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Publication Number:  FHWA-HRT-15-034    Date:  June 2015
Publication Number: FHWA-HRT-15-034
Date: June 2015

 

Strength Characterization of Open-Graded Aggregates for Structural Backfills

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FOREWORD

State and local transportation agencies frequently use crushed, manufactured open-graded aggregates (OGA) as structural backfill material for retaining walls, bridge foundations, and other ground improvement applications, yet their strength characteristics are not fully understood or applied. The friction angle is required to efficiently design for lateral earth pressures and bearing capacity, but because of the large size of the standard American Association of State Highway and Transportation Officials (AASHTO) OGAs, this parameter cannot be measured with standard testing equipment. Instead, current practice is to select a low, default friction angle, which can lead to over-conservative, less cost-effective designs. To address this gap, research was initiated by the Federal Highway Administration's Turner-Fairbank Highway Research Center (TFHRC) to establish a knowledge base on the most commonly used AASHTO OGAs. The study included a systematic approach to fully characterize the strength parameters utilizing the large scale direct shear and triaxial devices in the TFHRC geotechnical laboratory. Relationships between other important soil parameters on the friction angle, as well as the impact of different automated testing devices and data interpretation methods, were also investigated.  This report presents the results of this research and will assist State and local transportation agencies, researchers, and design consultants in gaining confidence on the use of higher friction angles for structural backfills to optimize the design of geotechnical features.

Jorge E. Pagán-Ortiz
Director, Office of Infrastructure
Research and Development

Notice

This document is disseminated under the sponsorship of the U.S. Department of Transportation in the interest of information exchange. The U.S. Government assumes no liability for the use of the information contained in this document. This report does not constitute a standard, specification, or regulation.

The U.S. Government does not endorse products or manufacturers. Trademarks or manufacturers' names appear in this report only because they are considered essential to the objective of the document.

Quality Assurance Statement

The Federal Highway Administration (FHWA) provides high-quality information to serve Government, industry, and the public in a manner that promotes public understanding. Standards and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its information. FHWA periodically reviews quality issues and adjusts its programs and processes to ensure continuous quality improvement.

 

Technical Report Documentation Page

1. Report No.

FHWA-HRT-15-034

2. Government Accession No. 3 Recipient's Catalog No.
4. Title and Subtitle

Strength Characterization of Open-Graded Aggregates for Structural Backfills

5. Report Date

June 2015

6. Performing Organization Code

HRDI-40

7. Author(s)

Nicks, J.E., Gebrenegus, T., Adams, M.T.

8. Performing Organization Report No.

 

9. Performing Organization Name and Address

Turner-Fairbank Highway Research Center
6300 Georgetown Pike
McLean, VA 22101

10. Work Unit No. (TRAIS)

11. Contract or Grant No.

N/A

12. Sponsoring Agency Name and Address

Office of Infrastructure R&D
FHWA Research, Development and Technology
6300 Georgetown Pike
McLean, VA 22101

13. Type of Report and Period Covered

Technical

14. Sponsoring Agency Code

 

15. Supplementary Notes

None.

16. Abstract

Open-graded aggregates are common in road and bridge construction because they are easy to place, and they have the advantages of very low fine content, free-draining characteristics, low frost heave potential, and simple quality assurance testing. They are a suitable alternative to well-graded aggregate blends in many applications. Another key benefit is their strength, but this attribute it is often not accounted for in design. This report presents strength characteristics of 16 open-graded aggregates commonly selected as structural backfills and discusses the impact of different test and data interpretation methods. Results of large-scale direct shear and large diameter triaxial tests indicate higher strengths for these materials than typical default values typically assumed in design. It was also observed that the mean grain size, sphericity, angularity, and void ratio play a role in the measured friction angles. This information will help designers better understand these backfills and aid in the cost-effective design of retaining walls and bridge foundations.

17. Key Words

Friction angle, strength, open-graded aggregate, structural backfill, retaining wall, bridge foundation

18. Distribution Statement

No restrictions. The report will be available to the public at FHWA: www.fhwa.dot.gov/research or NTIS: www.ntis.gov.

19. Security Classification
(of this report)

Unclassified

20. Security Classification
(of this page)

Unclassified

21. No. of Pages

149

22. Price

N/A

Form DOT F 1700.7 Reproduction of completed page authorized

SI* (Modern Metric) Conversion Factors

TABLE OF CONTENTS

LIST OF FIGURES

Figure 1. Chart. Effect of porosity and compaction on the shear strength of granular materials (modified from Rowe, 1962)
Figure 2. Chart. The theoretical determination of the drained shear strength for sands based on the three components that comprise the mobilized friction angle (after Lee and Seed, 1967)
Figure 3. Chart. Secant (symbol for friction angle's) and tangent (symbol for friction angle't) friction angle illustration for DS testing
Figure 4. Graph. Sieve analysis results of tested aggregates
Figure 5. Photo. Mold (0.1 ft3) for funnel and vibratory table tests
Figure 6. Photo. LVDT and mold set-up for vibratory table tests
Figure 7. Photo. Repose angle cylinder test
Figure 8. Photo. Aggregate pile to determine repose angle
Figure 9. Chart. Relationship between repose angle and mean aggregate size
Figure 10. Diagram. AIMS angularity index classification ranges
Figure 11. Diagram. AIMS texture classification ranges
Figure 12. Diagram. AIMS sphericity index classification ranges
Figure 13. Chart. Angularity results from AIMS2
Figure 14. Chart. Texture results from AIMS2
Figure 15. Chart. Sphericity results from AIMS2
Figure 16. Photo. SDS test device at TFHRC
Figure 17. Chart. Shear stress versus horizontal displacement for No. 57 from SDS tests
Figure 18. Chart. Shear stress versus horizontal displacement for No. 5 from SDS tests
Figure 19. Chart. Shear versus normal stress for No. 57 using SDS
Figure 20. Chart. Deformation behavior of No. 57 during SDS testing
Figure 21. Chart. Friction angle versus dilation angle for No. 57 using SDS
Figure 22. Chart. Relationship between friction and dilation angles in SDS testing.
Figure 23. Chart. Summary of SDS testing
Figure 24. Photo. LSDS device at TFHRC
Figure 25. Chart. Vertical displacement calibration for LSDS device
Figure 26. Chart. Global MC envelope for all AASHTO OGAs tested under both dry and saturated conditions
Figure 27. Chart. Relationship between friction and dilation angles in LSDS testing.
Figure 28. Chart. Summary of LSDS testing under dry conditions
Figure 29. Chart. Summary of LSDS testing under saturated conditions
Figure 30. Chart. Comparison of the effective tangent and secant friction angles for LSDS testing
Figure 31. Diagram. Overview of the adopted procedure for LDTX testing
Figure 32. Chart. The effective deviator stress versus axial strain for No. 57 from LDTX testing
Figure 33. Chart. Volumetric strain versus axial strain for No. 57 from LDTX testing
Figure 34. Chart. Graphical method to determine membrane penetration correction and the effect of aggregate size on volume reduction during LDTX testing
Figure 35. Chart. Effect of confining stress on volume reduction for AASHTO No. 68 sample during LDTX testing
Figure 36. Chart. Relationship between p' and q values in LDTX testing series
Figure 37. Chart. Relationship between friction and dilation angles in LDTX testing.
Figure 38. Chart. Summary of LDTX testing
Figure 39. Chart. Comparison of the effective tangent and secant friction angles for LDTX testing
Figure 40. Chart. Difference between LSDS and LDTX as a function of mean aggregate size
Figure 41. Chart. Relationship between LSDS and LDTX friction angles
Figure 42. Chart. Relationship between LSDS and LDTX effective secant friction angles
Figure 43. Chart. Relationship between LDTX and LSDS measured dilation angles
Figure 44. Chart. Relationship between LSDS and SDS measured friction angles
Figure 45. Chart. Relationship between LSDS and SDS measured dilation angles
Figure 46. Chart. Relationship between measured tangent and CV friction angles.
Figure 47. Chart. Measured friction angles under dry and saturated LSDS testing using the MC approach
Figure 48. Chart. Measured friction angles under dry and saturated LSDS testing using the ZDA approach
Figure 49. Chart. Relationship between measured friction angles under dry and saturated conditions in the LSDS device
Figure 50. Chart. Relationship between tangent friction angle and median grain size
Figure 51. Chart. Relationship between CV friction angle and median grain size
Figure 52. Chart. Relationship between secant friction angle and median grain size for LDTX testing
Figure 53. Chart. Relationship between tangent friction angle and Cu
Figure 54. Chart. Relationship between CV friction angle and Cu
Figure 55. Chart. Relationship between tangent friction angle and maximum unit weight
Figure 56. Chart. Relationship between CV friction angle and maximum unit weight
Figure 57. Chart. Relationship between CV friction angle and repose angle
Figure 58. Chart. Relationship between tangent friction angle and angularity
Figure 59. Chart. Relationship between CV friction angle and angularity
Figure 60. Relationship between initial void ratio and tangent friction angle
Figure 61. Chart. Relationship between CV friction angle and texture
Figure 62. Chart. Relationship between tangent friction angle and sphericity
Figure 63. Chart. Relationship between CV friction angle and sphericity
Figure 64. Chart. Relationship between tangent friction angle and angularity in LSDS testing for AASHTO No. 8 aggregates
Figure 65. Chart. Relationship between CV friction angle and angularity in LSDS testing for AASHTO No. 8 aggregates
Figure 66. Chart. Relationship between tangent friction angle and sphericity in LSDS testing for AASHTO No. 8 aggregates
Figure 67. ZDA Approach for LDTX and LSDS testing.
Figure 68. Photo. No. 5 aggregate sample
Figure 69. Photo. No. 56 aggregate sample
Figure 70. Photo. No. 57 aggregate sample
Figure 71. Photo. No. 6 aggregate sample
Figure 72. Photo. No. 67 aggregate sample
Figure 73. Photo. No. 68 aggregate sample
Figure 74. Photo. No. 7 aggregate sample
Figure 75. Photo. No. 78 aggregate sample
Figure 76. Photo. No. 8A aggregate sample
Figure 77. Photo. No. 8B aggregate sample
Figure 78. Photo. No. 8C aggregate sample
Figure 79. Photo. No. 8D aggregate sample
Figure 80. Photo. No. 8E aggregate sample
Figure 81. Photo. No. 89 aggregate sample
Figure 82. Photo. No. 9 aggregate sample
Figure 83. Photo. No. 10 aggregate sample

LIST OF TABLES

Table 1. Reported friction angles from DS and TX testing
Table 2. Bearing capacity factors
Table 3. Impact of friction angle on geotechnical constants in design
Table 4. Selected AASHTO M43-05 (ASTM D448) aggregate designations
Table 5. OGAs tested
Table 6. Aggregate gradation and classification
Table 7. Unit weight of aggregates
Table 8. Repose angle of aggregates
Table 9. Summary of AIMS2 average results for angularity, texture, and sphericity
Table 10. Ottawa test results using SDS device and MC approach
Table 11. SDS test results using MC approach
Table 12. SDS test results using the ZDA approach
Table 13. LSDS resistance calibration values
Table 14. LSDS Test results using MC approach
Table 15. LSDS test results using ZDA approach
Table 16. LDTX test results for Ottawa sand and AASHTO No. 68
Table 17. LDTX test results using MC approach
Table 18. LDTX test results using ZDA approach
Table 19. LSDS versus LDTX measured friction angles using MC approach
Table 20. LSDS versus LDTX measured friction angles using ZDA approach
Table 21. SDS versus LSDS friction angles using MC approach
Table 22. SDS versus LSDS friction angles using ZDA approach
Table 23. Summary of LSDS and LDTX testing
Table 24. No. 5 gradation
Table 25. No. 5 density
Table 26. No. 5 repose angle
Table 27. No. 5 standard DS results
Table 28. No. 5 LSDS results-dry
Table 29. No. 5 LSDS results-saturated
Table 30. No. 5 LDTX results
Table 31. No. 56 gradation
Table 32. No. 56 density
Table 33. No. 56 repose angle
Table 34. No. 56 standard DS results
Table 35. No. 56 LSDS results-dry
Table 36. No. 56 LSDS results-saturated
Table 37. No. 57 LDTX results
Table 38. No. 57 gradation
Table 39. No. 57 density
Table 40. No. 57 repose angle
Table 41. No. 57 standard DS results
Table 42. No. 57 LSDS results-dry
Table 43. No. 57 LSDS results-saturated
Table 44. No. 57 LDTX results
Table 45. No. 6 gradation
Table 46. No. 6 density
Table 47. No. 6 repose angle
Table 48. No. 6 standard DS results
Table 49. No. 6 LSDS results-dry
Table 50. No. 6 LSDS results-saturated
Table 51. No. 6 LDTX results
Table 52. No. 67 gradation
Table 53. No. 67 density
Table 54. No. 67 repose angle
Table 55. No. 67 standard DS results
Table 56. No. 67 LSDS results-dry
Table 57. No. 67 LSDS results-saturated
Table 58. No. 67 LDTX results
Table 59. No. 68 gradation
Table 60. No. 68 density
Table 61. No. 68 repose angle
Table 62. No. 68 standard DS results
Table 63. No. 68 LSDS results-dry
Table 64. No. 68 LSDS results-saturated
Table 65. No. 68 LDTX results
Table 66. No. 7 gradation
Table 67. No. 7 density
Table 68. No. 7 repose angle
Table 69. No. 7 standard DS results
Table 70. No. 7 LSDS results-dry
Table 71. No. 7 LSDS results-saturated
Table 72. No. 7 LDTX results
Table 73. No. 78 gradation
Table 74. No. 78 density
Table 75. No. 78 repose angle
Table 76. No. 78 standard DS results
Table 77. No. 78 LSDS results-dry
Table 78. No. 78 LSDS results-saturated
Table 79. No. 78 LDTX results
Table 80. No. 8A gradation
Table 81. No. 8A density
Table 82. No. 8A repose angle
Table 83. No. 8A standard DS results
Table 84. No. 8A LSDS results-dry
Table 85. No. 8A LSDS results-saturated
Table 86. No. 8B gradation
Table 87. No. 8B LSDS results-dry
Table 88. No. 8B LSDS results-saturated
Table 89. No. 8C gradation
Table 90. No. 8C LSDS results-dry
Table 91. No. 8C LSDS results-saturated
Table 92. No. 8D gradation
Table 93. No. 8D LSDS results-dry
Table 94. No. 8D LSDS results-saturated
Table 95. No. 8E gradation
Table 96. No. 8E density
Table 97. No. 8E LSDS results-dry
Table 98. No. 8E LSDS results-saturated
Table 99. No. 8E LDTX results
Table 100. No. 89 gradation
Table 101. No. 89 density
Table 102. No. 89 repose angle
Table 103. No. 89 standard DS results
Table 104. No. 89 LSDS results-dry
Table 105. No. 89 LSDS results-saturated
Table 106. No. 89 LDTX results
Table 107. No. 9 gradation
Table 108. No. 9 density
Table 109. No. 9 repose angle
Table 110. No. 9 standard DS results
Table 111. No. 9 LSDS results-dry
Table 112. No. 9 LSDS results-saturated
Table 113. No. 9 LDTX results
Table 114. No. 10 gradation
Table 115. No. 10 density
Table 116. No. 10 repose angle
Table 117. No. 10 standard DS results
Table 118. No. 10 LSDS results-dry
Table 119. No. 10 LSDS results-saturated
Table 120. No. 10 LDTX results

LIST OF ABBREVIATIONS

 

AASHTO American Association of State Highway and Transportation Officials
AIMS Aggregate imaging measurement system
AIMS2 Second generation aggregate imaging measurement system
CC Consolidation curve
COV Coefficient of variation
CV Constant volume
DS Direct shear
FHWA Federal Highway Administration
FM Fineness modulus
LDTX Large diameter triaxial
LSDS Large-scale direct shear
LVDT Linear variable differential transformer
MC Mohr-Coulomb
MSE Mechanically stabilized earth
NAVFAC Naval Facilities Engineering Command
OG Open-graded
OGA Open-graded aggregate
PS Plane strain
SDS Standard direct shear
TFHRC Turner-Fairbank Highway Research Center
TX Triaxial
ZDA Zero dilation angle

LIST OF SYMBOLS

α Angle of the modified (stress-path) failure envelope
Δ(σ'1 - σ'3) Change in effective deviator stress due to the membrane correction
Δh Change in horizontal displacement
Δv Change in vertical displacement
ε1 Axial strain
εV Volumetric strain
σ'1 Effective major principal stress
σ'3 Effective minor principal stress
σ'c Effective confining stress
σ'c,1 Low-range effective confining stress
σ'c,2 Mid-range effective confining stress
σ'c,3 High-range effective confining stress
σ'd Deviator stress
σ'n Effective normal stress
𝜏 Shear strength
𝜏f Shear stress at failure
ϕ Friction angle
ϕ DS Friction angle measured from direct shear testing
ϕ TX Friction angle measured from triaxial testing
ϕ' Effective friction angle
ϕ'µ True friction angle
ϕ'cv Constant volume effective friction angle
ϕ'cv,dry Constant volume effective friction angle under dry conditions
ϕ'cv,LDTX Constant volume effective friction angle from LDTX testing
ϕ'cv,LSDS,dry Constant volume effective friction angle from LSDS testing under dry conditions
ϕ'cv,sat Constant volume effective friction angle under saturated conditions
ϕ'cv,SDS Constant volume effective friction angle from SDS testing
ϕ'Dense Friction angle for aggregates tested at a dense state
ϕ'f Dilation corrected friction angle
ϕ'Loose Friction angle for aggregates tested at a loose state
ϕ'Medium Friction angle for aggregates tested at a medium dense state
ϕ's Secant, or peak effective friction angle
ϕ's,LDTX Effective secant friction angle from LDTX testing
ϕ's,LSDS,sat Effective secant friction angle from LDTX testing
ϕ't,LDTX Effective tangent friction angle from LDTX testing
ϕ't,LSDS,dry Effective tangent friction angle from LSDS testing under dry conditions
ϕ't,LSDS,sat Effective tangent friction angle from LSDS testing under saturated conditions
ϕ't,SDS Effective tangent friction angle from SDS testing
ϕ't Tangent effective friction angle
ψ Dilation angle
ψmax Maximum dilation angle
A Nonlinear material constant (Charles and Watts 1980)
Ac Corrected area
Ao Original sample area
a Experimental constant
b Nonlinear material constant (Charles and Watts 1980)
c Cohesion
c' Effective cohesion determined from the Mohr-Coulomb approach
c'SDS Effective cohesion from standard direct shear testing
Cc Coefficient of curvature
Cu Coefficient of uniformity
d10 Particle diameter at which 10 percent of the sample is finer, by mass
d30 Particle diameter at which 30 percent of the sample is finer, by mass
d50 Median grain size; particle diameter at which 60 percent of the sample is finer, by mass
d60 Particle diameter at which 60 percent of the sample is finer, by mass
d85 Particle diameter at which 85 percent of the sample is finer, by mass
dε1 Incremental axial strain
dε3 Incremental lateral strain
dmax Maximum diameter of the aggregate sample
D Sample diameter
Dc Specimen diameter at the end of the consolidation phase
Em Young's modulus for the membrane material
F Area correction actor
h horizontal displacement
Ka Active earth pressure coefficient
L Distance from intersection of bisector line and consolidation curve for the membrane penetration correction
n Porosity
Ny Bearing capacity factor
Nc Bearing capacity factor
Nq Bearing capacity factor
p' Effective mean stress path
q Shear stress path
t90 Time needed to achieve 90 percent of the total consolidation
tm Thickness of the membrane

 

 

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Turner-Fairbank Highway Research Center | 6300 Georgetown Pike | McLean, VA | 22101