17 - UFC 3-250-11 Soil Stabilization And Modifications For Pavements.pdf

UFC 3-250-11

30 November 2020

UNIFIED FACILITIES CRITERIA (UFC)

SOIL STABILIZATION AND

MODIFICATION FOR PAVEMENTS

APPROVED FOR PUBLIC RELEASE; DISTRIBUTION UNLIMITED

SOIL STABILIZATION AND MODIFICATION FOR PAVEMENTS

Any copyrighted material included in this UFC is identified at its point of use.

Use of the copyrighted material apart from this UFC must have the permission of the copyright holder.

Use of the name or mark of any specific manufacturer, commercial product, commodity, or service in this UFC does not imply endorsement by the Department of Defense.

U.S. ARMY CORPS OF ENGINEERS (Preparing Activity)

NAVAL FACILITIES ENGINEERING COMMAND

AIR FORCE CIVIL ENGINEER CENTER

Record of Changes (changes are indicated by \1\ ... /1/)

Change No. Date Location

This UFC supersedes UFC 3-250-11, Soil Stabilization for Pavements, dated January 16, 2004.

FOREWORD

The Unified Facilities Criteria (UFC) system is prescribed by MIL-STD 3007 and provides planning, design, construction, sustainment, restoration, and modernization criteria, and applies to the Military Departments, the Defense Agencies, and the DoD Field Activities in accordance with USD (AT&L) Memorandum dated 29 May 2002. UFC will be used for all DoD projects and work for other customers where appropriate. All construction outside of the United States is also governed by Status of Forces Agreements (SOFA), Host Nation Funded Construction Agreements (HNFA), and, in some instances, Bilateral Infrastructure Agreements (BIA).

Therefore, the acquisition team must ensure compliance with the most stringent of the UFC, the SOFA, the HNFA, and the BIA, as applicable.

UFC are living documents and will be periodically reviewed, updated, and made available to users as part of the Services’ responsibility for providing technical criteria for military construction. Headquarters, U.S. Army Corps of Engineers (HQUSACE), Naval Facilities Engineering Command (NAVFAC), and Air Force Civil Engineer Center (AFCEC) are responsible for administration of the UFC system. Defense agencies should contact the preparing Service for document interpretation and improvements. Technical content of UFC is the responsibility of the cognizant DoD working group. Recommended changes with supporting rationale may be sent to the respective DoD working group by submitting a Criteria Change Request (CCR) via the Internet site listed below.

UFC are effective upon issuance and are distributed only in electronic media from the following source:

• Whole Building Design Guide web site http://www.wbdg.org/ffc/dod.

Refer to UFC 1-200-01, DoD Building Code, for implementation of new issuances on projects.

AUTHORIZED BY:

CHRISTINE T. ALTENDORF, PhD, P.E., SES

R. DAVID CURFMAN, P.E., SES

Chief Engineer

Chief, Engineering and Construction Naval Facilities Engineering Command U.S. Army Corps of Engineers

NANCY J. BALKUS, P.E., SES

Deputy Director of Civil Engineers DCS/Logistics, Engineering & Force Protection (HAF/A4C) HQ United States Air Force

MICHAEL McANDREW Deputy Assistant Secretary of Defense (Construction) Office of the Assistant Secretary of Defense (Sustainment) http://www.wbdg.org/pdfs/ufc_implementation.pdf http://www.wbdg.org/ffc/dod

REVISION SUMMARY SHEET

Document: UFC 3-250-11, Soil Stabilization and Modification for Pavements

Superseding: This UFC supersedes UFC 3-250-11, Soil Stabilization for Pavements, dated January 16, 2004.

Description: This UFC documents the standard practice of stabilizing and modifying soil for use in pavement and operating surfaces. It provides guidance for improving the engineering properties of soils used for pavement base courses, subbase courses, and subgrades by the use of additives mixed into the soil to effect the desired improvement.

This UFC is also applicable to roads, airfields, and construction platforms having a stabilized surface layer.

Reasons for Document: This update brings the document in compliance with UFC 1- 300-01, Criteria Format Standard. Editorial changes were made to improve readability, correct typographical errors, and update outdated references.

Impact: Cost impact is negligible; improved guidance typically results in improved performance and reduced lifecycle cost.

Unification Issues: None

XX XXX 2020

i

CONTENTS

CHAPTER 1 INTRODUCTION

1-1 PURPOSE

1-2 SCOPE

1-3 APPLICABILITY

1-4 USES OF STABILIZATION/MODIFICATION

1-4.1 Quality Improvement

1-4.2 Thickness Reduction

1-4.3 Operating Surfaces

1-5 DEFINITIONS

1-6 REFERENCES

CHAPTER 2 SELECTION OF ADDITIVE

2-1 FACTORS TO CONSIDER

2-1.1 Soil Types and Additives

2-1.2 Screening Tests for Organic Matter and Sulfates

2-1.3 Traditional Additives

2-1.4 Non-Traditional Additives

2-1.5 Selection of Candidate Additives

2-2 USE OF STABILIZED SOILS IN FROST AREAS

2-2.1 Frost Considerations

2-2.2 Limitations

2-2.3 Construction Cutoff Dates

2-3 THICKNESS REDUCTION FOR BASE AND SUBBASE COURSES

2-4 DRYING WET SOILS

2-4.1 Chemicals

2-4.2 Lime

2-4.3 Amounts

2-4.4 Factors

CHAPTER 3 DETERMINING STABILIZER CONTENT

3-1 STABILIZATION WITH PORTLAND CEMENT

3-1.1 Types of Portland Cement

3-1.2 Water for Hydration ii

3-1.3 Gradation Requirements

3-1.4 Cement Content for Modifying Soils

3-1.5 Cement Content for Stabilized Soil

3-2 STABILIZATION WITH LIME

3-2.1 Types of Lime

3-2.2 Gradation Requirements

3-2.3 Lime Content for Lime-Modified Soils

3-3 STABILIZATION WITH LIME-FLY ASH (LF) AND LIME-CEMENT-FLY

ASH (LCF)

3-3.1 Types of Fly Ash

3-3.2 Evaluation of Fly Ash

3-3.3 Gradation Requirements

3-3.4 Selection of Lime-Fly Ash Content for LF and LCF Mixtures

3-3.5 Selection of Cement Content for LCF Mixtures

3-4 STABILIZATION WITH BITUMEN AND POLYMER EMULSION

3-4.1 Types of Bituminous-Stabilized Soils

3-4.2 Types of Bitumen

3-4.3 Soil Gradation

3-4.4 Mix Design

3-4.5 Polymer Emulsion

3-5 STABILIZATION WITH LIME-CEMENT AND LIME-BITUMEN

3-5.1 Lime-Cement

3-5.2 Lime-Asphalt

3-5.3 Lime Treatment of Expansive Soils

3-6 STABILIZATION WITH FIBERS

CHAPTER 4 CONSTRUCTION PROCEDURES

4-1 CONSTRUCTION WITH PORTLAND CEMENT

4-1.1 General Construction Steps

4-1.2 Mixing Equipment

4-1.3 Equipment for Handling and Spreading Cement

4-1.4 Construction

4-1.5 Special Construction Problems

4-2 CONSTRUCTION WITH LIME

4-2.1 Lime Stabilization Methods iii

4-2.2 Construction Steps

4-3 CONSTRUCTION WITH LIME-FLY ASH (LF) AND LIME-CEMENT-FLY

ASH (LCF)

4-4 CONSTRUCTION WITH BITUMEN OR POLYMER EMULSION

4-4.1 Equipment for Mixed-in-Place Materials

4-4.2 Mixed-in-Place Construction

4-4.3 Central Plant Mix Construction

4-5 CONSTRUCTION WITH FIBERS

CHAPTER 5 QUALITY CONTROL

5-1 PURPOSE

5-2 CEMENT STABILIZATION

5-2.1 Pulverization

5-2.2 Cement Content

5-2.3 Moisture Content

5-2.4 Uniformity of Mixing

5-2.5 Compaction

5-2.6 Curing

5-3 LIME STABILIZATION

5-3.1 Pulverization and Scarification

5-3.2 Lime Content

5-3.3 Uniformity of Mixing

5-3.4 Compaction

5-3.5 Curing

5-4 LIME-FLY ASH (LF) AND LIME-CEMENT-FLY ASH (LCF)

5-5 BITUMINOUS AND POLYMER STABILIZATION

5-5.1 Moisture Content

5-5.2 Viscosity of Asphalt

5-5.3 Asphalt and Polymer Content

5-5.4 Uniformity of Mixing

5-5.5 Aeration

5-5.6 Compaction

5-5.7 Curing

APPENDIX A BEST PRACTICES

A-1 THICKNESS CRITERIA-STABILIZED SOIL LAYERS

iv

A-2 PH TEST ON SOIL-CEMENT MIXTURES

A-3 DETERMINATION OF SULFATE IN SOILS - GRAVIMETRIC AND

TURBIDIMETRIC METHOD

APPENDIX B GLOSSARY

B-1 ACRONYMS

B-2 TERMS

APPENDIX C REFERENCES

FIGURES

Figure 2-1 Gradation Triangle for Aid in Selecting Commercial Stabilizing Agent .. 8

Figure A-1 Example Standard Curve for Spectrophotometer

TABLES

Table 2-1 Minimum Unconfined Compressive Strength for Cement-, Lime-, Lime- Cement, and Lime-Cement-Fly Ash-Stabilized Soils

Table 2-2 Durability Requirements

Table 2-3 Guide for Selecting a Stabilizing Additive

Table 3-1 Gradation Requirements for Cement-Stabilized Base and Subbase Courses

Table 3-2 Cement Requirements for Various Soils

Table 3-3 Recommended Gradations for Bituminous-Stabilized Subgrade Materials

Table 3-4 Recommended Gradations for Bituminous-Stabilized Base and Subbase Materials

Table 3-5 Emulsified Asphalt Requirements

Table 3-6 Swell Potential of Soils

Table A-1 Equivalency Factors for Stabilized Material

CHAPTER 1 INTRODUCTION

1-1 PURPOSE.

This UFC documents the standard practice for improving the engineering properties of soils used for operating surfaces, pavement base courses, subbase courses, and subgrades by the use of additives mixed into the soil to effect the desired improvement. It also provides criteria and guidance for the stabilization and modification of soils for pavements, operating surfaces, and construction platforms. This UFC is also applicable to roads, airfields, and construction platforms having a stabilized surface layer. These soil improvements consist of applying a chemical such as lime, cement, or bitumen to a layer of soil or aggregate that will be included within a pavement area. Stabilization or modification measures discussed herein are useful to limit expansive soil’s ability to expand and contract due to changes in moisture content, increase the soil’s shear strength to prevent or minimize pumping, or to bind soils or aggregates.

1-2 SCOPE.

This UFC prescribes the appropriate type(s) of additive to be used with different soil types, procedures for determining a design treatment level for each type of additive, and standard construction practices for incorporating the additive into the soil. This UFC provides criteria and guidance on stabilization or modification using portland cement, lime, lime-fly ash, lime-cement-fly ash, bitumen, polymer, lime-cement, lime-bitumen, or geo-synthetic fibers.

1-3 APPLICABILITY.

Soil stabilization and modification are optional. The installation, in conjunction with the designers of record, determines whether to include soil stabilization or modification in the pavement design. Apply the requirements in this UFC when soil stabilization or modification is included in the design. These requirements are dependent upon each individual site’s soil conditions.

1-4 USES OF STABILIZATION/MODIFICATION.

Pavement design is based on the premise that the specified structural quality will be achieved for each layer of material in the pavement system. Each layer resists shearing, avoids excessive deflections that cause cracking within the layer or in overlying layers, and prevents excessive permanent deformation. As the quality of a soil layer is increased, the ability of that layer to distribute the load over a greater area is increased so a reduction in the required thickness of the soil and surface layers is achieved.

1-4.1 Quality Improvement.

The most common improvements achieved through stabilization/modification include better soil gradation, reduced plasticity index (PI) or swelling potential, and increased durability and strength. Stabilization may be used to provide a working platform for construction.

1-4.2 Thickness Reduction.

The strength of a soil layer can be improved through the use of additives to permit a reduction in design thickness of the stabilized material compared with an unstabilized or unbound material. Procedures for designing and evaluating pavements that include stabilized soils are presented in UFC 3-250-01, Paving Design for Roads and Parking Areas, UFC 3-250-03, Standard Practice Manual for Flexible Pavements, and UFC 3-260-02, Pavement Design for Airfields. The design thickness of a base or subbase course can be reduced if the stabilized material meets the specified gradation, strength, stability, and durability requirements indicated in this UFC for the particular type of material. See Appendix A for examples of reducing design thickness when using stabilized materials.

1-4.3 Operating Surfaces.

Stabilized materials can be used to construct operating surfaces. These cases are typically for temporary construction platforms, roads, or airfields where poor soils exist and conventional pavements cannot be feasibly constructed. For roads and airfields, adhere to the strength, durability, and construction criteria in this UFC.

1-5 DEFINITIONS.

Appendix B contains a list of acronyms and definitions.

1-6 REFERENCES.

Appendix C contains a list of references used in this document. The publication date of the code or standard is not included in this document. Unless otherwise specified, the most recent edition of the referenced publication applies.

CHAPTER 2 SELECTION OF ADDITIVE

2-1 FACTORS TO CONSIDER.

In the selection of a stabilizer, the factors to consider 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, the type and availability of the additive, the cost, and environmental conditions.

2-1.1 Soil Types and Additives.

There may be more than one candidate stabilizer applicable for an individual soil type;

however, there are general guidelines that make specific stabilizers more desirable based on soil granularity, plasticity, or texture. Portland cement, for example, is used with a variety of soil types; however, since it is imperative the cement be mixed intimately with the fines fraction (<

0.074 mm), avoid plastic soils unless steps are taken to reduce the plasticity. Generally, well-graded granular materials that possess fines to produce homogenous mixtures are best suited for portland cement stabilization. Lime will react with soils of medium to high plasticity to lower its plasticity, increase workability, reduce swell potential, and increase shear strength. In some cases, adding lime prior to adding soil cement will improve the workability of the soil cement.

Lime is used to stabilize a variety of materials, including weak subgrade soils, transforming them into a "working table" or subbase; with marginal granular base materials (i.e., clay-gravels and "dirty" gravels) lime forms a strong, high-quality base course. Fly ash is a pozzolanic material—it reacts with lime and is therefore almost always used in combination with lime in soils that have little or no plastic fines. The use of portland cement with lime and fly ash for added strength is often required. This combination of lime-cement-fly ash (LCF) has been used successfully in base course stabilization. Asphalt, bituminous, or polymer materials are used for waterproofing and gaining strength. Generally, soils suitable for asphalt or polymer stabilization are silty sands and granular materials because thoroughly coating all the soil particles is desirable. Fibers may be used in combination with some additives to improve strength and crack resistance, but they severely limit workability after mixing with soil.

Do not create a solid, rigid, or brittle surface that will lead to reflective cracking of the surface pavement when designing stabilized base and subbase layers for pavements. Each type of stabilization method has limitations and appropriate uses; therefore, create and test a representative mix design for each individual scenario of the planned field operations. Evaluate each stabilized soil layer to establish the appropriate thickness of stabilized soil to prevent undesired effects to the finished pavement surfaces. Include a bond breaker when overlaying a stabilized layer using portland cement.

2-1.1.1 Portland Cement Stabilization.

Cement stabilization utilizes mixing dry portland cement powder into the subbase soils to increase the shear strength within the treated layer. Typically, this is accomplished by using a dosage rate of 3 to 6 percent by weight portland content and mixing it approximately 1 to 2 feet (0.3 to 0.6 m) in depth. These are general estimations; the procedure to determine the dosage rate is described in paragraph 3-1 and its subparagraphs. Use layered elastic pavement design procedures described in UFC 3-260-02 to determine the depths to which mixing must occur.

2-1.1.2 Lime Stabilization or Modification.

Lime modification utilizes mixing dry hydrated lime (powder) or quicklime (granulated) into the subgrade/subbase soils to lower the PI of the soil to control the shrink/swell potential of the soil. Lime stabilization utilizes mixing lime or quicklime into the subgrade, select fill, and or subbase material to increase the pH to generate a pozzolanic reaction that increases the soil’s shear strength. This requires fine-grained soils with a minimum fraction of silt and clay to be reactive and is not effective on granular materials. Typically, lime modification is performed on the upper layer of subgrade, select fill, and/or subbase and mixing is approximately 0.75 to 2 feet (0.2 to 0.6 m) in depth. These are general estimations; the procedure to determine the dosage rate is described in paragraph 3-2 and its subparagraphs. Use layered elastic pavement design procedures described in UFC 3-260-02 to determine the depths to which mixing must occur.

2-1.1.3 Lime-Fly Ash or Lime-Cement-Fly Ash Stabilization.

Fly ash acts similarly to portland cement and is used to reduce the quantity of lime or portland cement necessary to achieve the desired stabilization/modification effect. Fly ash is a cheaply obtained industrial byproduct used for cost reduction and can stabilize more coarse gradations than lime alone. Fly ash physical and chemical properties can vary widely. Investigate each locally available product for its applicability to the specific project. Additional information is in paragraph 3-3.

2-1.1.4 Bitumen or Polymer Stabilization.

Bitumen and polymer stabilization increases adhesive properties, the shear strength of the soils, and decreases the effects of water on the treated soils. Bitumen and polymers are not effective against freeze-thaw action and testing the durability of the finished product is not possible. Additional information is in paragraph 3-4.

2-1.1.5 Lime-Cement or Lime-Bitumen Stabilization.

A lime-cement and lime-bitumen combination is used primarily when lime is needed to control the shrink/swell potential of soils and the treated soil has a low shear strength. Portland cement-bitumen is used to increase the shear strength to a desired minimum strength.

Bitumen and polymers are not effective against freeze-thaw action and testing the durability of the finished product is not possible. Additional information is in paragraph 3-5.

2-1.1.6 Geosynthetic Fiber Stabilization.

Geosynthetic fiber stabilization increases the shear strength of the soils. It is not effective in controlling shrink/swell characteristics of soils. Installation of the fibers can be problematic in the field. It requires material-specific experience and attentive quality control and quality assurance to ensure proper construction. Additional information is in paragraph 3-6.

2-1.2 Screening Tests for Organic Matter and Sulfates.

The presence of organic matter and/or sulfates frequently has deleterious effects on stabilized soil or the additives used, preventing the stabilization process from occurring. Perform testing for detection of these materials if their presence is suspected.

2-1.2.1 Organic Matter.

A soil may be acidic, neutral, or alkaline and still respond well to cement treatment. Although certain types of organic matter, such as undecomposed vegetation, may not adversely influence stabilization, organic materials such as humic acid act as hydration retarders and reduce strength. When such organics are present, they inhibit the normal hardening process. A pH test to determine the presence of organic material is presented in Appendix A. If the pH of a 10:1 mixture (by weight) of soil and cement 15 minutes after mixing is at least 12.0, it is probable that any organics present will not interfere with normal hardening. Refer to ACI 230.1R, Report on Soil Cement, for additional guidance. Limit soil organic content to a maximum of 2 percent by weight and a minimum pH of 5.3 or more prior to the addition of soil cement, lime, or fly ash.

2-1.2.2 Sulfates.

Cements in contact with sulfates can generate a reaction that is expansive in nature and can damage aboveground, below ground, and nearby facilities if present with cement, lime, or fly ash-stabilized soils. The resistance to sulfate attack differs for cement-treated coarse-grained and fine-grained soils and is a function of sulfate concentrations. If sulfate concentration exceeds 0.1 percent by weight do not use cement or fly ash stabilization without prior authorization by the Pavements Discipline Working Group (DWG). Use the procedure in Appendix A or the current version of ASTM C1580, Standard Test Method for Water-Soluble Sulfate in Soil, to determine the concentrations of sulfate in the existing soil and groundwater.

Do not design, specify, construct, or install soil cement, lime, or fly ash for soils containing more than 0.1 percent sulfate.

2-1.3 Traditional Additives.

2-1.3.1 Cement.

Portland cement can be used either to modify and improve the quality of the soil or transform the soil into a cemented mass with increased strength and durability. Cement can be used effectively as a stabilizer for a wide range of materials; however, the soil should have a PI less than 30 or the stabilization effects will be minimal. For coarse-grained soils, the amount passing the No. 4 (4.75 mm) sieve should be greater than 45 percent or the stabilization effects will be minimal. The amount of cement used depends on whether the soil is to be modified or stabilized.

When designing, specifying, or constructing using cement to stabilize base material for support of a rigid pavement, a bond breaker between the pavement concrete and the cement-stabilized base material to prevent adhesion at that interface is required. Adhesion of the concrete pavement to the cement-stabilized base results in unacceptable cracking. An asphalt emulsion bond breaker is not recommended because once the emulsion begins to dry and harden it does not perform as a bond breaker and instead causes further adherence problems. To create a bond break, install a layer of #89 stone meeting ASTM C33, Standard Specification for Concrete Aggregates, specifications between 0.25 and 0.5 inches (6.3 and 12.7 mm) in thickness. Then install a fabric meeting the requirements of AASHTO M288, Standard Specification for Geosynthetic Specification for Highway Applications, class I fabric with elongation less than 50 percent at the specified strengths, or install a double layer of liquid membrane-forming compound on the stabilized base prior to the placement of concrete. Do not design, specify, or install a layer of plastic or rubberized sheet material as a bond breaker without prior approval of the Pavements DWG. Do not design, specify, or construct PCC pavement slabs with joint spacing greater than 13 feet (4 m) over cement-stabilized base.

2-1.3.2 Lime.

Experience shows that lime will react with many medium-, moderately fine-, and fine-grained soils to produce decreased plasticity, increased workability, reduced swell, and increased strength. Consider using lime to stabilize soils classified according to the Unified Soil Classification System (USCS) as CH, CL, MH, ML, OH, OL, SC, SM, GC, GM, SW-SC, SP- SC, SM-SC, GW-GC, GP-GC, ML-CL, and GM-GC. Consider lime with all soils having a PI greater than 12 and more than 25 percent of the soil passing the No. 200 (0.075 mm) sieve.

Do not design, specify, or construct lime-stabilized subgrade, subbase, or base soils with a sulfate content greater than 0.1 percent.

2-1.3.3 Fly Ash.

Fly ash, when mixed with lime, can effectively stabilize most coarse- and medium-grained soils; however, the PI cannot be greater than 25 and remain effective. Soils classified by the USCS as SW, SP, SP-SC, SW-SC, SW-SM, GW, GP, GP-GC, GW-GC, GP-GM, GW-GM, GC-GM, and SC-SM can be stabilized with fly ash. Some sources of fly ash may contain high amounts of sulfates. In some soils, this may lead to the formation of ettringite, which may swell excessively. Do not design, specify, or construct fly ash-stabilized subgrade, subbase, or base soils with a sulfate content greater than 0.1 percent.

2-1.3.4 Bituminous.

Most bituminous soil stabilization has been performed with asphalt cement, cutback asphalt, and asphalt emulsions. Soils that can be stabilized effectively with bituminous materials usually contain less than 30 percent passing the No. 200 (0.075 mm) sieve and have a PI less than

10. Soils classified by the USCS as SW, SP, SW-SM, SP-SM, SW-SC, SP-SC, SM, SC, SM- SC, GW, GP, SW-GM, SP-GM, SW-GC, GP-GC, GM, GC, and GM-GC can be effectively stabilized with bituminous materials, provided the above-mentioned gradation and plasticity requirements are met.

2-1.3.5 Combination.

Combinations of lime and cement are often acceptable expedient stabilizers. Lime can be added to the soil to increase the soil’s workability and mixing characteristics as well as reduce its plasticity. Cement or fly ash can then be mixed into the soil to provide rapid strength gain.

Combinations of lime and asphalt are often acceptable stabilizers. The lime addition may prevent stripping at the asphalt-aggregate interface and increase the mixture’s stability. Do not design, specify, or construct lime cement combination-stabilized subgrade, subbase, or base soils with a sulfate content greater than 0.1 percent.

2-1.4 Non-Traditional Additives.

2-1.4.1 Polymers.

Polymer emulsions (sometimes referred to as latex) can be used for soil stabilization. The most common types of polymer emulsions used for soil stabilization are polyvinyl acetate-based copolymers. These materials have excellent adhesive characteristics and provide stiffness, toughness, and water resistance. Polymer emulsions typically consist of 45 to 60 percent polymer content by weight and cure mainly by water loss. These materials may be limited by shelf life. Do not mix polymer emulsions with gray water or salt water for dilution.

Soils that can be stabilized with polymer emulsions are similar to those that can be stabilized by bitumen.

2-1.4.2 Fibers.

Numerous fiber types (e.g., polypropylene, polyvinyl alcohol, nylon, and natural) and morphologies (monofilament, fibrillated, and tape) are available. Most soils can be stabilized with fibers; however, mixing may be difficult in highly plastic soils. This difficulty limits the use of fibers for stabilizing fine-grained soils.

2-1.4.3 Non-traditional Additives.

A variety of other non-traditional soil stabilization and modification additives, such as acids, lignin derivatives, enzymes, tree resin emulsions, and silicates, are available from the commercial sector. These additives vary widely in their chemical composition and action mechanism, may be liquid or solid, and are often touted to be applicable for most soils.

Research studies in this area have demonstrated that many non-traditional soil additives have little to no benefit for granular soil types. Sandy and gravelly soils are often problematic for stabilization, requiring a binder such as cement, polymer, or asphalt emulsion to provide cohesion between soil grains. Medium- and fine-grained soils are likely to benefit most from the other non-traditional stabilizers; however, these are treated on a case-by-case basis. Do not design, specify, or construct otherwise stabilized subgrade, subbase, or base soils without prior approval by the Pavements DWG.

2-1.4.4 Non-traditional Combinations.

Soil blended with combinations of traditional and non-traditional additives has been shown to be more effective than soil blended with a single additive. Polymer emulsions and hydraulic cements with and without synthetic fibers combine well, as do synthetic fibers and hydraulic cements. These combinations have shown promise in laboratory testing and field demonstrations. A synergism exists between emulsions and hydraulic cement: the water in the emulsion hydrates the cement and the polymer adds flexibility, water resistance, and crack resistance. The use of fibers improves the resistance of the soil to deformation and resists the tensile stresses that lead to cracking. However, fibers are difficult to use during construction as they reduce workability and are difficult to place in wind as they blow around and even be blown completely out of the construction area.

2-1.5 Selection of Candidate Additives.

2-1.5.1 Candidate Stabilizers.

The selection of candidate stabilizers is made using Figure 2-1 and Table 2-3. The soil gradation triangle in Figure 2-1 is based upon the soil grain-size characteristics. The triangle is divided into areas of soils with similar grain size and therefore pulverization characteristics.

The selection process is continued with Table 2-3, which indicates candidate stabilizers for each area shown in Figure 2-1, with restrictions-based percent passing the No. 200 (0.075

mm) sieve, on grain size and/or PI. Also provided in the second column of Table 2-3 is a listing of soil classification symbols applicable to the area determined from Figure 2-1. This is an added check to ensure the proper area has been selected. Thus, the soil is characterized in terms of grain-size distribution and Atterberg limits to initiate the additive selection process.

Data required to enter Figure 2-1 are (1) percent material passing the No. 200 (0.075 mm) sieve and (2) percent material passing the No. 4 (4.75 mm) sieve but retained on the No. 200 (0.075 mm) sieve (i.e., total percent material between the No. 4 (4.75 mm) and the No. 200 (0.075 mm) sieves). The triangle is entered with these two values and the applicable area (1A, 2A, 3, etc.) is found at their intersection. The area determined from Figure 2-1 is then found in the first column of Table 2-3 and the soil classification is checked in the second column.

Candidate stabilizers for each area are indicated in the third column and restrictions for the use of each material are presented in the subsequent columns.

Figure 2-1 Gradation Triangle for Aid in Selecting Commercial Stabilizing Agent

2-1.5.2 Restrictions.

These restrictions prevent the use of stabilizing agents that are not applicable for the particular soil type under consideration. For example, assume a soil classified as an SC, with 93 percent passing the No. 4 (4.75 mm) sieve and 25 percent passing the No. 200 (0.075 mm) sieve with a liquid limit (LL) of 20 and a plastic limit of 11. Thus, 68 percent of the material is between the No. 4 and No. 200 (0.075 mm) sieves and the PI is 9. Entering Figure 2-1 with the values of 25 percent passing the No. 200 (0.075 mm) sieve and 68 percent between the No. 4 (4.75

mm) and No. 200 (0.075 mm) sieves, the intersection of these values is found in area 1-C.

Then, going to the first column of Table 2-3, we find area 1-C and verify the soil classification, SC, is included in the second column. From the third column, all four stabilizing materials are

0 10 20 30 40 50 60 70 80 90 100

P er ce nt b y

W ei gh t, S an d (P as si ng N o.

an d R et ai ne d on N o.

S ie ve

Percent by Weight, Fine (Passing No. 200 Sieve)

Area 3 Fine-Grained

Soils

Area 2C Gravels

Area 1C Sands

A re a 1A

A re a 1B

A re a 2B

A re a 2A found to be potential candidates. The restrictions in the following columns are now examined.

Bituminous/polymer stabilization is acceptable since the PI does not exceed 10 and the amount of material passing the No. 200 (0.075 mm) sieve does not exceed 30 percent.

However, note that the soil barely qualifies under these criteria and bituminous/polymer stabilization is probably not the first choice. The restrictions under portland cement indicate the PI is less than the equation indicated in footnote b. Since the PI, 9, is less than that value, portland cement is a candidate material. The restrictions under lime indicate that the PI not be less than 12; therefore, lime is not a candidate material for stabilization. The restrictions under LCF stabilization indicate that the PI does not exceed 25; thus, LCF is also a candidate stabilizing material. At this point, the designer makes the final selection based on other factors, such as availability of material, economics, etc. Once the type of stabilizing agent to be used is determined, samples are prepared and tested in the laboratory to develop a design mix meeting minimum engineering criteria for field stabilization.

2-1.5.3 Fibers.

Fibers can be used in many soil types because they provide mechanical stabilization.

Monofilament, tape, and fibrillated fibers are most common for improving soils. They provide immediate improvements in load bearing and crack resistance by enhancing strain resistance.

Fibers mixed into soil without adhesives (i.e., cement) rely on mechanical interlock, friction, and fiber entanglement between themselves and soil particles to anchor the fiber in place.

Thus, a high surface area fiber (such as a tape fiber) may be more efficient in a fine-grained clay soil than in a coarse-grained soil such as gravel, where fewer soil grains are in contact with the fiber. Longer fibers generally result in improved performance but hamper construction.

They are more difficult to mix and considerable “balling” may occur, diminishing efficiency.

After mixing, workability is sharply reduced, making blade work and grading difficult.

2-2 USE OF STABILIZED SOILS IN FROST AREAS.

2-2.1 Frost Considerations.

Pavement systems may experience two general types of freeze-thaw action. Cyclic freeze-thaw occurs in the material when freezing occurs as the advancing frost line moves by and thawing subsequently occurs. Heaving conditions develop when a quasi-equilibrium frost line condition is established in the stabilized material layer. If the material is frost-susceptible, the static frost line situation provides favorable conditions for moisture migration and subsequent ice lens formation and heaving. While bituminous, polymer, portland cement, lime, and LCF stabilization are the most common additives, other stabilizers may be used for pavement construction in areas of frost design but only with approval from the Pavements DWG.

Additives and cold weather placement techniques are also available for portland cement stabilization and are addressed in Chapter 4.

2-2.2 Limitations.

In frost areas, stabilized soil is used in one of the upper elements of a pavement system only if the cost is justified by the reduced pavement thickness. Use a soil treatment with a lower degree of additive than that indicated for stabilization (i.e., soil modification) in frost areas only with caution and after intensive tests because weakly cemented material usually has less capacity to endure repeated freezing and thawing than firmly cemented material. A possible exception is modifying a soil that will be encapsulated within an impervious envelope as part of a membrane-encapsulated-soil-Iayer pavement system. A soil unsuitable for encapsulation due to excessive moisture migration and thaw weakening may be made suitable by moderate amounts of a stabilizing additive. Test modified materials to ascertain the desired improvement is durable through repeated freeze-thaw cycles. Do not stabilize soils to improve the strength at the expense of making the soil more susceptible to ice segregation.

2-2.3 Construction Cutoff Dates.

Construct materials stabilized with cement, lime, or LCF early enough during the construction season to allow the development of adequate strength before the first freezing cycle begins.

The rate of strength gain is substantially lower at 50 °F (10 °C) than at 70 °F or 80 °F (21 °C or 27 °C). Chemical reactions will not occur rapidly for (1) lime-stabilized soils when the soil temperature is less than 60 °F (16 °C) and is not expected to increase for one month or (2) cement-stabilized soils when the soil temperature is less than 40 °F (4 °C) and is not expected to increase for one month. In frost areas, set a construction cutoff date well in advance of the onset of freezing conditions (e.g., 30 days) because it is not adequate to protect the mixture from freezing during a seven-day curing period as required by the applicable guide specifications if the lime or cement reactions continue past the seven-day period.

2-3 THICKNESS REDUCTION FOR BASE AND SUBBASE COURSES.

Ensure the stabilized base and subbase course materials meet the requirements of gradation, strength, and durability to qualify for reduced layer thickness design. Gradation requirements are presented in paragraphs 3-1 through 3-6 covering design with each type of stabilizer.

Unconfined compressive strength and durability requirements for bases and subbases treated with cement, lime, lime-cement, lime-fly ash (LF), and LCF are indicated in Tables 2-1 and 2-2, respectively. For bituminous-stabilized materials to qualify for reduced thickness, ensure they meet strength requirements in UFC 3-260-02. Polymer emulsion materials qualify for reduced thickness when they meet a minimum unconfined compressive strength of 400 psi (2.76 MPa).

Ensure all stabilized materials except those treated with bitumen and polymer emulsion meet minimum durability criteria to be used in pavement structures. There are no durability criteria for bituminous- or polymer emulsion-stabilized materials since it is assumed they will be waterproof if properly designed and constructed.

Table 2-1 Minimum Unconfined Compressive Strength for Cement-, Lime-, Lime- Cement, and Lime-Cement-Fly Ash-Stabilized Soils

Stabilized Soil Layer Minimum Unconfined Compressive Strength psi (MPa)a Flexible pavement Rigid pavement

Base course 750 (5.17) 500 (3.45)

Subbase course, select material or subgrade 250 (1.72) 200 (1.38) a Unconfined compressive strength determined at 7 days for cement stabilization and 28 days for lime, LF ash, or LCF ash stabilization.

Table 2-2 Durability Requirements

Type of Soil Stabilized Maximum Allowable Weight Loss After 12

Wet-Dry or Freeze-Thaw Cycles (Percent of Initial Specimen Weight)

Granular, PI < 10 11

Granular, PI > 10 8

Silt 8

Clays 6

2-4 DRYING WET SOILS.

2-4.1 Chemicals.

The chemicals of choice for drying are oxides of calcium and magnesium, generally referred to as quicklime. Depending on the source, they may be further defined as high-calcium quicklime, magnesium quicklime, or dolomitic quicklime. These substances contain metal oxides (e.g., calcium oxide, CaO) that react with water to form the metal hydrate (e.g., CaO⋅(OH)2). As quicklime hydrates with water in the soil, it releases considerable heat (i.e., is exothermic), which in turn evaporates more water and further reduces the moisture content of the soil.

Because this reaction is fast and exothermic, it is effective for drying soils down to freezing temperatures.

2-4.2 Lime.

Lime is a generic term used to cover a variety of materials. Only quicklime is effective for drying soils. Hydrated lime has already combined with water and will not draw moisture into new chemical forms. It will have certain chemical reactions (cation exchange) with some clay minerals that will make them less plastic and easier to dry by conventional methods, but this is much different from the avid drying of quicklime that is not related to soil type. Be aware that “agricultural lime” has no value for engineering stabilization or drying uses. Do not construct stabilized soil materials using agricultural lime.

Any product that contains free quicklime (CaO) can be used as a drying agent. The more CaO it contains, the more effective it will be as a drying agent. Portland cement, fly ash (particularly high calcium Class C ashes), and kiln dust are examples of common industrial materials with significant CaO content that can be used for soil drying. Their effectiveness for drying is directly dependent on their free CaO content. Quicklime is a caustic material; hence, normal safety precautions for such materials need to be followed (long sleeves and pants, rubber gloves, goggles, and mask). Quicklime is a relatively granular material and does not dust like the much finer hydrated lime. While safety precautions are required to handle this material, they are within the range of precautions used on construction sites. Quicklime is a commonly used industrial and construction material and the safety precautions required are reasonable to comply with in the field to ensure worker health and safety. Delivery is typically by dump truck and mixing is typically with a conventional field rotary mixer. Thorough mixing enhances uniform drying.

2-4.3 Amounts.

Amounts for use are generally by trial and error. Take a known weight of a wet soil sample and mix with a known amount of quicklime and see visually and texturally if it dries the wet soil sample. The reaction with CaO is quick and results will be evident very rapidly. Repeat but vary the amount of additive. This is a practical way to ascertain if non-quicklime materials such as portland cement, fly ash, or kiln dust have free CaO to be effective to dry the soil. A qualitative assessment of a simple test of this sort is generally all that is justified for military engineering applications. This will also provide a rough initial estimate of the amount of additive to use for drying.

2-4.4 Factors.

Three factors are involved with drying of the soil: chemical hydration of CaO, exothermic-reaction-caused evaporation, and normal evaporation because of ambient conditions.

Therefore, trying to assign exact formulas for how much quicklime to add to gain a specific reduction in moisture content is not justified. It is better to make multiple additions to close in on the desired moisture content. One to two percent quicklime for each percent reduction in desired moisture content is a good starting point.

Table 2-3 Guide for Selecting a Stabilizing Additive

Area Class (b) Type of Stabilizer Restriction (LL and PI) RESTRICTION (% PASS NO. 200) (c) REMARK

1A SW or SP

1. Polymer/bituminous emulsion and cement

2. Portland cement

3. Lime-cement-fly ash

4. 2 and 3 with fiber (a)

PI not to exceed 25 3 & 4. Lime requires at least 25% passing the No.

200 (0.075 mm) sieve

Soils near or above their optimum moisture content may require drying prior to emulsion stabilization.

1B

SW-SM or SP-SW or SW-SC or

SP-SC

1. Polymer/bituminous emulsion and cement

2. Portland cement and fiber (a)

3. Lime and fiber (a)

4. Lime-cement-fly ash and fiber (a)

5. 2, 3, and 4 with fiber (a)

1. PI not to exceed 10

2. PI not to exceed 30

3. PI not less than 12

4. PI not to exceed 25

3, 4, 5. Lime requires at least 25% passing the No.

200 (0.075 mm) sieve

Soils near or above their optimum moisture content may require drying prior to emulsion stabilization.

1C SM or SC or SM-SC

1. Polymer/bituminous emulsion and cement

2. Portland cement and fiber (a)

3. Lime and fiber (a)

4. Lime-cement-fly ash and fiber (a)

5. 2, 3, and 4 with fiber (a)

1. PI not to exceed 10

2. (b)

3. PI not less than 12

4. PI not to exceed 25

1. Not to exceed 30% by weight 3, 4, 5. Lime requires at least 25% passing the No.

200 (0.075 mm) sieve

Soils near or above their optimum moisture content may require drying prior to emulsion stabilization.

2A GW or GP

1. Polymer/bituminous emulsion and cement

2. Portland cement and fiber (a)

3. Lime-cement-fly ash and fiber (a)

4. 2 and 3 with fiber (a)

PI not to exceed 25 3 & 4. Lime requires at least 25% passing the No.

200 (0.075 mm) sieve

Well-graded material only. Material to contain at least 45% by weight of material passing No. 4 (4.75 mm) sieve. Soils near or above their optimum moisture content may require drying prior to emulsion stabilization.

2B

GW-GM or GP-GM or GW-GC or

GP-GC

1. Polymer/bituminous emulsion and cement

2. Portland cement and fiber (a)

3. Lime and fiber (a)

4. Lime-cement-fly ash and fiber (a)

5. 2, 3, and 4 with fiber (a)

1. PI not to exceed 10

2. PI not to exceed 30

3. PI not less than 12

4. PI not to exceed 25

3, 4, 5. Lime requires at least 25% passing the No.

200 (0.075 mm) sieve

Well-graded material only. Material to contain at least 45% by weight of material passing No. 4 (4.75 mm) sieve. Soils near or above their optimum moisture content may require drying prior to emulsion stabilization.

2C GM or GC or GM-GC

1. Polymer/bituminous emulsion and cement

2. Portland cement and fiber (a)

3. Lime and fiber (a)

4. Lime-cement-fly ash and fiber (a)

5. 2, 3, and 4 with fiber (a)

1. PI not to exceed 10

2. (c)

3. PI not less than 12

4. PI not to exceed 25

1. Not to exceed 30% by weight 3, 4, 5. Lime requires at least 25% passing the No.

200 (0.075 mm) sieve

Well-graded material only. Select Material to contain at least 45% by weight of material passing No. 4 (4.75 mm) sieve.

CH or CL or MH or ML or OH or OL or

ML-CL

1. Portland cement and fiber (a)

2. Lime (a)

3. 2 with fiber (a)

1. LL less than 40 and PI less than 20

2. PI not less than 12

2 & 3. Lime requires at least 25% passing the No.

200 (0.075 mm) sieve

Organic and strongly acidic soils falling within this area are not susceptible to stabilization by conventional means.

(a) Monofilament polypropylene fiber – Length and denier will vary depending on soil type

(b) Soil classification corresponds to ASTM D2487-17. Restriction on liquid limit (LL) and plasticity index (PI) is in accordance with ASTM D4318-17e1.

(c) PI ≤ 20 + 50 – percent passing No. 200 (0.075 mm) sieve

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CHAPTER 3 DETERMINING STABILIZER CONTENT

3-1 STABILIZATION WITH PORTLAND CEMENT.

Portland cement can be used either to modify and improve the quality of the soil or to transform the soil into a cemented mass with increased strength and durability. The amount of cement used will depend upon whether the soil is to be modified or stabilized.

3-1.1 Types of Portland Cement.

Several different types of cement have been successfully used for stabilizing soils.

Type I normal portland cement and Type IA air-entraining cements were used extensively in the past with similar results. At the present time, Type II cement has greater sulfate resistance obtained while the cost is often the same. High early strength cement (Type III) has been found to give higher strength in certain soils and cures much faster than Type I or Type II. Type III cement has a finer particle size than the other cement types. Chemical and physical property specifications for portland cement can be found in ASTM C150, Standard Specification for Portland Cement.

3-1.2 Water for Hydration.

Potable water is normally used for cement stabilization, although seawater has been found to be satisfactory.

3-1.3 Gradation Requirements.

Gradation requirements for cement-stabilized base and subbase courses are in Table 3-1.

Table 3-1 Gradation Requirements for Cement-Stabilized Base and Subbase Courses

Type Course Sieve Size Percent Passing

Base

1.5 inch (37.5 mm) 100

0.75 inch (19 mm) 70–100 No. 4 (4.75 mm) 45–70 No. 40 (0.425 mm) 10–40 No. 200 (0.075 mm) 0–20

Subbase

1.5 inch (37.5 mm) 100 No. 4 (4.75 mm) 45–100 No. 40 (0.425 mm) 10–50 No. 200 (0.075 mm) 0–20

3-1.4 Cement Content for Modifying Soils.

3-1.4.1 Improved Plasticity.

In a majority of cases, lime is preferred over soil cement to reduce the PI or control expansive behavior. If lime is not locally or economically available for a project, it is possible to use cement to reduce the plasticity. Do not design, specify, or install soil cement to reduce the PI of a soil without prior authorization by the Pavements DWG.

The amount of cement required to improve the quality of the soil through modification is determined by the trial-and-error approach, as described below.

To reduce the PI of the soil, prepare successive samples of soil-cement mixtures prepared at different treatment levels and the PI of each mixture determined in accordance with ASTM D4318, Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils. The minimum cement content that yields the desired PI is selected but, since it was determined based upon the minus No. 40 (0.425 mm) sieve material, this value is adjusted to find the design cement content based upon total sample weight expressed as:

𝐴𝐴 = 100𝐵𝐵𝐵𝐵

where

A = design cement content, percent total weight of soil B = percent passing No. 40 (0.425 mm) sieve size, expressed as a decimal C = percent cement required to obtain the desired PI of minus No. 40 (0.425

mm) sieve material, expressed as a decimal

3-1.4.2 Improved Gradation.

If the objective of modification is to improve the gradation of a granular soil through the addition of fines then conduct a particle size analysis (ASTM D7928, Standard Test Method for Particle-Size Distribution (Gradation) of Fine-Grained Soils Using the Sedimentation (Hydrometer) Analysis) on samples at various treatment levels to determine the minimum acceptable cement content.

3-1.4.3 Reduced Swell Potential.

Portland cement may reduce swell potential of certain expansive soils. However, portland cement is not as effective as lime in reducing swell potential and may be considered too expensive for this application. Determining cement content to reduce the swell potential of fine-grained plastic soils can be accomplished by molding several samples at various cement contents and soaking the specimens along with untreated specimens for four days. The lowest cement content that eliminates the swell potential or reduces the swell characteristics to the minimum is the design cement content.

Procedures for measuring swell characteristics of soils are found in ASTM D4546-14e1, Standard Test Methods for One-Dimensional Swell or Collapse of Soils. Check the cement content found to accomplish soil modification to determine whether it provides an unconfined compressive strength great enough to qualify for a reduced thickness design in accordance with criteria established for soil stabilization.

3-1.4.4 Frost Areas.

Cement-modified soil may also be used in frost areas but, in addition to the procedures for mixture design described in paragraphs 3-1.5.1 through 3-1.5.5, cured specimens are subjected to the 12 freeze-thaw cycles prescribed by ASTM D560, Standard Test Methods for Freezing and Thawing Compacted Soil-Cement Mixtures (but omitting wire-brushing). Follow the determination of frost design soil classification by means of standard laboratory freezing tests. If cement-modified soil is used as subgrade, its frost susceptibility, determined after freeze-thaw cycling, is used as the basis of the pavement thickness design if the reduced subgrade design method is applied.

3-1.5 Cement Content for Stabilized Soil.

Determine the design…