Characterizing the Headcut Erodibility Index (Kh) 3) for Cohesive Soils PART 2 – SITES EARTH...

PART 2 – SITES EARTH SPILLWAY EVALUATION

B. Earth Spillway Integrity Analysisb. Characterization of geologic materials:

iii. Characterizing the Headcut Erodibility Index (Kh)Characterizing the Headcut Erodibility Index (Kh)3) for Cohesive Soils3) for Cohesive Soils

PART 2 – SITES EARTH SPILLWAY EVALUATION

B. Earth Spillway Integrity Analysisb. Characterization of geologic materials:

iii. Characterizing the Headcut Erodibility Index (Kh)Characterizing the Headcut Erodibility Index (Kh)3) for Cohesive Soils3) for Cohesive Soils

SPECIALTY WORKSHOP: SPECIALTY WORKSHOP: SITES TRAINING AND SITES TRAINING AND

INTRODUCTION TO WINDAM INTRODUCTION TO WINDAM

ASDSO Dam Safety 2008ASDSO Dam Safety 2008

ObjectivesObjectives State the common range of the Kh State the common range of the Kh

parameter for cohesive soilsparameter for cohesive soils Identify plot of rate of headcut migration Identify plot of rate of headcut migration

versus Kh parameter and explain versus Kh parameter and explain sensitivity of plot to values of Khsensitivity of plot to values of Kh

Using example soil data, assign a value to Using example soil data, assign a value to the Kh parameter accuratelythe Kh parameter accurately

Identify tests used to characterize soils for Identify tests used to characterize soils for estimating the Kh parameterestimating the Kh parameter

Phase III Erosion ProcessPhase III Erosion Process Phase 3 is the headcut advance that occurs after Phase 3 is the headcut advance that occurs after

establishment in Phase IIestablishment in Phase II

Where dx/dt = rate of headcut advanceWhere dx/dt = rate of headcut advance

A = hydraulic attackA = hydraulic attackAA00 = Attack threshold (no movement below value) = Attack threshold (no movement below value)C = Proportionality Coefficient C = Proportionality Coefficient

= -0.79 x ln(K= -0.79 x ln(Khh) + 3.04 for K) + 3.04 for Khh < 18.2 and < 18.2 and C = 0.75 for Kh > 18.2C = 0.75 for Kh > 18.2

0AACdt

dx

Other Terms in dx/dt equation for Other Terms in dx/dt equation for Phase IIIPhase III

31

qHA

31

21

0 101

233189

hh KKA

ln(

.exp

Headcut - classicalHeadcut - classical

EXPANDING HEADCUT

Rate of migration of headcut primarily a function of soil Kh

value

HEADCUT MIGRATION RATE

dX/dt

HEADCUT MIGRATION

dx/dt = C (A - Adx/dt = C (A - Aoo))

dX/dt = rate of headcut migration,

C = material-dependent advance rate coefficient,

A = hydraulic attack, and

Ao = material-dependent threshold.

Headcut Advance Rate Versus Kh

0

10

20

30

40

50

60

70

80

0.00 2.00 4.00 6.00 8.00 10.00

Kh Value

Ad

van

ce R

ate

of H

ead

cut,

feet

per

ho

ur

Unit Dischargecfs per foot = 100

Overfall Height (foot) = 12

Chapter 52, Appendix BChapter 52, Appendix B

First, determine if material is soil First, determine if material is soil or rockor rock

ASTM D2488 – provides limited ASTM D2488 – provides limited guidance.guidance.

Refer to Chapter 52 for Refer to Chapter 52 for Information on RocksInformation on Rocks

Typical values for the KhTypical values for the Kh FactorFactor

0.2

0.01

0.5

10

Hard Rock

Soft or Jointed Rock

Weathered Rock

Soil

BackgroundBackground

a

rs

nsh J

JJ

J

RQDMK

MMss is the primary term that affects is the primary term that affects

the value of Kh for soil materials. the value of Kh for soil materials.

The other terms are more The other terms are more

important for rock. An exception important for rock. An exception

might be blocky clays.might be blocky clays.

Available Tools For Estimating Available Tools For Estimating KhKh

NEH 628, Chapter 52, NEH 628, Chapter 52, Appendix BAppendix B

Soil Catalog on SITES CDSoil Catalog on SITES CD

Chapter 52, Appendix B - SoilsChapter 52, Appendix B - Soils

If the PI greater than 10 – If the PI greater than 10 –

CohesiveCohesive

If the PI is If the PI is << 10, soil is 10, soil is

CohesionlessCohesionless

Headcut MigrationHeadcut Migration

The rate of headcut migration is The rate of headcut migration is very sensitive to values of the very sensitive to values of the Kh factor in the lower ranges of Kh factor in the lower ranges of the value.the value.

Equations for headcut advance Equations for headcut advance put into spreadsheet to put into spreadsheet to evaluate sensitivityevaluate sensitivity

Demonstration of Influence of Kh on Demonstration of Influence of Kh on predicted rate of migration of headcutpredicted rate of migration of headcut

Examples developed from equations for Examples developed from equations for

headcut migration shown in Chapter 51, headcut migration shown in Chapter 51,

Part 628, NRCSPart 628, NRCS First example is for relatively low unit First example is for relatively low unit

discharge of 20 cubic feet per second per discharge of 20 cubic feet per second per

foot width of spillway – Condition for foot width of spillway – Condition for

critical depth of 2 feetcritical depth of 2 feet 2 feet

Headcut Advance Rate Versus Kh

0

5

10

15

20

25

30

35

40

0.0 0.5 1.0 1.5 2.0

Kh Value

Ad

va

nc

e R

ate

of

He

ad

cu

t, f

ee

t p

er

ho

ur

Unit Dischargecfs per foot = 20

Overfall Height (foot) = 8Soi

l

Wea

ther

ed

Roc

k

Poor Quality Rock

0.2

Available Tools ForAvailable Tools For Estimating K Estimating Khh

SpreadsheetSpreadsheet Uses soil properties from field and Uses soil properties from field and

laboratory tests or table estimateslaboratory tests or table estimates NEH 628, Chapter 52, Appendix NEH 628, Chapter 52, Appendix

BB Soil CatalogueSoil Catalogue

Examples of Catalog SoilsExamples of Catalog Soils

Catalogue Soils are on SITES CDCatalogue Soils are on SITES CD

Soil CatalogSoil Catalog

Existing catalog for soils (assuming Existing catalog for soils (assuming that Kh value is that Kh value is << 0.2) is ten samples 0.2) is ten samples

Ten soils in catalog have Kh values Ten soils in catalog have Kh values from 0.01 to 0.17.from 0.01 to 0.17.

Catalog listings with Kh values of 0.2 Catalog listings with Kh values of 0.2 and above are weathered shale – and above are weathered shale – these have Kh values of 0.2 – 0.5. these have Kh values of 0.2 – 0.5. Five ListingsFive Listings

Soil CatalogueSoil Catalogue

Soil descriptions are very briefSoil descriptions are very brief Some wording confusing – example Some wording confusing – example

index value based on laboratory index value based on laboratory strengthstrength notation for nonplastic ML notation for nonplastic ML soil from Oklahomasoil from Oklahoma

Discussion on methods used for Discussion on methods used for back-computation of Kback-computation of Khh value for value for soilssoils

No soils in range of 0.05 to 0.10

No soils in range of 0.12 to 0.16

Kh – 0.05

Twin Caney 17-34, KS

CL, firm, soil fill;headcut

Kh – 0.05

East Fork Pond River 7B, KY

Debris fill; spillway exit – (probably CL)

Kh – 0.10

East Fork Pond River 9A, KY

CL, firm, soil cover in headcut; upper material only considered in Kh determination

Kh – 0.16

Misteguay 4, MI

CL, stiff, glacial till; headcut

Material Strength Number, MMaterial Strength Number, Mss

For Cohesive SoilsFor Cohesive Soils MMss = 0.78 (UCS) = 0.78 (UCS)1.091.09, where UCS is , where UCS is

unconfined compressive strength unconfined compressive strength – Use Table 52.3– Use Table 52.3

Needlessly complicated – use Needlessly complicated – use simpler methods (later)simpler methods (later)

Confusion on terms related to Confusion on terms related to unconfined compressive strengthunconfined compressive strength

The distinction is important because it is a The distinction is important because it is a factor of 2factor of 2

1AP

qu

P

2uq

qu

Confusion on terms related to Confusion on terms related to unconfined compressive strengthunconfined compressive strength When an unconfined compression When an unconfined compression

test is performed in a laboratory, test is performed in a laboratory, the value usually reported is the the value usually reported is the unconfined compressive stress unconfined compressive stress applied to the sample at failureapplied to the sample at failure

When a field vane shear test is When a field vane shear test is performed, the value reported is performed, the value reported is usually the shear strength of the usually the shear strength of the soil (soil (1/21/2 the q the quu strength). strength).

Conditions for unconfined Conditions for unconfined compressive strengthcompressive strength

Should unconfined compression tests Should unconfined compression tests be routinely performed at the insitu be routinely performed at the insitu water content or should the samples water content or should the samples be saturated prior to testing? Values be saturated prior to testing? Values will depend strongly on this factor. will depend strongly on this factor. The unconfined strength of a The unconfined strength of a saturated sample could be ½ that of a saturated sample could be ½ that of a sample tested at 90 percent of sample tested at 90 percent of saturation. saturation.

Conditions for unconfined Conditions for unconfined compressive strengthcompressive strength

Unconfined compression tests Unconfined compression tests are inadvisably performed on are inadvisably performed on low PI soils. High strengths may low PI soils. High strengths may be measured from negative pore be measured from negative pore pressures that occur during the pressures that occur during the test. Soils with PI’s less than 15 test. Soils with PI’s less than 15 should be tested in a submersed should be tested in a submersed condition to avoid this problemcondition to avoid this problem

Table 52-3Table 52-3

Consistency Unconfined Compressive

Strength (kPa)

Unconfined Compressive Strength (psf)

Ms

Very soft < 40 < 835 < 0.02

Soft 40 – 80 835-1,670 0.02 – 0.05

Firm 80 – 150 1,670 – 3,131 0.05 – 0.10

Stiff 150 – 300 3,131 – 6,265 0.10 – 0.20

Very stiff 300 – 625 6,265 – 13,053 0.20 – 0.45

Relation of consistency to Relation of consistency to unconfined compressive strengthunconfined compressive strength

Table 5 in Chapter 52 shows Table 5 in Chapter 52 shows different range of unconfined different range of unconfined strengths for consistency than strengths for consistency than other standard geotechnical other standard geotechnical referencesreferences

See Table from Terzaghi as See Table from Terzaghi as ExampleExample

Table 52-3Table 52-3

Consistency Table 52-3 Unconfined

Compressive Strength (psf)

TerzaghiUnconfined

Compressive Strength (psf)

Very soft < 835 < 500

Soft 835-1,670 500 – 1,000

Firm 1,670 – 3,131 1,000 – 2,000

Stiff 3,131 – 6,265 2,000 – 4,000

Very stiff 6,265 – 13,053 4,000 – 8,000

Confusion on units related to Confusion on units related to unconfined compressive strengthunconfined compressive strength Table 52-3 employs units of kilopascals Table 52-3 employs units of kilopascals

(kPa) while Table 5 in Appendix B uses (kPa) while Table 5 in Appendix B uses

units of mega Pascals (mPa). units of mega Pascals (mPa).

While this is only a conversion factor of While this is only a conversion factor of

1,000, confusion could still result, 1,000, confusion could still result,

particularly when laboratories report particularly when laboratories report

results in pounds per square foot or results in pounds per square foot or

pounds per square inchpounds per square inch

Undrained Shear StrengthUndrained Shear Strength

Only applicable for quick Only applicable for quick loading of slowly permeable loading of slowly permeable saturated soilssaturated soils

Undrained Shear Undrained Shear StrengthStrength

Mistake to perform tests on Mistake to perform tests on more permeable material such more permeable material such as siltsas silts

Unrealistically high strengths on Unrealistically high strengths on these materials from negative these materials from negative pore pressures during load pore pressures during load applicationapplication

2,000 psf

Undrained Strength, c = qu/2, psf

Very Stiff Clays with PI > 12, Overconsolidated – Undrained Shear Strength > 2,000 psf

0 0.33 0.67 1.0

Undrained Shear Strength Scale, after Terzaghi and Peck, 2nd Edition, page 30


2,000 psf


Stiff Clays with PI > 12 overconsolidated Undrained Shear Strength = 1,000 - 2,000 psf

0 0.33 0.67 1.0

1,000 psf


500psf


Medium Clays with PI > 12, Slightly Overconsolidated –

Undrained Shear Strength = 500-1,000 psf

0 0.33 0.67 1.0

1,000 psf

1.00.67


500psf


Soft Clays with PI > 12Normally Consolidated

Undrained Shear Strength = 250-500 psf

0 0.33

250psf



Very Soft Clays with PI > 12, Under Consolidated Undrained Shear Strength < 250 psf

0 0.33 0.67 1.0

250psf

Distinguish between Distinguish between Unconfined Unconfined CompressiveCompressive Strength (qu) and Strength (qu) and Undrained ShearUndrained Shear Strength (su or c) Strength (su or c)

2u

uq

cS

Torvane measures Undrained Torvane measures Undrained ShearShear Strength Strength

2u

uq

cS

Pocket Penetrometer measures Pocket Penetrometer measures unconfined compressive strength unconfined compressive strength

(qu)(qu)

2u

uq

cS

3 = 0

Load

Strain rate = 1-2%/min.

Unconfined compression testUnconfined compression test

1

1 ALoad

qu

2uqc

2u

uq

cS

Unconfined compression test Unconfined compression test ssuu = “undrained shear strength” (c = “undrained shear strength” (c

value)value) qquu = “unconfined compressive strength” = “unconfined compressive strength”

1 @ failure = q u3 = 0

c = su

= c = su = ½*qu

Factors in erodibility of Factors in erodibility of overconsolidated clay soilsoverconsolidated clay soils

What is importance What is importance of blocky structure of blocky structure and slickensides?and slickensides?

Is there available Is there available case history data?case history data?


What is importance of blocky structure and slickensides?

Is there available case history data?


Not all overconsolidated clays may Not all overconsolidated clays may have open blocky structurehave open blocky structure

Example is chalks from Alabama and Example is chalks from Alabama and MississippiMississippi

Index properties of these weathered Index properties of these weathered chalks may be similar to those of chalks may be similar to those of overconsolidated clays with fissured overconsolidated clays with fissured blocky structureblocky structure

How do you account for differenceHow do you account for difference

ConsistencyConsistency Very SoftVery Soft SoftSoft MediumMedium StiffStiff V/Stiff to V/Stiff to

HardHard

Field Estimate of ConsistencyField Estimate of Consistency

Rule of ThumbRule of Thumb

Thumb will Thumb will penetrate soil penetrate soil more than 1-more than 1-inch. Extrudes inch. Extrudes between fingers between fingers when squeezed when squeezed in fistin fist


Thumb will Thumb will penetrate soil penetrate soil more than 1-more than 1-inch. Extrudes inch. Extrudes between fingers between fingers when squeezed when squeezed in fistin fist


ConsistencyConsistency Very SoftVery Soft SoftSoft MediumMedium StiffStiff V/Stiff to HardV/Stiff to Hard



Thumb will Thumb will penetrate soil penetrate soil about 1-inch. about 1-inch. Easily molded in Easily molded in fingersfingers


Thumb will Thumb will penetrate soil penetrate soil about 1-inch. about 1-inch. Easily molded in Easily molded in fingersfingers





Thumb will not Thumb will not penetrate soil, penetrate soil, but will indent but will indent about 1/4 inch. about 1/4 inch. Molded by finger Molded by finger pressurepressure


ConsistencyConsistency

Very SoftVery Soft

SoftSoft

MediumMedium

StiffStiff

V/Stiff to HardV/Stiff to Hard


Thumb will not Thumb will not indent soil, but indent soil, but soil can be soil can be indented with indented with thumbnail. thumbnail.


Thumb will not Thumb will not indent soil, but indent soil, but soil can be soil can be indented with indented with thumbnail. thumbnail.





Can only be Can only be marked with marked with knife - not knife - not indented with indented with thumbnailthumbnail. .

SummarySummary

The rate of upstream migration of a The rate of upstream migration of a headcut predicted by the SITES headcut predicted by the SITES program is extremely sensitive in the program is extremely sensitive in the range of values of the Kh parameter range of values of the Kh parameter which are typical for soilswhich are typical for soils

Cohesive soils have typical values of Cohesive soils have typical values of Kh between 0.05 and 0.2Kh between 0.05 and 0.2

Characterizing the Headcut Erodibility Index (Kh) 3) for Cohesive Soils PART 2 – SITES EARTH...

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Transcript of Characterizing the Headcut Erodibility Index (Kh) 3) for Cohesive Soils PART 2 – SITES EARTH...