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FiguresFigure 1.1. Structural layers in flexible and rigidpavements 3Figure 5.5. Mixing portland cement and native soil witha reclaimer 32Figure 1.2. Cation exchange 5Figure 5.6. Roadway reclaimer 33Figure 1.3. Flocculation and agglomeration 5Figure 5.7. Vibratory sheepsfoot tamping roller 33Figure 1.4. Cementitious hydration 5Figure 5.8. Smooth drum roller 33Figure 1.5. Pozzolanic reactions 6Figure 1.6. Comparison of CSS with removal andreplacement Figure 6.1. Cement and subgrade being mixed withreclaimer 356Figure 6.2. Vibratory sheepsfoot roller compactingsubgrade 35Figure 2.1. Atterberg limits and phases of fine-grainedsoils 9Figure 2.2. PI of two types of clay soils after beingstabilized with lime or cement 11Figure 2.3. Effects of cement treatment on the Atterberglimits of plastic soils 12Figure 6.3. Reclaimers mixing the subgrade and cement 36Figure 6.4. Compacted subgrade 36Figure 6.5. Weak materials evidenced duringmodification 37Figure 2.4. Sieve analysis apparatus 12Figure 6.6. Mixer incorporating portland cement to adepth of 16 in. 37Figure 2.5. Hydrometer test apparatus 13Figure 6.7. Subgrade after cement application 38Figure 2.6. CBR testing machine 13Figure 2.7. Unconfined compressive strength testingapparatus Figure 6.8. Mixing and compacting cement andsubgrade 3814Figure 6.9. Compacting cement-stabilized subgrade 38Figure 6.10. Mixing cement and subgrade 39Figure 6.11. Constructing foundation for the schoolstructure 39Figure 2.8. Effect of cement on swelling according to aCBR test 15Figure 3.1. Decision tree for developing a CMS or CSSmix design 18Figure 3.2. Test pit 20Figure 3.3. Core sampling of pavement 20Figure 3.4. Auger sampling 21Figure 3.5. Shelby tube sampler 21Figure 3.6. Standard penetration sampler 21Figure 3.7. Dynamic cone penetrometer testing 21Figure 3.8. Falling weight deflectometer 22Figure 3.9. Automated plate load testing apparatus 22Figure 4.1. Mixture design steps for the design path 25Figure 4.2. Maximum dry density and optimum moisturecontent curve for a stabilized clay soil 27Figure 4.3. Example of unconfined compressive strengthversus cement content 28Figure 5.1. Moisture conditioning with a water truck 30Figure 5.2. Spreading dry portland cement 30Figure 5.3. Spreading portland cement slurry 30Figure 5.4. Dust control during portland cementapplication 31Figure 6.12. Completed foundation on cement-treatedsubgrade 39Figure B.1. Untreated soil samples in labeled sealedcontainers 47Figure B.2. Mixing additive and soil in the laboratory 47Figure B.3. Preparing a specimen just after compaction 47Figure B.4. Extruding a compacted specimen from amold 47Figure B.5. Weighing a compacted specimen 47Figure B.6. Measuring the diameter of a specimen 47Figure B.7. Measuring the height of a specimen using amicrometer dial assembly 47Figure B.8. Specimen in a sealed bag 47Figure B.9. Specimen in an oven 48Figure B.10. Oven-dried specimen before weighing andmeasuring 48Guide to Cement-Stabilized Subgrade Soilsvii

TablesTable 1.1. Key features of soil-cement products 4Table 2.1. AASHTO soil classification system 7Table 2.2. Comparisons between AASHTO soilclassification system and USCS 8Table 2.3. Three soil index properties compared to degreeof expansion 10Table 2.4. Effect of cement treatment on the PI and SL ofclay soils 11Table 2.5. Models relating material index and strengthproperties to Mr and CBR 14viiiGuide to Cement-Stabilized Subgrade SoilsTable 3.1. Uses for disturbed and undisturbed soilsampling 19Table 5.1. Typical cement spread rates (US customaryunits) 31Table 5.2. Typical cement spread rates (metric units) 31Table 5.3. Comparison of typical gradation requirementsfor CSS, CTB, and FDR 32

Executive SummaryThe primary purpose of this guide is to describe thecharacteristics, uses, and benefits of cement-stabilizedsubgrade (CSS) and present methods for geotechnicalevaluation, mix design, construction, and field testingthat will produce a satisfactory project. The materialin this guide updates and expands on the informationpresented in the Portland Cement Association’s 2008publication Guide to Cement-Modified Soil (CMS).The use of cement for subgrade soil treatment, whetherthrough CMS or CSS, can be an economical, fast, andsustainable solution to several soil problems encounteredbefore or during construction. Cement treatment canthus help reduce or maintain project timelines andbudgets and minimize the impacts of poor soil onpavement design. Additionally, in both rural and urbanenvironments cement treatment is usually a cost-effectiveand sustainable alternative to removing and replacing soil.CMS and CSS share many characteristics andapplications. Both treatments improve the physicalproperties of the native in situ soil, and both help extendthe life of a pavement by providing uniform support via adurable, stable, and typically non-expansive subgrade.However, to clarify the scope of this guide the twotreatments are distinguished as follows: CMS describes otherwise untreated soils that havebeen treated with a relatively small proportion ofportland cement to provide a stable working platform.The improvements offered by the treatment includereducing the plasticity and shrink/swell potential ofunstable, highly plastic, wet, or expansive soils as wellas increasing the bearing capacity. CSS not only provides all the benefits of CMS butalso substantially increases soil stiffness and strength tothe point where the treatment can provide structuralbenefits to pavement and building foundations.Unless stated otherwise, this guide primarily discusses CSS.While this document may not address all of a project’sspecific details, it provides guidance on the cementtreatment of subgrade soils. Among other topics, thisguide addresses soil classifications and properties,geotechnical evaluation, and the design, construction,and field testing of CSS. Throughout, this guide addressesthe importance of geotechnical oversight at the beginningof a project, during the mix design stage, and duringconstruction in order to ensure that the project meets itsintended purpose.The six chapters in this guide cover the followinginformation: Chapter 1. IntroductionChapter 1 covers the purpose of the guide, keyterminology, a comparison of soil-cement products,soil modification mechanisms, and the benefits and lifeexpectancy of CSS. Chapter 2. Materials and PropertiesChapter 2 covers basic information on soil types andproperties and explains how cement stabilization affectsengineering properties. Chapter 3. Geotechnical Evaluation and FieldSamplingChapter 3 covers the various types of sampling andtesting required to determine soil types and properties. Chapter 4. Mixture DesignChapter 4 covers the process used to determinethe proper cement content required to stabilize thesubgrade. Chapter 5. Construction, Field Inspection, andTestingChapter 5 discusses the process for constructing acement-stabilized subgrade, the equipment required,and the necessary field inspection and testing. Chapter 6. Case StudiesChapter 6 includes five case studies that describeprojects in which cement has been incorporated intothe subgrade to improve soil properties. Each casestudy includes project information and a detaileddiscussion of improvements.The following appendices are included at the end of thisguide: Appendix A. Suggested Construction Specificationfor Cement-Stabilized Subgrade SoilsAppendix A provides a guide specification coveringsubmittals, materials, equipment, constructioninspection, and testing requirements for constructingcement-stabilized subgrade soils. Appendix B. Mix Design Swell Test Method for SoilTreatment Using Additives (Cement or Other)Appendix B describes a test method for determiningthe extent to which engineering properties such asstrength and shrink/swell potential are improved forsoils treated with cement or other additives relative tountreated soil.Executive Summaryix

xGuide to Cement-Stabilized Subgrade Soils

Chapter 1. IntroductionThe primary purpose of this guide is to describe thecharacteristics, uses, and benefits of cement-stabilizedsubgrade (CSS) and present methods for geotechnicalevaluation, mix design, construction, and field testingthat will produce a satisfactory project. The materialin this guide updates and expands on the informationpresented in the Portland Cement Association (PCA)publication Guide to Cement-Modified Soil (CMS)(Halsted et al. 2008).Cement-Modified Soil (CMS)CMS is a compacted mixture of pulverized in situ soil,water, and small proportions of cement that results inan unbound or slightly bound material. The treatedmaterial is similar to a soil but has reduced plasticity anda lower susceptibility to moisture, resulting in a moreworkable material.To clarify the scope of this guide, CSS is distinguishedfrom CMS as follows: Improves the workability of subgrade soils and theirability to be used in construction CMS describes otherwise untreated soils that havebeen treated with a relatively small proportion ofportland cement to provide a stable working platform.The improvements offered by the treatment includereducing the plasticity and shrink/swell potential ofunstable, highly plastic, wet, or expansive soils andincreasing the bearing capacity. Reduces plasticity and shrink/swell volume changepotential CSS not only provides all of the benefits of CMS butalso substantially increases soil stiffness and strengthto the point where the treatment provides structuralbenefits to pavement and building foundations. Promotes soil dryingUnless otherwise noted specifically, this guide primarilydiscusses CSS. Uses on-site soil rather than costly removal andreplacement with select fill materialThis first chapter defines key terminology regardingcement-treated soil, compares four cement treatmentsin terms of their applications and benefits, describes themechanisms by which cement modifies soil, and thensummarizes the benefits and life expectancy of CSS.TerminologySoil-CementSoil-cement refers to a compacted engineered mixtureof soil, cement, and water designed and constructed forvarious pavement and geotechnical applications andcharacteristics. The term soil-cement can be consideredan umbrella term covering the four types of cementproducts defined in the following paragraphs.Note that throughout this guide, the term cementalways refers to portland cement, a hydraulic cementthat sets and hardens by reacting chemically with water(hydration).The principal benefits of CMS are as follows: Reduces moisture susceptibility and migration Increases the speed of construction on sites due to thereduced impact of rain Increases bearing capacity compared to untreated soil Provides a significant improvement to the workingplatform Provides a permanent soil modification (does not leach) Does not require any mellowing periodCement-Stabilized Subgrade (CSS)CSS is a compacted, engineered mixture of pulverizedin situ soil, water, and moderate proportions of cement(slightly more cement than CMS) that results in asemi-bound to bound material. The treated material hasstructural engineering properties similar to or better thanthose of a granular material.In addition to all of the benefits of CMS, CSS providesimproved shear and compressive strength and/or soilshrink/swell tendencies. The degree of improvementdepends on the quantity of cement used and the type ofsoil. Therefore, by the addition of varying amounts ofcement, it is possible to produce cement-stabilized subgradewith a wide range of engineering properties. Typical sevenday unconfined compressive strengths (UCS) for CSS rangefrom 100 to 300 psi (0.7 to 2.1 MPa).Chapter 1: Introduction1

CSS provides all the principal benefits of CMS inaddition to the following: Reduces moisture susceptibility and migration Improves bearing strength and increases CaliforniaBearing Ratio (CBR) Potentially allows for a reduction in pavement thicknessor increased pavement life May be used to support other infrastructure,including rails, airfields, parking lots, and loading/intermodal facilitiesCement-Treated Base (CTB)Cement-treated base (CTB) is a fully bound, compacted,engineered mixture of aggregate, water, and sufficientcement to meet the project-specified minimum durabilityand strength requirements. CTB can be mixed in placeusing on-site soils or mixed in a central plant or pugmillusing selected aggregate. Because of the better aggregateselection available for CTB, it typically uses about thesame quantity of cement as CSS; however, CTB resultsin a stronger, more durable, more frost-resistant layerwithin the pavement structure. Typical seven-day UCSfor CTB range from 300 to 600 psi (2.1 to 4.1 MPa).More detailed information about CTB can be found inthe PCA publication Guide to Cement-Treated Base (CTB)(Halsted et al. 2006).The principal benefits of CTB are as follows (Halsted etal. 2006): Provides a stiffer and stronger base than an unboundgranular base Requires a thinner section for roadway bases comparedto an unbound granular base Provides a moisture-resistant base Provides an erosion-resistant base Improves freeze/thaw durability Provides high strength, even when saturatedFull-Depth Reclamation (FDR)A special case of CTB, full-depth reclamation (FDR) is aprocess that involves pulverizing and blending an existingdistressed asphalt roadway surface and its underlyingbase and/or subgrade materials. Cement is mixed withthe pulverized material, compacted, and cured, resultingin a new homogenous and stabilized base. With respect2Guide to Cement-Stabilized Subgrade Soilsto performance, this method compares favorably tothe complete removal and replacement of a distressedasphalt pavement and underlying granular base material.More detailed information about the benefits, design,and construction of FDR can be found in the PCApublication Guide to Full-Depth Reclamation (FDR) withCement (Reeder et al. 2017).FDR provides all the principal benefits of CTB inaddition to the following: Increases structural capacity for existing roadwayscompared to non-stabilized sections Increases durability compared to granular basematerials Provides substantial cost savings compared to removaland replacement Reduces environmental impacts compared to removaland replacement Reduces truck traffic and improves safety compared toremoval and replacement Allows for faster construction time compared toremoval and replacementComparison of CMS, CSS, CTB, and FDRGiven that subgrade soils are an integral part of anypavement system design (Figure 1.1) and poor subgradesupport is the principal cause of pavement distress, thebenefits offered by any of the four cement treatmentsdescribed above can be crucial to long-term pavementperformance if undesirable subgrade soils are encounteredwithin project limits.CSS and CMSDespite their similarities, CMS and CSS are differenttreatments. The primary purpose of CSS is to improvethe engineering properties of the native in situ subgradesoil so that it behaves similarly to or better than anenhanced, untreated aggregate base with uniformsupport. While CMS also improves the properties ofthe existing subgrade, CSS provides all the benefits ofCMS in addition to reducing moisture susceptibility,permeability, volume change potential, and plasticity andimproving bearing strength. The improved strength thatCSS provides are often considered in pavement designanalysis, resulting in reduced thicknesses for the overlyingpavement courses.

CP Tech CenterFigure 1.1. Structural layers in flexible and rigid pavementsTo determine whether CSS or CMS is a viable candidatefor a project, a routine geotechnical investigation shouldbe performed during the planning stages to determinethe soil profile and various properties of the underlyingmaterial, including moisture content, plasticitycharacteristics, swell potential, and bearing capacity. Ifremediation is required, this information can also helpdetermine the amount of cement necessary to remedy thesoil. Chapter 2 discusses the recommended laboratorytests in greater depth.CSS and CTBThe main difference between CSS and CTB is thatCSS involves mixing cement into an existing finegrained material to improve the properties of theexisting subgrade soil, while CTB provides a higher endstructural base layer and usually, but not always, consistsof predominantly coarse-grained material. CTB is alsodesigned to have higher strengths than CSS and to beresistant to frost and erosion.Because CTB typically consists of select aggregate,it typically requires a similar or lower percentage ofcement compared to CSS in order to achieve the desiredstrength and durability properties. The percentage ofcement required for both CSS and CTB depends onthe type of soil or aggregate, the desired engineeringcharacteristics, and the design of the overall pavementsystem. For instance, a lower percentage of cementwould be required to provide the desired engineeringproperties for a subgrade or base material that consists ofa well-graded aggregate compared to a subgrade or basematerial that consists of a significant amount of clayeyor silty soil.In terms of applications, CSS alone may be used as thedirect support material for concrete pavements, while forflexible pavements an aggregate base is typically placedupon the CSS before placement of the pavement. LikeCSS, CTB may also be used as the direct support materialfor concrete pavements, though an interlayer of asphalt/aggregate base or a geofabric between the CTB and theconcrete may also be included. Unlike CSS, CTB may beused as the direct support material for flexible pavements;however, in such cases precautions, such as limiting thestrength of the CTB or incorporating microcracking intothe CTB, should be considered in order to reduce thepotential for reflective cracking in the

The primary purpose of this guide is to describe the characteristics, uses, and benefits of cement-stabilized subgrade (CSS) and present methods for geotechnical evaluation, mix design, construction, and field testing that will produce a satisfactory project. The material in this guide up