• The energy potential for water is expressed in terms of total head, where total head is equal to the sum of the elevation head, he, and the pressure head, hp.

• Flow occurs because of differences in total head.

20

Total Head IllustratedTotal Head Illustrated

s he

hp

he

hp

Datum

ds dh

Figure 3 – Total Head and Total Head Gradient

21

Darcy’s Law for FlowDarcy’s Law for Flow

The flow law relating the dischargevelocity, v, to the driving potential (total head or hydraulic gradient).

where K (cm/sec) = permeability

= total head gradient.

ds

dhKv

ds

dhKv

22

Validity of Darcy’s LawValidity of Darcy’s Law

Laminar Flow Zone

Total Head Gradient

Transition Zone

Turbulent Flow Zone

1

K

Vel

ocity

, v

23

Soil Shear StrengthSoil Shear Strength

• Soil shear strength is the maximum shear stress that a soil can withstand.

• Soil shear strength is determined using Mohr-Coulomb shear strength parameters.

24

Mohr-Coulomb Failure EnvelopeMohr-Coulomb Failure Envelope

3f 3f1f3f1f 1f

Normal Stress, Figure 5 – Mohr-Coulomb Failure Envelope

Sh

ear

Str

ess,

c

= c + tan(

25

Failure ConditionsFailure Conditions• Unconsolidated Undrained (UU or Q)

– Failure that occurs rapidly during or shortly after construction.

• Consolidated Undrained (CU or R)– Failure that occurs rapidly after

the soil has had time to consolidate.

• Consolidated Drained (CD or S)– Failure that occurs slowly after the soil has had time to consolidate.

26

Comparison of Shear Comparison of Shear StrengthsStrengths

0

0.5

1

1.5

2

2.5

3

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Normal Stress

Sh

ea

r S

tre

ss

UU CU CD

27

Slope Stability – Method of Slope Stability – Method of SlicesSlices

28

Slope Stability AnalysisSlope Stability Analysis

• The many different analysis methods available differ by the assumptions that are made concerning the side forces and the equilibrium conditions that are used.

• Commercial software programs are capable of analyzing many trial surfaces under many different conditions.

29

Laboratory TestingLaboratory Testing

30

Consolidation TestConsolidation Test • A soil specimen placed in a rigid ring is

inundated in water in order to saturate the soil.

• Incremental load test - the total stress is increased by doubling the applied loads and vertical deformations are measured over time for each load increment.

Reference: ASTM 2435 - 90

31

Specimen SaturationSpecimen Saturation

• It is difficult to verify that the test specimen is saturated.

• The time rate of consolidation is very sensitive to degree of saturation.

• Small seating pressures should be applied to the specimen during

saturation to prevent swelling.

32

End of Primary ConsolidationEnd of Primary Consolidation

• Test results are dependent on the load duration.

• Load increments should be held until primary consolidation is completed.

• It is difficult to verify by measuring

pore water pressure that the time for primary consolidation is reached.

33

Load DurationLoad Duration

• Typically loads are applied for equal 24-hour periods and the load time behavior is evaluated to determine if primary consolidation is reached.

• If necessary, apply the loads in equal increments of mulitples of 24-hour periods.

34

Unload-Reload CyclesUnload-Reload Cycles

• It may be difficult to estimate the maximum past consolidation pressure or the recompression index because of sample disturbance or if the soil is overconsolidated.

• An unload-reload cycle can be applied to the soil to improve the results.

35

Deformation-Time Deformation-Time BehaviorBehavior

• The coefficient of consolidation, cv, is used to compute consolidation rate.

• C is used to compute the secondary consolidation settlement.

• For either calculation, it is necessary to select an appropriate stress range when selecting the coefficient.

36

Coefficient of Coefficient of ConsolidationConsolidation

0.50

0.52

0.54

0.56

0.58

0.60

0.62

0.64

0.66

0.68

0.70

0 0.2 0.4 0.6 0.8 1

Coefficient of Consolidation, (cm2/min)

Av

era

ge

Vo

id R

ati

o

Square Root of Time Fitting Method

Log of Time Fitting Method

37

Coefficient of PermeabilityCoefficient of Permeability

0.50

0.52

0.54

0.56

0.58

0.60

0.62

0.64

0.66

0.68

0.70

1.00E-08 5.10E-07 1.01E-06 1.51E-06 2.01E-06

Coefficient of Permeability, (cm/sec)

Ave

rag

e V

oid

Rat

io

Square Root of Time Fitting Method

Log of Time Fitting Method

v

cv

Ca

435.0

e

acK wvv

1

38

Permeability TestingPermeability Testing

• Falling head permeability tests are conducted on fine-grained soils using flexible wall permeameters.

• Triaxial compression cells are used instead of permeameters, in which the load piston is used to measure change in length of the test specimen.

Reference: ASTM D 5084 - 90

39

Triaxial CellTriaxial Cell

40

Apparatus CapabilitiesApparatus Capabilities

• Apply cell pressure to the cell fluid in order to apply total stress, 3.

• Apply pore water pressure (back pressure) to top and bottom of test specimen to saturate the specimen and to develop a total head gradient

for permeability testing.

41

Back Pressure SaturationBack Pressure Saturation

• Increase cell pressure in small increments, , to specimen and measure the change in pore water, u.

• Soil is assumed close to saturation if

• Saturation can require several days for fine-grained soils.

95.0

u

B

42

Required Back PressureRequired Back Pressure

Required Back Pressure

In

itia

l Deg

ree

of

Sat

ura

tio

n

43

11Recommened Maximum Recommened Maximum H.G. To Prevent Soil H.G. To Prevent Soil

DisturbanceDisturbance K (cm/s) Hydraulic Gradient

1x10-3 to 1x10-4 2

1x10-4 to 1x10-5 5

1x10-5 to 1x10-6 10

1x10-6 to 1x10-7 20

Less than 1x10-7 301ASTM D 5084 - 90

44

Check for Validity of Check for Validity of Darcy’s LawDarcy’s Law

• Measure permeability of soil at three hydraulic gradients.

• Values of permeability should be within about 25%.

45

Accuracy of Permeability Accuracy of Permeability MeasurementsMeasurements

• Soil permeability is very sensitive to any disturbance or stress change that would affect the soil skeleton.

• Great care should be taken when obtaining undisturbed specimens

and when preparing laboratory-compacted specimens.

46

Acceptable Zone for Acceptable Zone for Minimizing PermeabilityMinimizing Permeability

105.0

110.0

115.0

120.0

125.0

130.0

5 10 15 20 25Molding Water Content

Dry

Un

it W

eig

ht

Acceptable Zone

Zero Air Voids

Specified Range

Qian X., Koerner R.M. and Gray D.H. (2002), "Geotechnical Aspects of Landfill Design and Construction," Prentice Hall, Upper Sadle River, NY 07458.

47

Triaxial Compression Triaxial Compression TestsTests

• Back pressure saturation technique is used for consolidated tests.

• Effective consolidation pressure is equal to the cell pressure minus the applied back pressure.

• Load the specimens at prescribed rates for undrained and drained testing.

48

Unconsolidated Undrained Unconsolidated Undrained Triaxial CompressionTriaxial Compression

• Soil specimens are not back pressure saturated before testing.

• If test specimens are not saturated, then the compressive strength willdepend on the cell pressure, i.e. ≠ 0.

• It is necessary to test representative samples at representative total

stress.

Reference: ASTM D 2850 - 87

49

Consolidated Undrained Consolidated Undrained Triaxial CompressionTriaxial Compression

• Soil specimens are back pressure saturated, consolidated to a predetermined effective stress and loaded to failure undrained.

• It is possible to determine both total stress and effective stress

parameters by measuring pore water pressures.

Reference: ASTM D 2850 - 87

50

Total and Effective Stress Total and Effective Stress Parameters (CU)Parameters (CU)

'3f 3f'3f'1f1f3f '1f 1f

Normal Stress,

She

ar S

tres

s,

c

Uf Uf

c'

'

51

Comparison of Effective Comparison of Effective Stress ParametersStress Parameters

• The effective stress parameters obtained from the consolidated undrained and consolidated drained tests are not equal.

• It is assumed that c’ = 0 for the consolidated drained test.

• The consolidated undrained test can be loaded to failure in less time.

52

Other Shear TestsOther Shear Tests

• Shear strength parameters can be determined from direct shear tests or torsional ring shear tests.

• It is difficult to control drainage conditions with the direct shear apparatus.

• Torsional ring shear tests are not performed very often.

53

Geotechnical AnalysisGeotechnical Analysis

54

Consolidation SettlementConsolidation Settlement• Primary and secondary consolidation

settlement is computed assuming one- dimensional strain. • The stress increases under landfills are high. • Foundation soils that are suitable because of low permeability may undergo large settlements making it difficult to accurately predict differential settlements.

55

Differential SettlementDifferential Settlement

Differential Settlement Resulting in Negative Drainage

Positive Drainage

Negative Drainage

Positive Drainage

Negative Drainage

High Stress Increases

Low Stress Increases

Low Stress Increases

56

Stability of Excavations Stability of Excavations with Hydrostatic Upliftwith Hydrostatic Uplift

Factor of Safety for Stability of Excavations With Hydrostatic Uplift

W

h

U = h w B

F.S = W / U

B

57

Flow into an ExcavationFlow into an Excavation

Velocity Vectors Showing Flow into an Excavation

58

Hydraulic GradientsHydraulic Gradients

• For sites with artesian pressure, high hydraulic gradients develop which

will result in high seepage velocities if the depth of excavation is large.• Soil erosion occurs if the hydraulic gradients are high enough.• Generally the hydraulic gradient should be less than 1.

59

Flow Through Liner SystemsFlow Through Liner Systems

• Advection – Movement of leachate caused by hydraulic gradients.

• Diffusion – Movement of leachate

caused by concentration gradients.

• For low permeability liner systems, movement of leachate is governed by diffusion.

60

“ “One Foot of Head of LeachateOne Foot of Head of Leachate””

61

Stability Analysis for Stability Analysis for Drained ConditionsDrained Conditions

• Effective stress parameters are used to analyze slopes for long term

stability.

• Effective stress parameters obtained from consolidated undrained triaxial compression tests with pore water pressure measurements are used for drained conditions.

62

Stability Analysis for Stability Analysis for Construction Conditions Construction Conditions

• Total stress parameters are used to analyze slopes for conditions with

excess pore water pressure at the onset of loading or unloading.

• Total stress parameters are obtained from unconsolidated undrained triaxial compression tests.

63

Analysis for Unsaturated Analysis for Unsaturated Soils Soils

• Stability analysis for shallow failure mechanisms for unsaturated conditions.

• Allow for additional soil shear strength using a shear strength parameter

that accounts for the increase of shear strength due to soil matric suction.

64

Stability Analysis for Stability Analysis for Earthquake ConditionsEarthquake Conditions

• Slopes are analyzed for earthquakes using quasi-seismic analysis.

• Slopes are analyzed for drained and undrained conditons.

• Failures occuring during earthquakes experience undrained failures?

65

Limitations of Stability Limitations of Stability Analysis Analysis

• Slopes with adequate factors of safety should be safe from large mass

movements.

• The analyses, however, do not preclude against deformations within masses or along interfaces.

• Generally, there is less deformation if the factors of safety are higher.

66

ClosureClosure

• Fundamental Soil Mechanics concepts were presented.

• Details of soil testing were discussed.

• Various aspects of Geotechnical analysis for waste management facilities were discussed.

67

Thanks for Coming.

I hope that you enjoy your visit to the University of Toledo

You are all invited to our Banyas Soil Mechanics Laboratory in NI 1024 for some light refreshments.