Characterizing the Headcut Erodibility Index (Kh) 3) for Cohesive Soils PART 2 – SITES EARTH SPILLWAY EVALUATION B. Earth Spillway Integrity Analysis b. Characterization of geologic materials: iii. Characterizing the Headcut Erodibility Index (Kh) 3) for Cohesive Soils SPECIALTY WORKSHOP: SITES TRAINING AND INTRODUCTION TO WINDAM ASDSO Dam Safety 2008

Objectives State the common range of the Kh parameter for cohesive soils State the common range of the Kh parameter for cohesive soils Identify plot of rate of headcut migration versus Kh parameter and explain sensitivity of plot to values of Kh Identify plot of rate of headcut migration versus Kh parameter and explain sensitivity of plot to values of Kh Using example soil data, assign a value to the Kh parameter accurately Using example soil data, assign a value to the Kh parameter accurately Identify tests used to characterize soils for estimating the Kh parameter Identify tests used to characterize soils for estimating the Kh parameter

Phase III Erosion Process Phase 3 is the headcut advance that occurs after establishment in Phase II Where dx/dt = rate of headcut advance Phase 3 is the headcut advance that occurs after establishment in Phase II Where dx/dt = rate of headcut advance A = hydraulic attack A 0 = Attack threshold (no movement below value) C = Proportionality Coefficient = x ln(K h ) for K h 18.2

Other Terms in dx/dt equation for Phase III

Headcut - 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 - A o ) dX/dt = rate of headcut migration, C = material-dependent advance rate coefficient, A = hydraulic attack, and A o = material-dependent threshold.

Chapter 52, Appendix B First, determine if material is soil or rock First, determine if material is soil or rock ASTM D2488 – provides limited guidance. ASTM D2488 – provides limited guidance. Refer to Chapter 52 for Information on Rocks Refer to Chapter 52 for Information on Rocks

Typical values for the Kh Factor Hard Rock Soft or Jointed Rock Weathered Rock Soil

Background M s is the primary term that affects the value of Kh for soil materials. The other terms are more important for rock. An exception might be blocky clays. M s is the primary term that affects the value of Kh for soil materials. The other terms are more important for rock. An exception might be blocky clays.

Available Tools For Estimating Kh NEH 628, Chapter 52, Appendix B NEH 628, Chapter 52, Appendix B Soil Catalog on SITES CD Soil Catalog on SITES CD

Chapter 52, Appendix B - Soils If the PI greater than 10 – Cohesive If the PI greater than 10 – Cohesive If the PI is < 10, soil is Cohesionless If the PI is < 10, soil is Cohesionless

Headcut Migration The rate of headcut migration is very sensitive to values of the Kh factor in the lower ranges of the value. The rate of headcut migration is very sensitive to values of the Kh factor in the lower ranges of the value. Equations for headcut advance put into spreadsheet to evaluate sensitivity Equations for headcut advance put into spreadsheet to evaluate sensitivity

Demonstration of Influence of Kh on predicted rate of migration of headcut Examples developed from equations for headcut migration shown in Chapter 51, Part 628, NRCS Examples developed from equations for headcut migration shown in Chapter 51, Part 628, NRCS First example is for relatively low unit discharge of 20 cubic feet per second per foot width of spillway – Condition for critical depth of 2 feet First example is for relatively low unit discharge of 20 cubic feet per second per foot width of spillway – Condition for critical depth of 2 feet 2 feet

Soil Weathered Rock Poor Quality Rock 0.2

Available Tools For Estimating K h Spreadsheet Spreadsheet Uses soil properties from field and laboratory tests or table estimates Uses soil properties from field and laboratory tests or table estimates NEH 628, Chapter 52, Appendix B NEH 628, Chapter 52, Appendix B Soil Catalogue Soil Catalogue

Examples of Catalog Soils Catalogue Soils are on SITES CD Catalogue Soils are on SITES CD

Soil Catalog Existing catalog for soils (assuming that Kh value is < 0.2) is ten samples Existing catalog for soils (assuming that Kh value is < 0.2) is ten samples Ten soils in catalog have Kh values from 0.01 to Ten soils in catalog have Kh values from 0.01 to Catalog listings with Kh values of 0.2 and above are weathered shale – these have Kh values of 0.2 – 0.5. Five Listings Catalog listings with Kh values of 0.2 and above are weathered shale – these have Kh values of 0.2 – 0.5. Five Listings

Soil Catalogue Soil descriptions are very brief Soil descriptions are very brief Some wording confusing – example index value based on laboratory strength notation for nonplastic ML soil from Oklahoma Some wording confusing – example index value based on laboratory strength notation for nonplastic ML soil from Oklahoma Discussion on methods used for back- computation of K h value for soils Discussion on methods used for back- computation of K h value for soils

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, M s For Cohesive Soils For Cohesive Soils M s = 0.78 (UCS) 1.09, where UCS is unconfined compressive strength – Use Table 52.3 M s = 0.78 (UCS) 1.09, where UCS is unconfined compressive strength – Use Table 52.3 Needlessly complicated – use simpler methods (later) Needlessly complicated – use simpler methods (later)

Confusion on terms related to unconfined compressive strength The distinction is important because it is a factor of 2 The distinction is important because it is a factor of 2 P  ququ

Confusion on terms related to unconfined compressive strength When an unconfined compression test is performed in a laboratory, the value usually reported is the unconfined compressive stress applied to the sample at failure When an unconfined compression test is performed in a laboratory, the value usually reported is the unconfined compressive stress applied to the sample at failure When a field vane shear test is performed, the value reported is usually the shear strength of the soil ( 1/2 the q u strength). When a field vane shear test is performed, the value reported is usually the shear strength of the soil ( 1/2 the q u strength).

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

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

Table 52-3 Consistency Unconfined Compressive Strength (kPa) Unconfined Compressive Strength (psf) Ms Very soft< 40< 835< 0.02 Soft40 – , – 0.05 Firm80 – 1501,670 – 3, – 0.10 Stiff150 – 3003,131 – 6, – 0.20 Very stiff300 – 6256,265 – 13, – 0.45

Relation of consistency to unconfined compressive strength Table 5 in Chapter 52 shows different range of unconfined strengths for consistency than other standard geotechnical references Table 5 in Chapter 52 shows different range of unconfined strengths for consistency than other standard geotechnical references See Table from Terzaghi as Example See Table from Terzaghi as Example

Table 52-3 ConsistencyTable 52-3 Unconfined Compressive Strength (psf) Terzaghi Unconfined Compressive Strength (psf) Very soft< 835< 500 Soft835-1, – 1,000 Firm1,670 – 3,1311,000 – 2,000 Stiff3,131 – 6,2652,000 – 4,000 Very stiff6,265 – 13,0534,000 – 8,000

Confusion on units related to unconfined compressive strength Table 52-3 employs units of kilopascals (kPa) while Table 5 in Appendix B uses units of mega Pascals (mPa). Table 52-3 employs units of kilopascals (kPa) while Table 5 in Appendix B uses units of mega Pascals (mPa). While this is only a conversion factor of 1,000, confusion could still result, particularly when laboratories report results in pounds per square foot or pounds per square inch While this is only a conversion factor of 1,000, confusion could still result, particularly when laboratories report results in pounds per square foot or pounds per square inch

Undrained Shear Strength Only applicable for quick loading of slowly permeable saturated soils Only applicable for quick loading of slowly permeable saturated soils

Undrained Shear Strength Mistake to perform tests on more permeable material such as silts Mistake to perform tests on more permeable material such as silts Unrealistically high strengths on these materials from negative pore pressures during load application Unrealistically high strengths on these materials from negative pore pressures during load application

2,000 psf Undrained Strength, c = q u /2, psf Very Stiff Clays with PI > 12, Overconsolidated – Undrained Shear Strength > 2,000 psf Undrained Shear Strength Scale, after Terzaghi and Peck, 2 nd Edition, page 30

2,000 psf Undrained Strength, c = q u /2, psf Stiff Clays with PI > 12 overconsolidated Undrained Shear Strength = 1, ,000 psf ,000 psf

Undrained Shear Strength Scale, after Terzaghi and Peck, 2 nd Edition, page psf Undrained Strength, c = q u /2, psf Medium Clays with PI > 12, Slightly Overconsolidated – Undrained Shear Strength = 500-1,000 psf ,000 psf

Undrained Shear Strength Scale, after Terzaghi and Peck, 2 nd Edition, page psf Undrained Strength, c = q u /2, psf Soft Clays with PI > 12 Normally Consolidated Undrained Shear Strength = psf psf

Undrained Shear Strength Scale, after Terzaghi and Peck, 2 nd Edition, page 30 Undrained Strength, c = q u /2, psf Very Soft Clays with PI > 12, Under Consolidated Undrained Shear Strength < 250 psf psf

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

Torvane measures Undrained Shear Strength

Pocket Penetrometer measures unconfined compressive strength (qu)

 3 = 0 Load Strain rate = 1- 2%/min. Unconfined compression test

 Unconfined compression test Unconfined compression test s u = “undrained shear strength” (c value) s u = “undrained shear strength” (c value) q u = “unconfined compressive strength” q u = “unconfined compressive strength”  failure = q u  3 = 0 c = s u  = c = s u = ½*q u 

Factors in erodibility of overconsolidated clay soils What is importance of blocky structure and slickensides? What is importance of blocky structure and slickensides? Is there available case history data? Is there available case history data?

Factors in erodibility of overconsolidated clay soils What is importance of blocky structure and slickensides? Is there available case history data?

Factors in erodibility of overconsolidated clay soils Not all overconsolidated clays may have open blocky structure Not all overconsolidated clays may have open blocky structure Example is chalks from Alabama and Mississippi Example is chalks from Alabama and Mississippi Index properties of these weathered chalks may be similar to those of overconsolidated clays with fissured blocky structure Index properties of these weathered chalks may be similar to those of overconsolidated clays with fissured blocky structure How do you account for difference How do you account for difference

Consistency Very Soft Very Soft Soft Soft Medium Medium Stiff Stiff V/Stiff to Hard V/Stiff to Hard Field Estimate of Consistency Rule of Thumb Thumb will penetrate soil more than 1- inch. Extrudes between fingers when squeezed in fist Rule of Thumb Thumb will penetrate soil more than 1- inch. Extrudes between fingers when squeezed in fist

Field Estimate of Consistency Consistency Consistency Very Soft Very Soft Soft Soft Medium Medium Stiff Stiff V/Stiff to Hard V/Stiff to Hard Consistency Consistency Very Soft Very Soft Soft Soft Medium Medium Stiff Stiff V/Stiff to Hard V/Stiff to Hard Rule of Thumb Thumb will penetrate soil about 1-inch. Easily molded in fingers Rule of Thumb Thumb will penetrate soil about 1-inch. Easily molded in fingers

Field Estimate of Consistency Consistency Consistency Very Soft Very Soft Soft Soft Medium Medium Stiff Stiff V/Stiff to Hard V/Stiff to Hard Consistency Consistency Very Soft Very Soft Soft Soft Medium Medium Stiff Stiff V/Stiff to Hard V/Stiff to Hard Rule of Thumb Thumb will not penetrate soil, but will indent about 1/4 inch. Molded by finger pressure

Field Estimate of Consistency Consistency Very Soft SoftMediumStiff V/Stiff to Hard Rule of Thumb Thumb will not indent soil, but soil can be indented with thumbnail. Rule of Thumb Thumb will not indent soil, but soil can be indented with thumbnail.

Field Estimate of Consistency Consistency Consistency Very Soft Very Soft Soft Soft Medium Medium Stiff Stiff V/Stiff to Hard V/Stiff to Hard Consistency Consistency Very Soft Very Soft Soft Soft Medium Medium Stiff Stiff V/Stiff to Hard V/Stiff to Hard Rule of Thumb Can only be marked with knife - not indented with thumbnail.

Summary The rate of upstream migration of a headcut predicted by the SITES program is extremely sensitive in the range of values of the Kh parameter which are typical for soils The rate of upstream migration of a headcut predicted by the SITES program is extremely sensitive in the range of values of the Kh parameter which are typical for soils Cohesive soils have typical values of Kh between 0.05 and 0.2 Cohesive soils have typical values of Kh between 0.05 and 0.2