THE FULL-DEPTH RECLAMATION COMMITTEE OF THE ASPHALT RECYCLING & RECLAIMING ASSOCIATION (ARRA) PRESENTS SOIL STABILIZATION

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3 THE FULL-DEPTH RECLAMATION COMMITTEE OF THE ASPHALT RECYCLING & RECLAIMING ASSOCIATION (ARRA) PRESENTS SOIL STABILIZATION Driving on a Solid Foundation A S P H A LT R E C Y C L I N G & R E C L A I M I N G A S S O C I AT I O N SOIL STABILIZATION 1

4 ACKNOWLEDGEMENTS The Board of Directors of ARRA along with the Full-Depth Reclamation Committee gratefully acknowledge the contributions of all members involved with the production of this publication. This publication provides a reference on the state-of-the-practice in Soil Stabilization as it is used on various projects such as roadways, parking lots, airfields, landfills, and earthwork related to building construction. PREFACE This publication is not meant to be an exhaustive treatise on all aspects of Soil Stabilization. It should be supplemented with other authoritative sources such as AASHTO, ASTM, FHWA references and specifications, and specifications and procedures from various federal and state agencies as well as non-governmental agencies which utilize Soil Stabilization. Another source with a wealth of information to offer is ARRA, along with its many Contractor, Supplier, and Affiliate members who continue to perfect the many innovative rehabilitation processes available to specifying agencies around the world. This publication is designed to provide a comprehensive, though not exhaustive, overview of Soil Stabilization (1) processes; (2) additive selections; (3) project evaluation and design methodologies; and (4) quality control and quality assurance measures. Disclaimer The Asphalt Recycling & Reclaiming Association (ARRA) has taken every care and precaution during the preparation of this manual, recognizing that the state-of-the-art of soil stabilization is continually evolving. Mature engineering judgment and skill must be used to properly apply the information, guidelines, and principles contained within this manual, taking into account existing conditions, locally available materials, equipment, and expertise. ARRA can accept no responsibility for any defects or failures in the performance of any recycled materials, pavement structure, analysis or design results from an inappropriate application or use of information contained within this manual. The photographs, drawings, and/or processes used in the manual are for illustration purposes only. They do not imply preferential endorsement of any particular make, model, or process by ARRA. Copyright 2008, ARRA. All Rights Reserved. No part of this work may be reproduced in any form, or by any means, without the permission of ARRA. Design/Production by Carroll Design/ Printed in the USA 2 A C K N O W L E D G E M E N T S / P R E F A C E

5 TABLE OF CONTENTS INTRODUCTION What is Soil Stabilization? CHAPTER ONE The Soil Stabilization Process Portland Cement Lime Kiln Dust Class C Fly Ash Bituminous Materials CHAPTER TWO Stabilization Design Philosophy Subgrade Investigation and Sampling Stabilizer Evaluation Stabilizer Selection Summary A S P H A L T R E C Y C L I N G & R E C L A I M I N G A S S O C I A T I O N 3

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7 Soil Stabilization is the long-term physical and chemical alteration of soils to enhance their physical and engineering properties. Stabilization of in-place soils by incorporating available additives can increase the shear strength of a soil and/or control the shrink-swell properties of a soil, thus improving the load bearing capacity of a subgrade to support pavements and foundations. (ARRA) Stated more simply, the idea is to add something to a poor soil in order to turn it into a workable and better performing construction material a concept that has been around for a long time. In fact, the Romans as far back as 312 B.C. used a wide variety of readily available and inexpensive materials ranging from chalk and sand to powdered volcanic rock to improve the soils upon which they built the great Appian Way. They were so successful in their efforts that portions of the original Appian Way are still in use today. Fast-forward some 2,300 years, and we find that engineers have an even greater understanding of how a wide variety of stabilizing additives can be used to treat a wide range of inferior soils and unbound aggregates. These engineers also recognize that proper preconstruction design and testing is an important component of any stabilization effort as it allows for the determination of the proper additive and its amount to be used to achieve the desired engineering properties. BENEFITS OF THE STABILIZATION PROCESS CAN INCLUDE: Higher strength values including CBR, modulus, compressive strength, and resistance (R) values Increased load-spreading capabilities Reduction in plasticity Lower permeability Potential reduction of pavement thickness with proper design Utilization of in-place materials which eliminates excavation, material hauling and handling, and subgrade, subbase and/or base importation Improved compaction INTRODUCTION WHAT IS SOIL STABILIZATION? Providing for all-weather access onto and within projects sites Another form of soil treatment closely related to Soil Stabilization is Soil Modification, sometimes referred to as mud drying, soil conditioning, or soil amending. Modification is primarily intended to reduce moisture content and plasticity in order to expedite construction, whereas stabilization can substantially increase the shear strength of a material such that it can be incorporated into the project s structural design. SOIL STABILIZATION Appian Way S O I L S T A B I L I Z A T I O N I N T R O D U C T I O N 5

8 Equipment for the modification and stabilization processes typically includes the following, along with various other job-specific equipment: Additive spreaders/distributors Soil mixers (reclaimers) Portable pneumatic storage containers Water trucks Various sizes of padfoot/sheepsfoot and smooth drum compactors depending on lift thickness and project requirements Motor graders Through stabilization, projects are built more cost effectively, perform better, and ultimately last longer. With the equipment and additives available today, Soil Stabilization can be performed at depths up to 18 inches (450 mm) per lift. Because stabilization greatly improves the underlying materials, the result is often an overall reduction in the final surface materials, thereby helping to reduce the costs associated with typically the most expensive layer in a pavement structure. Through stabilization, projects are built more cost effectively, perform better, and ultimately last longer. 6 S O I L S T A B I L I Z A T I O N I N T R O D U C T I O N

9 Let s not overlook the sound environmental benefits that stabilization offers as well. By treating the soils found on the project, the amount of construction materials that need to be imported and/or removed and transported on local roads are reduced, with a subsequent reduction in damage to these roads, and a protection of natural resources through the reduction in use of virgin aggregate or soil sources. Vehicle emissions are also greatly reduced as less material hauling is required. Typical construction steps for Soil Stabilization are given below, although they may vary somewhat depending on the moisture content and composition of the in-place soil. Check and calibrate all equipment Shape the area to crown and grade Scarify, if necessary, to aid subsequent pulverization Distribute stabilizing additive in dry form with mechanical spreader or in slurry form from distributor truck equipped with agitation system Mix with traveling rotary mixer, adding water if necessary, until a homogeneous, friable mixture is obtained that will meet the specified pulverization requirements Compact stabilized material with appropriate compaction equipment Shape the area to final crown and grade Cure the stabilized soils as appropriate until the next pavement layer is constructed By treating the soils found on the project, the amount of construction materials that need to be imported and or removed and transported on local roads are reduced. Whether a project is large or small, urban or rural, complicated or simple, the reactions that occur when a specific additive is mixed into a specific type of soil are exactly the same today as they were during the days of the Roman Empire. S O I L S T A B I L I Z A T I O N I N T R O D U C T I O N 7

10 The goal of stabilization is to improve material properties for pavement or foundation design purposes or to overcome deficiencies in available construction materials. 8 T H E S O I L S T A B I L I Z A T I O N P R O C E S S

11 CHAPTER ONE THE SOIL STABILIZATION PROCESS The goal of stabilization is to improve material properties for pavement or foundation design purposes or to overcome deficiencies in available construction materials. Although ideal soils are sought during the design process, the reality of less than ideal soils throughout given project limits has brought about the need for ways to stabilize existing in-place materials. In the selection of a stabilizing additive, factors that must be considered are the type of soil to be stabilized, the purpose for which the stabilized layer will be used, the type of soil improvement desired, the required strength and durability of the stabilized layer, and the economic and environmental impacts. Due to the wide array of stabilizers available for Soil Stabilization, a detailed discussion of every one would not be possible in this brochure. This section will cover the more commonly used stabilizing disciplines with the intent of offering a good understanding of how these processes differ and compare to each other. PORTLAND CEMENT Incorporating portland cement into soils produces an engineered material most commonly referred to as soilcement. The best soil-cement product is the one best suited for its intended application modification or stabilization. Problems can occur during construction when silt and clay soils are encountered, particularly when wet, as they can be soft, plastic, and difficult to compact. Cement-Modified Soil (CMS) is used to improve the engineering properties and construction characteristics of silt and clay soils by reducing the plasticity and enhancing the compaction and strength The best soil-cement of the material. With 1 to 3% cement (by dry weight of soil) used to product is the one best modify the soil, the final product is an improved construction material. suited for its intended BENEFITS OF CMS INCLUDE: Improved constructability of marginal on-site soils application modification Reduced plasticity and improved strength or stabilization. Less susceptible to damaging effect of water An all-weather work platform Use of on-site soil rather than removal and replacement with expensive select fill material Soil-cement is also a commonly used term that applies to all hardened cement-stabilized materials that meet the project specified minimum durability and strength requirements. However, these materials use more cement than CMS resulting in a strong, durable, frost resistant layer for the pavement structure. Typical cement contents for mixed-in-place cement-stabilized materials range from 2 to 8%, resulting in recommended 7-day unconfined compressive strengths ranging from 200 up to 500 psi (1.4 to 3.4 MPa). SOIL STABILIZATION T H E S O I L S T A B I L I Z A T I O N P R O C E S S 9

12 LIME Adding lime to soil can have a number of beneficial effects, both during construction and over the life of the pavement or soil structure. LIME CAN EFFECTIVELY BE USED FOR: Soil Drying reducing the amount of moisture in the soil Soil Modification reducing the soil plasticity, aiding compaction, and increasing the soil strength, primarily for construction purposes Soil Stabilization increases in strength, plasticity reduction, and reduces expansion characteristics Lime Modification rapidly and significantly improves soil workability by reducing soil plasticity, making the soil more friable and easier to compact. Because lime chemically combines with water, it can be used very effectively to dry wet soils often within a matter of hours. The dry-up of wet soil at construction sites is one of the widest uses of lime for soil. Lime Modification rapidly and significantly improves soil workability by reducing soil plasticity, making the soil more friable and easier to compact. Lime modified soil is much stronger than untreated soil and can provide an all-weather working platform. Lime Modification works best in clay soils and may or may not be permanent. Between 1 and 4% lime (by dry weight of soil) is typically used for Soil Drying and Soil Modification. Lime Stabilization changes the characteristics of a wide range of soils to produce permanent long-term strength, plasticity reduction, and expansion reduction. Fine-grained soils containing clay are good candidates for subgrade and base stabilization, generally using 2 to 6% lime (by dry weight of soil). 10 T H E S O I L S T A B I L I Z A T I O N P R O C E S S

13 KILN DUST Cement Kiln Dust (CKD) and Lime Kiln Dust (LKD) are fine particles captured in the rotary kilns used to manufacture portland cement and lime. These particles are mixtures of dust from the limestone fed into the kilns and fly ash from the coal used to heat the kilns. Due to the presence of calcium oxide and other cementitious and pozzolanic ingredients, particle fineness, and subsequent ability to harden upon exposure to moisture, kiln dusts have been used successfully as binders in soil modification and stabilization. Soils with high moisture contents can be dried effectively and economically with kiln dust. Soils with high moisture contents can be dried effectively and economically with kiln dust. These materials react with water, chemically absorbing excess moisture while also producing heat that causes evaporation. Simply mixing a small percentage of kiln dust into wet and unworkable soils will bring the soil characteristics into specification, thereby eliminating the need to bring in fill material, typically resulting in substantial savings. Kiln dusts also work well as soil stabilizers. When clay soils are combined with kiln dust, the result is a more granular soil that has reduced plasticity and sensitivity to moisture changes, increased durability to wet-dry and freeze-thaw cycles, and increased compressive and load-bearing strength. Depending on their chemical composition which vary depending upon the kiln from which they were derived kiln dusts may be used alone or in combination with other additives in stabilized base mixtures, typically at rates from 3 to 10% (by dry weight of soil). When using kiln dusts in combination with other additives it is important to quantify each specific additive to ensure adherence to the project mix design. T H E S O I L S T A B I L I Z A T I O N P R O C E S S 11

14 CLASS C FLY ASH The self-cementing properties of sub-bituminous coal fly ashes (Class C fly ash) can be used effectively in both Soil Modification and Soil Stabilization applications. These applications include drying of soils to facilitate compaction and to reduce shrink-swell potential of high plasticity clays. Not to be confused with Class F fly ash which alone does not have cementitious properties, Class C fly ash has also been used to improve shear strength and subgrade support, and to reduce compressibility of both cohesive soils and granular materials. As a drying agent, Class C fly ash can reduce the moisture contents of soils to facilitate mechanical compaction. The reduction in moisture content occurs rapidly and moisture contents can be adjusted to the desired range by varying the amount of ash incorporated. Since the drying effect is rapid, this method can be of benefit when adverse weather conditions prevent moisture reduction by aeration or replacement with drier soils. As a stabilizing additive, Class C fly ash creates a more stable section for paving operations and reduces the potential for subgrade damage due to construction traffic or adverse weather conditions. As a stabilizing additive, Class C fly ash creates a more stable section for paving operations and reduces the potential for subgrade damage due to construction traffic or adverse weather conditions. The cementitious properties of a particular Class C fly ash depend upon the mineralogy of both the ash and the soil being stabilized. Typical Class C fly ash application rates range from 6 18% (by dry weight of soil). At the higher application rates, the resulting 7-day unconfined compressive strengths can approach 500 psi (3.4 MPa) or more. 12 T H E S O I L S T A B I L I Z A T I O N P R O C E S S

15 EXPANSION CHAMBER Hot Bitumen Cold Water Foamed Bitumen BITUMINOUS MATERIALS Bitumen stabilized materials create a flexible and fatigue resistant pavement layer. The addition of bituminous stabilizing additives is a proven cost-effective way of improving the strength and controlling the moisture of inferior in-place subgrade or subbase soils. In addition, bituminous stabilization can be combined with small amounts of portland cement or lime to decrease cure time, ameliorate moisture sensitivity (stripping) problems, and improve retained strength characteristics. Asphalt emulsions have been used to stabilize subgrade and subbase materials. This water based additive mixes and binds easily with soils and aggregates, and the cured asphalt increases the material s load bearing capacity, stiffness, and resistance to weathering. Materials for emulsion stabilization include gravel, sand, and silt. The amount of bitumen used will vary with the material type, local conditions, and intended use, and will normally range from 1 to 10% (by dry weight of soil). Another technique used to incorporate bitumen into in-place subgrade and subbase material is expanded (foamed) asphalt. Asphalt foaming occurs when small amounts of water come into contact with hot asphalt (see figure 1). Each tiny foam bubble is a carrier of a thin film of asphalt. Expanded asphalt material can be placed, shaped, compacted and opened to traffic immediately after mixing and remains workable for extended periods of time if kept moist. Figure 1 The addition of bituminous stabilizing additives is a proven cost-effective way of improving the strength and controlling the moisture of inferior in-place subgrade or subbase soils. T H E S O I L S T A B I L I Z A T I O N P R O C E S S 13

16 The design procedures for stabilized materials require thorough investigation and knowledge of both the pavement material to be stabilized and the stabilizing additives available. 14 S T A B I L I Z A T I O N D E S I G N P H I L O S O P H Y

17 There are two main components essential to the design and proper performance of stabilized subgrade soil materials the mix design prepared for the stabilized subgrade and the structural design of the pavement under which the stabilized subgrade will be incorporated. These two factors are interrelated, as the performance of the stabilized subgrade is dependent upon the thickness and composition of the pavement structure under which it will be used. Likewise, the structural design of the pavement system depends on the structural characteristics of the stabilized subgrade onto which it will be placed. The design procedures for stabilized subgrade materials require thorough investigation and knowledge of both the subgrade material to be stabilized and the stabilizing additives available. The structural design of pavements incorporating stabilized layers should be done in accordance with pavement design procedures recommended by the governing project authority. SUBGRADE INVESTIGATION AND SAMPLING The following information should be reviewed whenever a project location is being evaluated for subgrade stabilization work: The material characteristics including soil classification, Atterberg limits, and moisture content The uniformity of materials The moisture content of materials The presence of any culverts or drainage structures The presence of any utility fixtures (both in ground and overhead) Drainage problems CHAPTER TWO STABILIZATION DESIGN PHILOSOPHY It is important in any type of stabilization work that the materials are thoroughly examined and their interactions with the specific additive to be used in the stabilization work be properly supported by laboratory analysis and testing before any actual field work begins. Although a review of previous construction documents or history can provide useful information, actual project samples are essential for proper evaluation. STABILIZER EVALUATION In order to obtain some preliminary information regarding the type of soil stabilizer required for a particular subgrade material, particle size distribution and Atterberg limits are frequently used. The commonly accepted range of suitability of various types of stabilizers is often based on the percentage of material passing the No. 200 (0.075 mm) sieve and the plasticity index (PI) of that material. This approach simply provides a guide for a more detailed analysis with particular materials and particular stabilizers. SOIL STABILIZATION S T A B I L I Z A T I O N D E S I G N P H I L O S O P H Y 15

18 Table 1 The information provided in Table 1 gives initial guidance in the evaluation of a stabilizer type. Remember, this information should be taken as a broad guideline only! For further information, it is recommended that reference be made to trade literature and published research. Once the decision is made regarding the proper stabilizer(s) for a project, a mix design should be performed by a qualified Geotechnical Engineer to help ensure success. ASTM test procedures are available to help determine the stabilizer amount necessary for achieving adequate structural strength when added to in-place materials. Climatic conditions have a significant impact on the evaluation of stabilizing additives. For instance, in wetter areas where the moisture content of the in-place materials is high, it is important to ensure that the retained strength of the stabilized materials is acceptable and susceptibility to moisture variations is low. The recommended atmospheric and material temperatures and conditions for properly stabilizing soils are governed by the type of stabilizer being used. STABILIZER SELECTION Upon completion of all analysis and testing when making the final selection of a stabilizer for a particular project there may be a number of acceptable chemicals or admixtures that should react favorably with the project s soil conditions. The reality is that the determination that more than one additive will result in acceptable final desired project requirements is the case for most soil modification and stabilization projects. Climatic conditions have a significant impact on the evaluation of stabilizing additives. 16 S T A B I L I Z A T I O N D E S I G N P H I L O S O P H Y

19 In addition, there are cases where two or more stabilizers can be used together to improve the soil characteristics for a given project. The use of combinations, or blends, of the additives mentioned in this brochure is quite common. As with all stabilization additives, when using pre-blended combinations, care should be taken to ensure that proper ratios and consistency are maintained and that the specific additives can be individually quantified to ensure they meet the project mix design. Other stabilizing additives including slag, chlorides, microbes, enzymes, and numerous proprietary products have also been used with varying degrees of success to stabilize problem soils either alone or in combination. In many cases, the decision of which stabilizer to use is generally a financial one, especially when initial construction costs and life cycle costs of all the considered alternatives are compared. Other things to consider are the past performance of a chosen stabilizer in similar work as well as the availability of materials, equipment, and qualified construction personnel. In many cases, the decision as to which stabilizer to use is generally a financial one, especially when initial construction costs and life cycle costs of all the considered alternatives are compared. S T A B I L I Z A T I O N D E S I G N P H I L O S O P H Y 17

20 When faced with tight construction schedules, dwindling budgets, and problematic soils, engineers today are increasingly turning to soil stabilization as a viable solution. 18 S U M M A R Y

21 When faced with tight construction schedules, dwindling budgets, and problematic soils, engineers today are increasingly turning to soil stabilization as a viable solution. As discussed in this brochure, selecting the right stabilizing additive for the job is the top priority. Factors to be considered include: Type of material to be stabilized Proposed use of the stabilized material Availability of suitable equipment, materials, and personnel Relative costs SUMMARY Applying the correct additive through modification or stabilization can improve the engineering properties of the less than perfect subgrade material. Significant economic benefits are gained by accelerated construction times; by decreasing the amount of materials to be excavated and trucked off site; and limiting the amount of select fill (soil or granular) to be imported onto the site. With proper design, savings can also be realized through the possibility of reducing the thickness of the pavement materials and/or aggregate base section thickness. The degree of need for stabilization will vary greatly and should be evaluated on a case-by-case basis. Looking backwards, we can truly see that All Roads Lead to Rome when it comes to the historical application of soil stabilization to overcome construction challenges. This knowledge, coupled with improved additives and innovative construction personnel, will certainly ensure that we will continue to be Driving on a Solid Foundation for many years to come. SOIL STABILIZATION S U M M A R Y 19

22 NOTES 20 A S P H A L T R E C Y C L I N G & R E C L A I M I N G A S S O C I A T I O N

23 A S P H A LT R E C Y C L I N G & R E C L A I M I N G A S S O C I AT I O N ARRA is a non-profit international trade association of contractors, equipment manufacturers, suppliers, public officials, and engineers, engaged in the recycling and reclaiming of asphalt and other roadway materials working to build a stronger and safer network of highways, streets, and roads across the country and around the world. Soil Stabilization Driving on a Solid Foundation, was developed by the Full-Depth Reclamation Technical Subcommittee of the Committee on Recycling Education, and has been based on the collective experience and knowledge of its participating members. This brochure may be amended from time to time to reflect the latest in state-of-the-art technology. Additional copies of this and other ARRA publications can be obtained by contacting the Association headquarters, or by visiting us online at ASPHALT RECYCLING & RECLAIMING ASSOCIATION #3 Church Circle PMB 250 Annapolis, MD USA Voice: Fax: krissoff@arra.org SSM 2/08-10M A S P H A L T R E C Y C L I N G & R E C L A I M I N G A S S O C I A T I O N

24 ASPHALT RECYCLING & RECLAIMING ASSOCIATION #3 Church Circle PMB 250 Annapolis, MD USA Voice Fax