Establishment of Subgrade Undercut Criteria and Performance of Alternative Stabilization Measures By Roy H. Borden, Ph.D., P.E Ben M. Cote M. A. Gabr, Ph.D., P.E. Young Jin Park Sang Chul Pyo Brent R. Robinson. P.E. Department of Civil, Construction, and Environmental Engineering North Carolina State University In Cooperation with The North Carolina Department of Transportation North Carolina State University Raleigh, North Carolina August 25, 2010 i Technical Report Documentation Page 1. Report No. 2. Government Accession No. 3. Recipient’s Catalog No. FHWA/NC/2008-07 … 4. Title and Subtitle 5. Report Date August 25, 2010 Establishment of Subgrade Undercut Criteria and Performance of Alternative 6. Performing Organization Code Stabilization Measures … 7. Author(s) 8. Performing Organization Report No. Roy H. Borden, Ph.D., P.E, Ben M. Cote, M. A. Gabr, Ph.D., P.E., Young Jin … Park, Sang Chul Pyo, and Brent R. Robinson. P.E. 9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) Department of Civil, Construction, and Environmental Engineering … North Carolina State University, Raleigh, North Carolina 27695-7908 11. Contract or Grant No. … 12. Sponsoring Agency Name and Address 13. Type of Report and Period Covered North Carolina Department of Transportation Final Report Research and Analysis Group 1 South Wilmington Street 8/2007 – 12/2009 Raleigh, North Carolina 27601 14. Sponsoring Agency Code 2008-07 Supplementary Notes: 16. Abstract The main objectives of the research work were to develop a systematic approach for determining whether or not undercut is necessary, and to investigate the adequacy of stabilization measures as implemented in conjunction with the undercut approach. The research work encompassed small and large scale laboratory testing, limited field testing using DCP, and numerical analyses and modeling. The following milestones were achieved based on the work performed in this project: i. Establishment of undercut design criteria based on the magnitude of strength and modulus of the subgrade soils. The undercut criteria were based on meeting a deformation limit state of 1 inch for both pumping and rutting, but with the additional requirement that bearing resistance of subgrade is at least twice the applied tire pressure to minimize the potential for rutting. ii. Development of a procedure for the use of DCP to provide input data for undercut criteria on the basis of modulus and strength. The proposed procedures are based on the analysis of the DCP data using wave mechanics and offer the advantage of using the DCP data to discern the need for undercut with depth (by applying the proposed undercut criteria incrementally) at the various phases of design and construction. iii. Development of guidelines for specifying a stabilization measure to achieve adequate subgrade support. Five stabilization measures were investigated through the performance of 22 large scale laboratory tests. These included the use of select fill, aggregate base course (ABC), geogrids with ABC, geotextiles with ABC, and lime stabilization. The performance of each stabilization measure was investigated in the laboratory and through numerical analyses. iv. Demonstrating the applicability of the proposed measures in several field configurations. Four field cases were idealized from actual project sections. The field cases were numerically modeled with the stabilization measures, and observations were made regarding subgrade response under static, proof roll, and cyclic loading. v. Performance of a comparative cost analysis to illustrate the relative cost of each stabilization measure in relation to measured performance (surface deformation). The cost analyses were performed with results presented in a normalized form in an attempt to account for variables such as strength of the stabilization measure and subgrade deformation. 17. Key Words 18. Distribution Statement Aggregate, Cone, Criteria, Excavation, Experimental, Fill, Geosynthetics, Lime, Load, Model, Numerical, Soils, Stabilization, Undercut 19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price Unclassified Unclassified 304 Form DOT F 1700.7 (8-72) Reproduction of completed page authorized ii DISCLAIMER The contents of this report reflect the views of the authors, who are responsible for the fact and accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the North Carolina Department of Transportation. This report does not constitute a standard, specification or regulation. iii ACKNOWLEDGEMENTS The authors would like to thank the members of the NCDOT Geotechnical, Materials, and Construction divisions who worked on this project. The time, expertise and guidance of NCDOT engineers were invaluable to this project. Special thanks are due to the members of the steering committee: Njoroge W. Wainaina, P.E. Mrinmay Biswas, Ph.D., P.E. Kyung J. Kim, Ph.D., P.E. Ernest Morrison, P.E. John L. Pilipchuk, P.E., L.G. Chris Kreider, P.E. Mehdi Haeri Dean Argenbright, L.G. Chun Kun Su J. Dean Hardiste, P.E. Robert E. Capehart, P.E. Scott Hidden, P.E . iv TABLE OF CONTENTS Page No. LIST OF TABLES ............................................................................................................ xii LIST OF FIGURES .......................................................................................................... xv EXECUTIVE SUMMARY .............................................................................................. xx CHAPTER 1: INTRODUCTION ...................................................................................... 1 Problem Statement............................................................................................................ 2 Undercutting in Unstable Soils ........................................................................................... 3 Chemical Stabilization ........................................................................................................ 6 Geosynthetics in Subgrade Stabilization ............................................................................ 6 Objectives........................................................................................................................... 6 Scope of Work ................................................................................................................... 7 Phase I: Laboratory-Scale Testing Program ....................................................................... 8 Phase II: Pilot-Scale Testing Program ................................................................................ 8 Phase III: Numerical Modeling ........................................................................................... 9 Report Layout ................................................................................................................. 10 CHAPTER 2: LITERATURE REVIEW ......................................................................... 11 Geosynthetic Reinforcement of Paved and Unpaved Road Sections ......................... 11 Paved Roads: Unbound Layers and Subgrade ................................................................. 11 Large Scale Cyclic-Plate Load Testing ......................................................................... 14 Summary—Large Scale Cyclic Plate Load Testing ......................................................... 25 The Dynamic Cone Penetrometer (DCP) ..................................................................... 25 Development ..................................................................................................................... 25 Factors Affecting Results .................................................................................................. 26 DCP Correlations to Material Properties .......................................................................... 27 DCP Correlations to California Bearing Ratio (CBR) ...................................................... 27 Empirical Correlations between DCP and Young’s Elastic Modulus (E) ........................ 28 CBR and Resilient Modulus (MR) Correlations................................................................ 29 Use of Combined Equations to Predict Resilient Modulus (MR) from DCP .................... 29 Empirical Correlations between DCP and Resilient Modulus (MR) ................................. 29 v DCP for Compaction Evaluation ...................................................................................... 31 Chemical Stabilization .................................................................................................... 31 Improvement of soil properties by incorporation of lime or cement ................................ 32 Durability of stabilized soil ............................................................................................... 33 Numerical Analysis ......................................................................................................... 34 Criteria for establishing required magnitude of undercut ................................................. 38 Summary .......................................................................................................................... 42 CHAPTER 3: Test Materials Characterization ................................................................. 43 Subgrade soils .................................................................................................................. 43 California