Recommended Practice for Stabilization of Sulfate-Rich Subgrade Soils (2009)

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ACKNOWLEDGMENT This work was sponsored by the American Association of State Highway and Transportation Officials (AASHTO), in cooperation with the Federal Highway Administration, and was conducted in the National Cooperative Highway Research Program (NCHRP), which is administered by the Transportation Research Board (TRB) of the National Academies. COPYRIGHT PERMISSION Authors herein are responsible for the authenticity of their materials and for obtaining written permissions from publishers or persons who own the copyright to any previously published or copyrighted material used herein. Cooperative Research Programs (CRP) grants permission to reproduce material in this publication for classroom and not-for-profit purposes. Permission is given with the understanding that none of the material will be used to imply TRB, AASHTO, FAA, FHWA, FMCSA, FTA, Transit Development Corporation, or AOC endorsement of a particular product, method, or practice. It is expected that those reproducing the material in this document for educational and not-for-profit uses will give appropriate acknowledgment of the source of any reprinted or reproduced material. For other uses of the material, request permission from CRP. DISCLAIMER The opinion and conclusions expressed or implied in the report are those of the research agency. They are not necessarily those of the TRB, the National Research Council, AASHTO, or the U.S. Government. This report has not been edited by TRB.

i TABLE OF CONTENTS ACKNOWLEDGMENTS………………………………………………………………………..V ABSTRACT……………………………………………………………………………………..VI EXECUTIVE SUMMARY……………………………………………………………………..VII INTRODUCTION .......................................................................................................................... 1  BACKGROUND ............................................................................................................................ 1  CHEMISTRY OF ETTRINGITE AND THAUMASITE .............................................................. 3  MECHANISMS OF REACTION .................................................................................................. 4  Mechanism of Formation ............................................................................................................ 4  Mechanism of Expansion ........................................................................................................... 5  ETTRINGITE FORMATION IN STABILIZED SOILS ............................................................... 6  MISCONCEPTIONS REGARDING ETTRINGITE FORMATION IN SOILS ........................... 7  SOURCE OF SULFATES IN SOIL ............................................................................................... 7  THRESHOLD SULFATE LEVELS IN SOILS ............................................................................. 8  RISK IDENTIFICATION ............................................................................................................ 10  Geology and Mineralogy .......................................................................................................... 10  Climatic Characteristics and Drainage Features ....................................................................... 11  Soil Classification ..................................................................................................................... 11  Topography and Spatial Variability .......................................................................................... 12  Visual Inspection ...................................................................................................................... 13  Soil Investigation ...................................................................................................................... 13  CONTROLLING ETTRINGITE FORMATION ......................................................................... 14  Regulating Reactant Concentrations ......................................................................................... 14  Techniques for Treating Sulfate Rich Soils ............................................................................. 16  Traditional Lime Treatment ............................................................................................................... 18  Modified Treatment ........................................................................................................................... 18  Alternative Treatments ....................................................................................................................... 19 

ii SULFUR CHARACTERIZATION IN SOILS ............................................................................ 21  Water Soluble Sulfates ............................................................................................................. 21  Acid Soluble Sulfates ............................................................................................................... 21  Total Reduced Sulfur ................................................................................................................ 22  Total Sulfur ............................................................................................................................... 22  METHODS FOR SULFATE QUANTIFICATION IN SOILS ................................................... 22  TxDOT Method—Tex-620-J .................................................................................................... 23  U.S. Army and Air Force Method ............................................................................................ 23  Gravimetric Method ........................................................................................................................... 23  Turbidimetric Method ........................................................................................................................ 23  University of Texas Arlington (UTA) Method ......................................................................... 24  Ion Chromatography ................................................................................................................. 24  TXDOT Colorimetric Method ................................................................................................... 24  CDOT Colorimetric Method ..................................................................................................... 25  TxDOT Conductivity Method .................................................................................................. 25  Inductively Coupled Plasma Atomic Emission Spectroscopy .................................................. 25  EVALUATING TECHNIQUES USED IN AASHTO SULFATE TESTING ............................ 26  Gravimetric Techniques ............................................................................................................ 26  Colorimetric Techniques .......................................................................................................... 26  RECOMMENDATIONS TO MODIFY AASHTO TEST METHODS ...................................... 27  CONCLUSIONS .......................................................................................................................... 30  REFERENCES ............................................................................................................................. 33 STANDARD RECOMMENDED PRACTICE FOR STABILIZATION OF SULFATE- BEARING SUBGRADE SOILS………………………………………………………………36

iii LIST OF FIGURES FIGURE 1. Comparison of ettringite concentrations in soil (a) Predicted using phase diagram approach and (b) Measured using a Differential Scanning Calorimeter............................... 10 FIGURE 2. Variability in size of gypsum crystals found in soil. .................................................... 13 FIGURE 3. Effect of mellowing on sulfate content in soil. ............................................................ 16 FIGURE 4. Guideline for stabilizing sulfate rich soils with lime ................................................... 17

iv LIST OF TABLES TABLE 1. Level of risk associated with lime stabilization in sulfate-bearing clays. ....................... 9 TABLE 2. Comparison of available test methods for measuring sulfate concentrations in soil. ... 29

v ACKNOWLEDGMENTS The research reported herein was performed under NCHRP Project 20-07 by the Texas Transportation Institute, Texas A&M University. The Texas Transportation Institute was the contractor for this study. Dallas N. Little, E.B. Snead Chair Professor, Texas A&M University, was the principal investigator. The author of this report is Syam Nair, Graduate Research Assistant, Texas Transportation Institute. The work was done under the general supervision of Dr. Dallas Little.

vi ABSTRACT Calcium-based stabilizers (CaO-based stabilizers) develop a high pH environment when mixed with water during construction. This high pH condition and the presence of lime (CaO) introduce an environment favorable to the formation of two expansive minerals, ettringite and possibly thaumasite, when sulfate-bearing salts are present in soil. The success of a lime stabilization project depends heavily on the accuracy of predictions regarding the potential of the native soils to form these disruptive minerals. This report describes the nature of these minerals and the mechanism of their formation in soils stabilized with lime, cement, or fly ash. The key issue for the engineer in deciding how to properly stabilize sulfate-bearing soils is to quickly and efficiently determine: (1) the sulfate content of the soils and (2) the threshold quantity of sulfates likely to cause damage. This report addresses both issues but focuses on the methods used to quantify sulfate content. Quick and effective field techniques and more precise laboratory tests are compared and evaluated. Recommendations are presented regarding the appropriate testing protocol to use. Specific recommendations for changes to the American Association of State Highway and Transportation Officials (AASHTO) test method T-290-95 (methods A and B) are made. The report also outlines techniques for effective risk assessment including reconnaissance, topographical assessment, pedological and geological assessment, climatic assessment, and soil sampling plans. The report also outlines construction tests that can reduce risks for various levels of sulfate determined using a method such as AASHTO T-290-95 (revised) or equivalent.

vii EXECUTIVE SUMMARY In place soil treatment using calcium-based stabilizers is an economically feasible solution alternative to address strength deficiencies and problematic shrink/swell behavior of unstable subgrade soils. Soil instability may originate from the presence of clay or silt whose instability is normally triggered by a change in moisture content. Even though stabilization improves engineering properties, problems can arise when calcium-based stabilizers are used in soils rich in sulfate-bearing minerals. Stabilization of sulfate rich soils in the presence of excess moisture may lead to the formation of minerals such as ettringite and/or thaumasite and can cause distress in or even destruction of pavement structures due to heaving. For 1 mole of ettringite to form 6 moles of CaO, 1 mole of Al2O3, 3 moles of SO4, and 32 moles of water are required. Calcium ions are provided by lime, Portland cement, or fly ash; alumina is supplied by dissolution of oxyhydroxides and phyllosilicates; and sulfates are supplied by dissolution of gypsum, oxidation of sulfide, or ion migration of sulfate ions as water diffuses through the matrix. Two theories exist regarding the cause of expansion during ettringite formation. The first theory explains that expansion is due to topochemical formation of ettringite and the anisotropic growth of the crystals. The second theory suggests that expansion is due to absorption of water by ettringite crystals where water molecules can cause interparticle repulsion resulting in overall expansion of the matrix. It is reasonable to believe that either one of the above theories or a combination of both may result in expansive behavior when ettringite is present. The current methodology to identify risk of sulfate-induced damage is based on the concentration of available (usually water soluble) sulfates in the soil. Sulfate contents are generally expressed either in ppm (parts per million) or mg/kg (which is equal to ppm) or in a percent dry weight of soil (10,000 ppm or 10,000 mg/kg is equal to 1 percent by mass). Many cases are documented in the literature where soils with varying levels of sulfates, from above 1,000 ppm to 10,000 ppm, precipitate ettringite when treated with calcium-based stabilizers. Based on these observations, identifying the threshold level of sulfates required to form a deleterious amount of ettringite in a specific soil is complex. These observed variations are primarily due to the difference in the mineralogy of the soils as the type and percentage of minerals, primarily clay minerals, in the soil determines the release of aluminates required to form ettringite. The Texas Department of Transportation (TxDOT) recommends a safety limit of 0.2 percent by mass for soluble sulfate as a threshold separating a safe acceptable risk from low to moderate risk. This limiting value agrees with studies by other researchers. Risk assessment along the alignment of the proposed roadway is a key factor in deciding the fate of a project. Risk identification prior to soil exploration can help develop effective soil exploration and sampling techniques, define the scale of testing needed, and help decide the controls required during construction. Sulfate concentrations in soil exhibit high spatial heterogeneity and hence selecting locations to perform sulfate testing is critical. Identification of problem areas depends on the availability of techniques that can characterize important soil properties in a time effective manner. Although the sulfate concentrations in soil can vary for many reasons, identification of problematic locations can be achieved by careful consideration of certain selected features along the alignment. Several factors are key to determining risk as well as identifying where samples should be taken as well as how many should be taken. Many factors related to soil properties can influence ettringite formation in stabilized soils. This includes ion availability, source for water supply, and system pH history. Therefore,