SHEAR STRENGTH, CREEP AND STABILITY OF FIBER-REINFORCED SOIL SLOPES By GARRY HADEN GREGORY, P. Bachelor of Science Oklahoma City University Oklahoma City, Oklahoma 1991 Master of Science in Civil Engineering South Dakota School of Mines and Technology Rapid City, South Dakota 1993 Submitted to the Faculty of the Graduate College of the Oklahoma State University In partial fulfillment of the requirements for the Degree of DOCTOR OF PHILOSOPHY May, 2006 UMI Number: 3220239 UMI Microform 3220239 Copyright 2006 by ProQuest Information and Learning Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code.
ProQuest Information and Learning Company 300 North Zeeb Road P. Box 1346 Ann Arbor, MI 48106-1346 SHEAR STRENGTH, CREEP AND STABILITY OF FIBER-REINFORCED SOIL SLOPES Dissertation Approved: Dr. Snethen, Advisor Dr. Todd Halihan Dr.
Gordon Emslie, Dean of the Graduate College ii ACKNOWLEDGMENTS The author wishes to gratefully acknowledge the efforts and contributions of the following individuals and firms. I sincerely appreciate the guidance, assistance, and advice provided by my advisor, Dr. Snethen, and the opportunity to have learned from his excellent teaching in the classroom and during research. I also appreciate the contributions and encouragement, provided by my other committee members, Dr.
Garold Oberlender, Dr. Stephen Cross, and Dr. Todd Halihan, and for the opportunity to have learned much in their classes. I wish to express my appreciation to Synthetic Industries of Chattanooga, Tennessee for providing the fiber material used in the research testing, and to Mr.
David Chill of Fiber Soils, Inc. for providing previous laboratory test results on fiber-reinforced soil. I want to thank Mr. David Porter who fabricated the Direct Shear Creep devices in the Civil Engineering Machine Shop.
I also appreciate the support and funding provided by the author’s firm Gregory Geotechnical and the efforts of our dedicated staff during this study, especially my daughter Marisa Duran who provided assistance during the laboratory testing program. I owe special gratitude to my wife Jan for her unwavering support, encouragement, and friendship during this effort. TABLE OF CONTENTS Chapter Page I. 1 Scope of Research Study.
2 Format of Dissertation. 9 Related Published Literature. 14 Utilization of Existing Data. 15 Effective Fiber Length.
19 FRS Shear-Strength Formulation. LABORATORY TESTING PROGRAM. 29 Laboratory Test Series. 29 Sample and Specimen Terminology.
29 Quantities and Types. 32 Bulk Sample Preparation. 32 Clay Soil Sample. 32 Silty Sand Sample.
33 Index and Classification Tests. 35 Liquid and Plastic Limits Tests. 35 Percent Passing No. 35 iv Standard Proctor Tests.
35 Sieve Analysis Tests. 36 Maximum-Minimum Index Density Tests. 36 Specimen Preparation Prior to Compaction. 36 Moisture and Weight Preparation.
38 Compaction of Clay Specimens. 40 Triaxial Shear Specimens. 40 Direct Shear Specimens. 46 Storage of Specimens.
46 Moisture Content Stability During Storage. 47 Compaction of Sand Specimens. 48 Triaxial Shear Specimens. 48 Direct Shear Specimens.
50 Triaxial Shear Tests – Clay. 51 Mounting in Triaxial Cell. 52 Saturation and Consolidation. 55 Electronic Data Acquisition.
55 Inspection and Dissection of Specimens Following Test. 56 Direct Shear Tests – Clay. 56 Mounting in Direct Shear Box. 57 Saturation and Consolidation.
59 Electronic Data Acquisition. 60 Inspection and Dissection of Specimens Following Test. 61 Creep Tests – Clay. 62 Mounting in Creep Device.
63 Saturation and Consolidation. 64 Creep Shear Stage. 65 Electronic Data Acquisition. 66 Triaxial Shear Tests – Sand.
67 Consolidation and Saturation. 68 Dissection and Inspection of Specimens Following Test. 69 v Direct Shear Tests – Sand. 69 Test Type and Shear Stage.
69 Inspection and Dissection of Specimens Following Test. 70 Interface Shear Tests. 70 Specimen Preparation and Test Observation. CORRELATION AND ANALYSIS OF DATA.
75 Triaxial Shear Test Data. 75 Available Test Data. 75 Summary of Current Triaxial Test Data. 76 Summary of Previous AGT Laboratory Triaxial Test Data.
78 Direct Shear Test Data. 80 Summary of Direct Shear Test Data. 80 Creep Test Data. 82 Interface Test Data.
85 Correlation of Shear Strength with Conceptual Model. 86 Conceptual Model Calculations. 86 Frictional Strength Correlations for Current Test Results. 87 Frictional Strength Correlations for Previous AGT Test Results.
88 Cohesive Strength Correlations for Current Test Results. 89 Cohesive Strength Correlations for Previous AGT Test Results. 90 Calibration of Conceptual Model. 91 Decay Function for Large Fiber Content.
APPLICATION OF FRS IN SLOPE STABILITY. 95 Slope Stability Analysis of FRS Slopes. 96 Analysis Using Existing Computer Programs. 96 Analysis Using Modified Computer Programs.
CASE HISTORY PROJECTS. 98 PGBT Turnpike Project. 98 FRS Application in Project. 99 Slope Stability Analyses.
100 Project Related Testing. 102 vi Lake Ridge Parkway Slope Repair Project. 102 FRS Application in Project. 103 Obtaining Soil Samples for Research Testing.
103 Project Related Testing. 104 Slope Stability Analyses. CONCLUSIONS AND RECOMMENDATIONS. 117 Conclusions Regarding Laboratory Test Results.
118 Conclusions Regarding Conceptual Model Development. 118 Conclusions Regarding Application of Model. 119 Recommendations Regarding Project Applications. 120 Recommendations for Future Research.
127 LABORATORY TEST REPORTS. 192 SCHEMATIC DRAWINGS – DIRECT SHEAR CREEP DEVICE. 195 SLOPE STABILITY ANALYSIS - COMPUTER OUTPUT. 196 vii LIST OF TABLES Table Page 1.
Routine Laboratory Testing Program. Shear Strength and Creep Laboratory Testing Program. Approximate Test Durations. Summary of Triaxial Test Results.
Summary of Triaxial Test Results (1-Specimen Tests). AGT Soil Properties. Summary of AGT Triaxial Test Results. Summary of Direct Shear Test Results.
81 viii LIST OF FIGURES Figure Page 1. FRS Mixture with Sand. Normal Stress on Planar Reinforcement. Range of Potential Orientation About Fiber Longitudinal Axis.
Stress Distribution on Fiber Cross-Sectional Axis. Effective Fiber Length Across Shear Plane. Geometry of Fiber Distribution in Sphere Space. Rotation Point of FRS Strength Envelope.
Processing of Clay Sample. Clay Specimens Prior to Hydration. Spreading Fibers over Hydrated Clay Soil Specimen. Initial Hand Mixing of FRS Specimen.
Final Hand Mixing of FRS Specimen. Mixed FRS Specimen Ready for Storage or Compaction. Placement of Loose Specimen into Mold. Compaction With Metal Rod.
Rod Plunged to Near Bottom of Mold During Initial Compaction. Finishing Compaction With Piston and Guide Ring. Preparation for Compaction of Direct Shear Specimen. 47 ix Figure Page 19.
Completing Compaction of Direct Shear Specimen. Clay Specimen Storage Cooler. Preparation of Sand Specimen in Split Mold. Addition of Fibers to Sand Specimen During Compaction.
Compacted Sand Specimen After Removal of Split Mold. Preparation of FRS Sand Specimen in Direct Shear Box. FRS Specimen Mounted on Base of Triaxial Cell. Specimen With Membrane and Top Cap in Place.
Saturation/Consolidation Stage. Shear Stage of Triaxial Test on Clay Specimen. Test Data Display in Real Time on Computer Screen. Clay Triaxial Specimen Following Test.
Dissected Triaxial Clay Specimen With Exposed Fibers. Mounting of Clay Specimen in Direct Shear Box. Computer Controlled Direct Shear Machine. Dissected Direct Shear Clay Specimen With Exposed Fibers.
Direct Shear Creep Devices. Mounting Clay Specimen in Creep Device. Fully Mounted Creep Specimen With Water in Reservoir. Triaxial Test on Sand Specimen.
Large Scale Direct Shear Machines Used in Interface Tests. Real Time Data From Interface Shear Tests. Sheet Material on Bottom Shear Box After Interface Test. Plot of Creep Test Data in Semi-Log Form.
Plot of Creep Test Data in Arithmetic Form. Interface Shear Test Results – Fiber Material on Soils. Model Versus Current Test Results for Tan Ø. Model Versus AGT Test Results for Tan Ø.
Model Prediction Versus Current Test Results for c. Model Versus AGT Test Results for c. Fiber Content Versus Reduction Factor for Interface Coefficients. Spreading Fibers for FRS on PGBT Project.
Mixing FRS on PGBT Project. Dissected Field Specimen Following Triaxial Test. Mixer for Processing Fiber-Soil Specimen into Slurry. Sieving of Slurry to Extract Fibers.
Slope Failure on Lake Ridge Parkway. Slope Failure Scarp at Roadway Edge – Lake Ridge Pkwy. Slope Failure at Roadway Edge – Lake Ridge Pkwy. 111 xi Figure Page 58.
Initial Excavation for FRS Slope Repair – Lake Ridge Pkwy. Partially Used Fiber Supply Bag – Lake Ridge Pkwy. FRS Embankment Construction – Lake Ridge Pkwy. Down Slope View of Completed FRS Embankment – Lake Ridge.
Up Slope View of Completed FRS Embankment–Lake Ridge Pkwy. 116 xii NOMENCLATURE ASTM = American Society for Testing and Materials c = cohesion of raw soil Δc frs = corrected apparent increase in cohesive strength due to fiber c frs = the cohesion value of FRS d = diameter of fiber, or equivalent diameter for non-circular fiber FRS = fiber-reinforced soil FS = factor of safety fc = interaction coefficient related to the cohesive component of the shear strength fφ = interaction coefficient related to the frictional component of the shear strength (sometimes referred to as fφ tan φ = tan δ) Gs = Specific Gravity of fiber material K 0 = At-rest earth pressure coefficient l = length of fiber Le = effective length of an individual fiber nf = average number of fibers per unit volume NP = non plastic Nf = number of fibers intersecting the shear plane pcf = pounds per cubic foot xiii Nomenclature (continued) τ frsc = uncorrected apparent increase in cohesive shear strength due to fiber τ frsφ = apparent increase in frictional shear strength due to fiber tan φ frs = tangent of the friction angle for FRS Vr = fiber volume ratio (ratio of fiber volume to total volume of a unit mass of FRS) Wf = weight of fibers in a unit volume of FRS z = depth below ground surface γ = soil unit weight γw = unit weight of water φ = angle of shearing resistance of raw soil σh or σh = horizontal stress σv or σv = vertical stress xiv CHAPTER I INTRODUCTION Background The concept of fiber-reinforced soil (FRS) dates to ancient times when clay bricks were reinforced with straw fibers. This concept is also similar to natural root reinforcement of soil where micro root structure increases the apparent shear strength of the root reinforced zone compared to similar soil with no root structure. The use of geosynthetics (synthetic plastic materials) for reinforcement of soil structures has become well established in the past 20 years.
The geosynthetic reinforcement materials initially consisted mostly of geotextiles and geogrids, often referred to as planar reinforcement. Techniques for design and analysis of earth structures reinforced with planar geosynthetics are well developed, and have been presented extensively in the literature. The rapid increase in the use of planar geosynthetics led to the concept and development of synthetic fibers for soil reinforcement. The concept of using short synthetic fibers for soil reinforcement was the subject of several early research studies and was discussed in the literature (Andersland and Khattak, 1979; Hoare, 1979; Gray and Ohashi, 1983).
However, short synthetic fibers for soil reinforcement were not commercially available until about 1990 when a pilot 1 program of fiber research, production, and full-scale test projects was undertaken by a major geosynthetics manufacturer in the United States (Synthetic Industries, 1990). The author became involved in numerous projects consisting of fiber- reinforced embankments and related laboratory testing in 1994. Fiber-reinforced soil (FRS) has been used successfully on more than 50 embankment slopes in the United States in the past 15 years (Gregory and Chill, 1998, Gregory, 1999b, Chill 2006). The author has been involved in more that 15 of the FRS projects.
The geosynthetic fiber reinforcement has consisted of 1-inch to 2.75-inch (25- to 70-mm) length polypropylene fibers. These fibers, when mixed into the soil, significantly increase the apparent shear strength of the entire soil mass. An FRS mixture is illustrated in Figure 1.