Recommended Practice for Stabilization of Subgrade Soils and Base Materials (2009) / Chapter Skim
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From page 34... ...
32 Standard Recommended Practice for Stabilization of Subgrade Soils and Base Materials AASHTO Designation: R Draft (2008) American Association of State Highway and Transportation Officials 444 North Capitol Street N.W., Suite 249 Washington, D.C.
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33 Standard Recommended Practice for Stabilization of Subgrade Soils and Base Materials AASHTO Designation: R Draft (2008) INTRODUCTION Engineering design of pavement structures relies on the assumption that each layer in the pavement possesses the minimum specified structural quality to support and distribute the super imposed loads.
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From page 36... ...
34 T 27, Sieve Analysis of Fine and Coarse Aggregate T 87, Dry Preparation of Disturbed Soil and Soil Aggregate Samples for Test T 89, Determining the Liquid Limit of Soils T 90, Determining the Plastic Limit and Plasticity Index of Soils T 99, Moisture Density Relationship of Soils Using a 2.5-kg (5.5-lb) Rammer and a 305-mm (12-in.)
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From page 37... ...
35 ppm - Parts per million or milligram per liter; concentration of solute, sulfates, in milligrams per liter of solvent, water. Isomorphous substitution - Substitution of one atom by another of similar size within a crystal lattice and without disrupting the crystal structure of the mineral.
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36 introduce salts or organic material of a variety of species and concentrations. These processes create differences in soil compositions within soil groups which can influence the behavior of individual soils.
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37 processes also make the soil more friable and workable. Longer-term reactions involve interactions between free lime (Ca(OH)
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38 remixed after the mellowing periods to achieve a homogeneous mixture before compaction. The ultimate strength of a soil-cement mixture with an extended mellowing period may be lower when compared to mixtures where compaction is achieved before initial set.
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39 Cement kiln dust (CKD) generally contains between about 30 and 40 percent lime and about 20 to 25 percent pozzolanic material.
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40 should be performed in accordance with the AASHTO standard recommended practice for stabilizing sulfate rich soils. Subsurface investigations should be guided by the purpose, requirements, and geographical settings of the project location.
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41 Subsurface conditions identified at the individual test pits, boring holes or by examining open cut sections during subsurface investigations should be used as a guideline to decide the frequency of sampling. The uniformity of the soil and the potential for the soil profile to contain minerals that may cause a deleterious reaction with the stabilizer form the basis for determining the frequency of sample collection.
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42 Soil Exploration/Sampling Soil Classification/Sieve Analysis/Atterberg Limits Sulfate Test Refer Sulfate Guidelines Additive selection Mix Design Evaluation of Properties Proceed to Construction Acceptable Base Material No treatment unless required for project Additive selection Mix Design Evaluation of Properties < 25 % passing No. 200 ≥ 25 % passing No.
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43 Classification of soils should be done in accordance with AASHTO M 145. As a candidate for stabilization purposes, the soil should first be classified as either a subgrade category or base category material.
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45 Sieve Analysis < 25% Passing No. 200 sieve Base Material Atterberg Limits PI ≤ 12 PI ≥ 12 Lime Cement Asphalt (PI< 6)
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46 Portland cement is generally considered to be a good candidate stabilizer for soils with less than 35 percent passing the no. 200 sieve.
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47 in order to determine whether the strength and performance characteristics of the stabilized soil are satisfactory. Specific test methods required to validate the use of a selected stabilizer are discussed in later sections of this standard.
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48 Lime-soil mixtures should be fabricated following ASTM D 3551 for moisture density and compressive strength testing. For compressive strength testing, samples are not required to mellow before fabrication unless it is difficult to achieve satisfactory homogeneity during laboratory mixing.
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49 Note 6. This step is applicable only for expansive clay soils and sulfate bearing soils.
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50 The purpose of adding lime as a stabilizer for base materials is for lime to interact with the fine material to form a matrix and provide improved strength, stiffness, and durability for the aggregate base. Since the fine material (smaller than 75 µm material)
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51 If the soluble sulfate concentration of the soil to be treated is greater than 3,000 ppm, then the standard practice that deals with stabilization of high sulfate soils should be consulted before proceeding. The standard practice for stabilization of high sulfate soils identifies the risk associated with stabilization of these soils and describes steps to reduce the risk of treatment of these soils.
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52 Preparation and curing of samples for compressive strength testing should follow ASTM D 1632. The samples should be moist cured through out the curing periods and immersed in water for four hours prior to compression testing.
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53 Add incremental concentrations of cement to the base materials, generally starting with 1 percent by weight of the entire mixture and increasing in 1 percent increments to a maximum of 3 percent. Moisture density relationships for each aggregate-lime blend should be determined following either AASHTO T-99 or AASHTO T-180 based on agency requirements.
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54 Steps for developing effective mix designs in soils using class C fly ash are detailed in sub sections 10.5.4.1 through 10.5.4.4. Development of mix designs to meet the strength requirement is dependent on moisture density and moisture strength relationships.
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55 Add increasing concentrations of fly ash to the soil with the designated amount of lime, as determined in the preceding section, starting with 4 percent by weight of the mixture and at increments of 2 percent. Follow the steps outlined in sections 10.3.7 through 10.3.10 and 10.3.12 for compressive strength and durability testing (Note 9)
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56 Typically, a 7-day compressive strength of 400 psi is considered acceptable for field applications. But this requirement may vary with field conditions and among agencies.
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57 Little, D
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