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9 Spacing of test pits during soil investigations should be dependent on the geologic complexity of the project area. Frequency of sampling should be based on the uniformity of soil, intent and level of investigation required and the potential for detrimental reactions with the soil during chemical stabilization processes. A general recommendation on frequency of sampling based on varying soil conditions is given in Table 1. Table1. Guideline regarding spacing between sampling locations (11). Soil condition Frequency of sampling Uniform 0.5 to 1.0 mile Non-Uniform 0.25 to 0.5 mile Highly variable 1,000 ft to 0.25 mile Sulfate bearing 500 ft Exploration should be deep enough to identify all strata that can significantly influence the outcome of the stabilization project. The chemical stabilization operation seldom proceeds deeper than 12-inches. However, the material below this layer affects stabilization. The most important factor is the depth of the water table. This depth and its annual fluctuation will probably require a combination of soil borings and a study of pedological and geological data sources to establish. However, knowledge of the fluctuation of the water level with respect to the stabilized layer will help define the risk of and extent of intrusion of moisture into the stabilized layer through capillary rise. The potential for capillary rise into the stabilized layer will also help assess whether or not diffusion of deleterious salts into the stabilized layer are probable. The Texas Department of Transportation (TxDOT) recommend continuous material sampling to a depth of at least 15 feet in locations where water fluctuations are high (11). For cuts exceeding these depths, sampling should be done to the road bed depth plus an additional 2 feet. Samples should be collected every time there is a change in observed physical characteristics of the material. AASHTO R-13 recommends that the depth of exploratory borings or test pits for road beds be at least 1.5 m (5 feet) below the proposed subgrade elevation. The boring depths and spacing requirements mentioned above should not be considered as either a minimum or a maximum, but instead should be used as a guide. In locations where project construction or performance may be affected by water or where impervious materials block internal drainage, borings should be extended to a sufficient depth to determine the engineering and hydro geologic properties relevant to the project design. GUIDELINES FOR SOIL STABILIZATION Stabilization projects are site specific and require integration of standard test methods, analysis procedures and design steps to develop acceptable solutions. Many variables should be considered in soil treatment, especially if the treatment is performed with the intent of providing a long-term effect on soil properties. Soil-stabilizer interactions vary with soil type and so does the extent of improvement in soil properties. Hence developing a common procedure applicable for all types of stabilizers is not practical. Instead, a generalized, flowchart-based approach, which provides the steps that should be followed in stabilizer selection, is presented in Figure 1.
10 ⤠25 % passing No. 200 ⥠25 % passing No. 200 > 3000 ppm No Figure 1. Guideline for stabilization of soils & base materials for use in pavements (12). 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 < 3000 ppm Change Additive(s) if needed Yes No No Yes
11 Soil exploration and sampling should be performed as described in the preceding sections. The soil can be classified as either a subgrade category or base category material on the basis of AASHTO M145. A key decision factor in selecting the appropriate subgrade additive is the concentration of water soluble sulfates in the soil. Sulfate testing should be done in accordance with the modified version of AASHTO T 290 or equivalent. Soils with sulfate levels above 3,000 ppm may be considered problematic and should be addressed separately from the standpoint of additive selection all the way through mix design and construction. Sampling, testing, stabilizer selection, and mix design for these soils should follow the draft recommended practice for stabilizing sulfate-bearing soils (13). A second key factor to be considered when deciding on the type of stabilizer to be used is the concentration of organic matter in the soil. Organic contents can interfere with strength gain mechanisms and should be determined prior to proceeding with mix design with any calcium-based stabilizer. Base materials must satisfy plasticity and gradation requirements and restrictions that vary from state-to-state. As a typical example, the Texas Department of Transportation (TxDOT) specifies various classes of base materials in Item 247 of the Texas Standard Specifications (14). AASHTO M 147 also provides guidance in distinguishing among classes of base materials. Guidelines for Stabilizer Selection Soil characteristics including mineralogy, gradation and physio-chemical properties of fine- grained soils influence the soil-additive interaction. Hence stabilizer selection should be based on the effectiveness of a given stabilizer to improve the physio-chemical properties of the selected soil. The preliminary selection of the appropriate additive(s) for soil stabilization should consider: ⢠Soil consistency and gradation ⢠Soil mineralogy and composition ⢠Desired engineering properties ⢠Purpose of treatment ⢠Mechanisms of stabilization ⢠Environmental conditions and engineering economics Soil index properties (i.e., sieve analysis, Atterberg limit testing, and moisture density testing) should be determined based on laboratory testing of field samples. Soil samples should be prepared following AASHTO T 87. The initial processing of most soils involves thorough air drying or assisted drying at a temperature not to exceed 60oC. Aggregations of soil particles should be broken down into individual grains to the extent possible. A representative soil fraction should be selected for testing following AASTHO T 248. The required quantity of soil smaller than 0.425 mm (No. 40 sieve) should be used to determine the soil index properties. Liquid limit testing should be performed following AASHTO T 89 and plastic limit and plasticity index testing should be measured following AASHTO T 90. Lime Stabilization Lime has been found to react successfully with medium, moderately fine and fine grained soils causing a decrease in plasticity and swell potential of expansive soils, and an increase in their workability and strength properties. Research has proven that lime may be an effective stabilizer in soils with clay content as low as 7 percent and in soils with plasticity indices below 10 (15).
12 The National Lime Association recommends a plasticity index of 10 or greater in order for lime to be considered as a potential stabilizer whereas the U.S Army Corps of Engineers recommends a plasticity Index of 12 or greater for successful lime stabilization (6, 16). Based on AASHTO classification, soil types A-4, A-5, A-6, A-7 and some of A-2-6 and A-2-7 are suitable for stabilization with lime (17). Cement Stabilization Cement stabilization is ideally suited for well graded aggregates with a sufficient amount of fines to effectively fill the available voids space and float the coarse aggregate particles. General guidelines for stabilization are that the plasticity index should be less than 30 for sandy materials. For fine-grained soils, soils with more than 50 percent by weight passing 75µm sieve, the general consistency guidelines are that the plasticity index should be less than 20 and the liquid limit (LL) should be less than 40 in order to ensure proper mixing (6). A more specific general guideline based on the fines content is given in the equation below which defines the upper limit of P.I. for selecting soil for cement stabilization (17). 4 )075.0(%5020. mmthansmallerIP â+⤠Cement is appropriate to stabilize gravel soils with not more than 45 percent retained on the no. 4 sieve. The Federal Highway Administration recommends the use of cement in materials with less than 35 percent passing no. 200 sieve and a plasticity index (PI) less than 20 (18). Based on this system, soils with AASHTO classifications A-2 and A-3 are ideal for stabilization with cement, but certainly cement can be successfully used to stabilize A-4 through A-7 soils as well. The Portland cement Association (PCA) established guidelines to for stabilizing a wide range of soils from gravels to clays. Fly Ash Stabilization The literature lacks a clear direction in selection parameters for the use of fly ash in soil stabilization. However, the literature documents that a wide range of aggregates can be suitably stabilized with fly ash including sands, gravels, crushed stones and several types of slags. Fly ash can be used effectively to stabilize coarse grained particles with little or no fines. In coarser aggregates, fly ash generally acts as a pozzolan and/or filler to reduce the void spaces among larger size aggregate particles to float the coarse aggregate particles. After the appropriate amount of fly ash is added to coarse grained soils to fill the voids, optimize density, an activator is often used to maximize the pozzolanic reaction in the mixture. The activator content is generally in the range of 20 to 30 percent of the fly ash used to fill the voids. The activator is normally either lime or Portland cement, but lime kiln dust or cement kiln dust can also be used. Similarly, consider a clay soil that is stabilized with lime but the clay is not pozzolanically reactive. The addition of fly ash and lime can substantially increase strength in the blend due to the reactive pozzolans provided by the ash. In these fine-grained soils, fly ash is typically used in conjunction with lime or cement to enhance the reactivity of the fine-grained soil with lime or cement. Class C fly ash has been used alone to stabilize moderately plastic soils. The basis for stabilization is free lime that becomes available upon hydration of the ash. The large majority of this lime is combined with the silica and alumina, but upon hydration, just as in the hydration of Portland cement, cementitious products are formed which stabilize the soil. However during this
13 hydration process, just as in the hydration of cement, free lime is released, which can react pozzolanically with the clay. This reaction reduces clay particle plasticity and improves strength. Successful application is often achieved with fine grained, plastic soils, by first applying lime or cement to reduce plasticity and improve workability of the soil and then adding the fly ash to boost strength of the soil, lime blend. Again, the impact of a given class F (with activator) or a given class C fly ash without activator may be very different depending on the pozzolan content of each ash, the degree of self cementing property of the class C ash, etc. Hence, the superior filler cannot be determined before hand and without evaluation. Techniques for Stabilizer Selection A range of options are available for selecting soil stabilizers most of which are based on the soil classification following either the AASHTO or Unified classification system. A simple, but well accepted methodology by which to select the appropriate stabilizer is the Soil Stabilization Index System (SSIS). The methodology was developed by U.S Air Force, and is based on soil index properties: plasticity index and percent passing the no. 200 sieve (19). These laboratory tests are easy to perform and are necessary inputs for AASHTO and Unified systems. Both these characteristics can be effectively correlated to the engineering properties of the soil and therefore can be used to differentiate engineering applicability. Figures 2 (for soils) and 3 (for base materials) use these two index properties, PI and percent passing the no. 200 sieve (percent smaller than 75 μm), to identify the appropriate stabilizer (12). Once the stabilizer is selected, detailed laboratory tests to determine strength and performance characteristics of soils are required. Individual test methods required for mix design for three traditional stabilizers are discussed in the later sections of this report. Sieve Analysis ⥠25% Passing No. 200 sieve Subgrade Atterberg Limits PI < 15 PI ⥠3515 ⤠PI ⤠35 Cement Asphalt (PI< 6) Lime-Flyash (Class F) Flyash (Class C) Lime Lime - Cement Lime â Flyash (Class F) Flyash (Class C) Cement Lime Lime - Cement Lime-Flyash (Class F) Lime - Flyash (Class C) Figure 2. Decision tree for selecting stabilizers for use in subgrade soils (12). Figures 2 and 3 present a set of general guidelines for selecting candidate stabilizers for soil and base materials. Agencies, however, should alter or adjust these guidelines based on their own
14 unique experiences as tempered by local conditions. It is important to remember that Figures 2 and 3 are âguidelinesâ but the final selection should be based on a more specific analysis of the soils. These involve identifying the reactivity of the pozzolans in the clay with the selected stabilizers. For example, lime may be an ideal stabilizer for reactive plastic clay because the lime can immediately reduce plasticity due to cation exchange reactions. Pozzolanic reaction continues over time to further reduce plasticity and increase strength due to the formation of, primarily, calcium-silicate-hydrates. On the other hand, a different clay bearing soil may not be pozzolanically reactive, and, even though the application of lime initially reduces plasticity and improves workability, the desired strength gain does not develop. In this case the stabilizer of choice may have to be Portland cement or a combination of lime and fly ash or lime and cement. Sieve Analysis < 25% Passing No. 200 sieve Base Material Atterberg Limits PI ⤠12 PI ⥠12 Lime Cement Asphalt (PI< 6) Flyash (Class C) Lime Cement Lime-Cement Lime â Flyash (Class F) Flyash (Class C) Figure 3. Decision tree for selecting stabilizers for use in Base materials (12). The decision trees provide a first step toward stabilizer selection. Once a stabilizer is selected, detailed mixture design is recommended if stabilization is the objective. If modification is the objective, then verification tests are required to ensure that the objectives of reduction in plasticity and perhaps immediate strength gain requirements are met. As discussed earlier in this document, modification refers soil improvement that occurs in the short term, during or shortly after mixing (within hours) where as Stabilization is generally a longer term reaction and the degree of strength gain required to achieve stabilization varies based on the expectations of the user. Again, as discussed earlier a strength increase of at least 50 psi greater than that of the untreated soil fabricated and cured under the same conditions as the stabilized material is used in this document to define stabilization. This value was used by Thompson in the Illinois method of mix design for lime treated soils (20). The researchers on this project recommend that a method of moisture conditioning be included in all strength testing protocols. This research team recommends capillary soak as the form of moisture conditioning before strength testing. In the
