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
Undrained Shear Strength Scale, after Terzaghi and Peck, 2nd Edition, page 30
2,000 psf
Undrained Strength, c = qu/2, 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
Undrained Shear Strength Scale, after Terzaghi and Peck, 2nd Edition, page 30
500psf
Undrained Strength, c = qu/2, psf
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
Undrained Shear Strength Scale, after Terzaghi and Peck, 2nd Edition, page 30
500psf
Undrained Strength, c = qu/2, psf
Soft Clays with PI > 12Normally Consolidated
Undrained Shear Strength = 250-500 psf
0 0.33
250psf
Undrained Shear Strength Scale, after Terzaghi and Peck, 2nd Edition, page 30
Undrained Strength, c = qu/2, psf
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?
Factors in erodibility of Factors in erodibility of overconsolidated clay soilsoverconsolidated clay soils
What is importance of blocky structure and slickensides?
Is there available case history data?
Factors in erodibility of Factors in erodibility of overconsolidated clay soilsoverconsolidated clay soils
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
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
Field Estimate of ConsistencyField Estimate of Consistency
ConsistencyConsistency Very SoftVery Soft SoftSoft MediumMedium StiffStiff V/Stiff to HardV/Stiff to Hard
ConsistencyConsistency Very SoftVery Soft SoftSoft MediumMedium StiffStiff V/Stiff to HardV/Stiff to Hard
Rule of ThumbRule of Thumb
Thumb will Thumb will penetrate soil penetrate soil about 1-inch. about 1-inch. Easily molded in Easily molded in fingersfingers
Rule of ThumbRule of Thumb
Thumb will Thumb will penetrate soil penetrate soil about 1-inch. about 1-inch. Easily molded in Easily molded in fingersfingers
Field Estimate of ConsistencyField Estimate of Consistency
ConsistencyConsistency Very SoftVery Soft SoftSoft MediumMedium StiffStiff V/Stiff to HardV/Stiff to Hard
ConsistencyConsistency Very SoftVery Soft SoftSoft MediumMedium StiffStiff V/Stiff to HardV/Stiff to Hard
Rule of ThumbRule of Thumb
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
Field Estimate of ConsistencyField Estimate of Consistency
ConsistencyConsistency
Very SoftVery Soft
SoftSoft
MediumMedium
StiffStiff
V/Stiff to HardV/Stiff to Hard
Rule of ThumbRule of Thumb
Thumb will not Thumb will not indent soil, but indent soil, but soil can be soil can be indented with indented with thumbnail. thumbnail.
Rule of ThumbRule of Thumb
Thumb will not Thumb will not indent soil, but indent soil, but soil can be soil can be indented with indented with thumbnail. thumbnail.
Field Estimate of ConsistencyField Estimate of Consistency
ConsistencyConsistency Very SoftVery Soft SoftSoft MediumMedium StiffStiff V/Stiff to HardV/Stiff to Hard
ConsistencyConsistency Very SoftVery Soft SoftSoft MediumMedium StiffStiff V/Stiff to HardV/Stiff to Hard
Rule of ThumbRule of Thumb
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
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