DOI 10.1007/s11204-016-9386-4 Soil Mechanics and Foundation Engineering, Vol. 53, No. 3, July, 2016 (Russian Original No. 3, May-June, 2016)

TECHNOLOGY AND WORK PRODUCTION EVALUATION OF FREEZING-THAWING CYCLES FOR FOUNDATION SOIL STABILIZATION

A. U. Uzer

UDC 626.862

Department of CCEE, Iowa State University, Ames, USA

Freezing-thawing cycles in soils reduce considerably the foundation capacity of buildings and infrastructure. In recent years, development of nontraditional stabilizers has created hundreds of new products for soil stabilization. Fiber portions of biomass (such as lignin) can be considered as byproducts of the conversion process, and these byproducts are generally used to produce octane booster fuels, bio-based products, and other chemical products. The use of lignin-based biofuel co-products (BCPs) to stabilize pavement subgrade soil is an innovative idea and satisfies the needs of sustainable development in construction. A series of laboratory tests, including unconsolidated undrained direct shear test, freeze-thaw durability test, and scanning electron microscope tests, was conducted to evaluate the effect of BCP addition on shear strength performance for four different soils encountered in Iowa. The results of this study indicate that BCPs are beneficial in the soil stabilization of low-quality materials for use in road construction.

1. Introduction

Stabilization involves the applications of agents, energy, or both to bind soils together, primarily to improve shear strength and compressibility [1]. Bituminous adhesives or binders are common construction materials and are mostly derived from fossil fuels [2]. However, the requirement to reduce fossil fuel pollution requires the use of alternative sources to produce fuel. It is therefore necessary to develop the applications of bio renewable resources as energy. Pavement soil-stabilization with additives or admixtures improves and stabilizes pavement structure. Hydrated lime, Portland cement, and fly ash are the most commonly used additives. In the context of sustainable construction development, however, the use of byproducts and waste products as an alternative to strengthen pavement subgrade soil continues to gain attention. Lignin derived from biomass is known as natural 'glue'. Traditional and current uses for lignin and modified lignin include concrete admixtures, binders, and well drilling mud [3, 4]. The expansion of lignin application can provide an additional revenue stream for biorefineries [5]. The two main categories of lignin modified by various recovery technologies are sulfur lignin and sulfur-free lignin [6]. Most sulfite lignins, or lignosulfonates, are derived from the paper industry; however, lignins obtained from bio-based energy production are sulfur-free. Soil stability increases by adding sulfite lignin to clayey soils, causing dispersion of the clay fraction [7, 8]. The use of biofuel coproducts (BCPs) containing sulfur-free lignin in pavement soil stabilization has been examined in recent studies at Iowa State University [9, 10].

Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 3, p. 27, May-June, 2016. 202

0038-0741/16/5303-0202

©

2016 Springer Science+Business Media New York

TABLE 1 Soil classification Type 1 A-6(2) clayey sand (SC) Type 2 A-4(2) sandy silt with clay (CL-ML) Type 3 A-4(1) sandy silt with clay (CL-ML) Type 4 A-4(0) sandy silt (ML)

gravel (> 4.75 mm)

Grain size distribution, % sand (0.075–4.75 mm) silt and clay (< 0.075mm)

7.1

54.9

38.0

0.1

37.2

62.7

5.2

41.7

53.1

3.8

45.3

50.9

In the current study, a direct shear device will be used in the laboratory to determine the shear strength of a cohesionless soil. For a specific vertical confining stress, the maximum shear stress is obtained from the plot of the shear stress versus the horizontal displacement [11,19]. The direct shear test is the most appropriate test to use because of its simple setup and equipment operation and ability to test under different saturation, drainage, and consolidation conditions. These advantages have to be weighed against the difficulty of measuring pore-water pressure when testing in undrained conditions, and the possible spuriously high results from forcing the failure plane to occur in a specific location [12]. The direct shear test is conducted by forcing the sample to fail along a predefined plane while being subjected to normal load. This test enables determination of the angle of internal friction and cohesion by giving a direct measure of the shear force capacity under specific conditions. The shear stress in the shear box of the test is defined as the shear resistance developed within the sliding plane along a known section area of the sample [13]. In cold regions, freezing-thawing damage is one of the major problems in road construction and earthwork applications. The freezing and thawing of the soils can cause significant changes of the geotechnical properties [14, 15] of natural and stabilized soils, its cracking and spalling. The design and construction of earth structures influenced seasonally by subzero temperatures require the determination of the mechanical properties of the construction materials under appropriate thermal conditions [16]. A comprehensive laboratory experimental test program was performed; comparing the shear strength of BCP treated on four different representative Iowa soil types. The engineering properties of the soil samples are shown in Table 1. 2. Materials and Methods

An experimental study was carried out in three phases including the preliminary material characterization, compaction, and direct shear tests. Five steps were used to prepare each sample: soil preparation, soil-water additive mixing, molding, compaction, and curing. The collected natural soil was dried, then broken down to particle sizes to pass through a 4.75 mm sieve. To remove the initial water of the BCPs, the additives were dried at below 60°C, decreasing the BCP water content to nearly 0% of the total volume. After soil and additive preparation, soil and additives were mixed, and water was added to the soil and additive mixture. A uniform, homogeneous mixture was produced. The initial soil moisture content was determined from a sample of the mixture in accordance with ASTM standard [17]. Specifically designed and constructed the mold apparatuses (Fig. 1) were used to compact loose materials by static compaction [18, 19]. The apparatuses can be assembled to insert the 25 mm high spacer plug into the mold with removable collar. Loose mixture materials were placed in the mold and then were compacted into a 50 mm diameter and 25 mm high specimen by applying a static load on the spacer plug. The static loading method was followed in preparing specimens rather than dynamic loading. We performed and evaluated the Atterberg limit test, and standard Proctor test for pure soil (control group) and the soil-BCP A (containing lignin) mixture (Table 2). The tests were conducted in accordance with the ASTM standard [20]. 203

Fig. 1. Mold apparatuses for direct shear test.

TABLE 2 Soil type Pure soil 1 Soil 1+12% cement Soil 1+12% BCP A Pure soil 2 Soil 2+12% cement Soil 2+12% BCP A Pure soil 3 Soil 3+12% cement Soil 3+12% BCP A Pure soil 4 Soil 4+12% cement Soil 4+12% BCP A

Atterberg limits, % Proctor test Liquid limit Plasticity limit Plasticity index Optimum moisture Max. dry unit LL PL PI content (OMC), % weight, kN 32.8 17.4 15.4 14.4 16.95 36.0 11.1 24.9 17.5 17.02 76.0 39.0 37.0 16.3 14.31 29.1 22.9 6.2 18.2 15.99 31.5 25.1 6.4 18.0 16.02 67.8 39.4 28.4 18.9 14.23 27.5 22.2 5.3 13.5 17.83 32.7 27.0 5.7 14.5 18.19 72.7 36.4 36.3 15.8 14.15 17.2 15.1 2.1 12.0 18.03 19.8 18.6 1.2 14.0 17.99 58.3 44.3 14.0 18.8 15.11

The additive BCP A was obtained from the full-scale, wet-mill, corn-based ethanol plant of Grain Processing Corporation of Muscatine, Iowa [21]. We obtained alkaline-washed corn hulls through the process of corn-to-ethanol conversion and powdered this to generate BCP A. BCP A contains about 5% lignin, 50% hemicellulose, 20% cellulose, and 25% other components. These lignin-type components have a specific gravity of 2 and are not high-molecular-weight lignin, such as those found in wood, but they are specific to maize. A series of laboratory tests, including unconsolidated undrained direct shear test (DST), freezethaw durability test (FT), scanning electron microscopy (SEM) procedures [22], and X-ray diffraction (XRD) [23] were conducted for the lignin-treated sandy silt. The stabilization effect of a soil additive is measured in terms of the increase in shear strength capacity as indicated by DST [24-26]. This study also used DST testing as the basis for performance characterization. In the FT test, each set of treatment group combinations containing the same six specimens was recorded at the end of each cycle (the end of each thawing) until all 12 cycles had been completed. Microstructural analysis was also conducted to identify how these BCPs work in soil stabilization. The DST was performed in accordance with ASTM standard [27] after various curing periods (1, 7, and 28 days). The applied displacement shearing rate was 0.35 mm per minute for all samples. The DST was carried out to evaluate the shear properties of pure soil and soil treated with 12% of BCP A. Under normal stress levels of 0.069 (DS 10), 0.138 (DS 20), and 0.207 (DS 30) MPa, three different moisture contents (OMC-4%, OMC, and OMC+4) were evaluated with three different curing periods (1, 7, and 28 days). The FT was conducted in this study by simulating natural freeze-thaw cycles to evaluate the durability improvement for additive-modified soils in accordance with [28]. To investigate the effect of the BCPs on the durability of the four types of soils, mass losses were calculated after 12 freezing-thawing cycles. All specimens (50 mm diameter and 100 mm height) were prepared and placed on a saturated filter pad in an uncovered metal container and subjected to the 12 freeze-thaw cycles. To prepare the samples for the closed system of freezing and thawing cycles, specimens were placed in a digital refrigerator at −23°C for 24 h and then at +21°C for the thawing phase for 204

a

b

c

d

Fig. 2. DST results at different moisture contents: a) soil 1; b) soil 2; c) soil 3; d) soil 4.

23 h. The test required two identical specimens in compliance with ASTM D 560. The first specimen was used only to determine the average diameter and height for volume change evaluation at the end of each cycle, and the second was used to determine oven-dried mass for mass loss evaluation after the 12 cycles. Three repetitions were conducted to improve test reliability. Once the samples had changed shape considerably and became non-cylindrical based on visual examination, the volume measurements were terminated. After the entire set of 12 cycles, the other three specimens were oven-dried at 110°C. In this research, untreated and BCP-treated specimens with the 7-day curing period were subjected to SEM test to identify underlying mechanisms in sulfur-free lignin. 3. Experimental Results

The DST was carried out to evaluate the shear properties of pure soil and soil treated with 12% BCP A. Under normal stress levels (DS 10, 20, and 30), shear capacities of specimens with different moisture contents and different curing periods were measured by subjecting the samples to shear loads until they failed, with the results shown in Figure 2. We compared the shear strength results for three different OMCs, and the highest value was obtained in OMC-4% (Fig.2). For all soil types, a significant increase in shear strength was observed once 12% BCP A was added to the pure soil. The shear strength values increased up to two times for all soil types. The fine particle content influences the soil strength capacity of the soils. Soil 1 shows the highest strength capacity for all types of specimen results because it has the lowest clay content. Soil 2 shows the weakest strength results because it has the highest clay content. Soils 3 and 4 have similar fine particle contents and similar strength values. We conclude that the shear strength increase is linear in the early stages of curing (1 and 7 days), but this is not the case in the long term (28 days). Figure 2 shows that the curing period has an influence on the strength of treated soil, but it does not have any effect on the strength of pure soil. For soil treated with BCP A, the increase in curing time does not show any significant effect on the strength of soils 1 and 2, but it decreases the strengths of soils 3 and 4. The activity of the flocculation process with longer 205

a

b

c

d

Fig. 3. Average mass loss of specimens in: a) soil 1; b) soil 2; c) soil 3; d) soil 4;

) 1-day curing and

) 7-day curing.

curing times may explain this phenomenon. These results support the findings of other studies [29]. The maximum shear strength obtained for all types of soils and OMC-4 conditions was 0.396 MPa. This maximum strength was attained in soil 1 at OMC-4%. The images of the specimens treated with BCP A indicated good freeze-thaw resistance, and the specimens did not fully failed after 12 cycles, indicating that BCP A treatment can significantly improve the resistance of soil to damage from freeze-thaw cycles. The specimens treated for a 7-day curing period demonstrated better performance than specimens with a 1-day curing period for soils 2, 3, and 4; however, for soil 1, the curing periods did not significantly influence performance. A volume expansion was also noticeable. The soil specimens treated with 3%, 6%, and 12% cement were also evaluated using a freezethaw test. The recorded images showed that increased cement content and curing period time for all specimens reduced the degree of specimen failure during freeze-thaw cycles. Full failure occurred only at the end of the tenth cycle in the cured soil 2 specimens treated with 3% of cement. The cement-treated specimens showed the best performance, especially at 12% cement content, and in this case the specimens after 12 freeze-thaw cycles resembled the original specimens before testing. The specimens treated with BCP A were able to reduce the mass loss to as little as 7% and not higher than 24%. The specimens treated with BCP A had the highest volume expansion, greater than 30%, among all treatment group combinations. The volume expansion is related to its high plasticity. During the same cycles, the specimens treated with BCP A and cured for 1-day had 5% or more expansion than 7-day cured specimens treated with the same additive. The BCP A soil treatment improved the capability for reducing mass loss and was similar to the 6% cements treatment. However, the significant volume expansion could be a concern. BCP A showed little performance improvement over 12% cement, but was better than 3% cement. The tested co-products are promising additives for improving durability under freeze-thaw conditions, and each type has specific advantages. The visual evidence results of soil loss and volume change in freeze-thaw tests are presented in Fig. 3 for the four different tested soils. 206

Fig.4.

Scanning electron microscope images at 500 times magnification of 12% of BCP A-treated: a) soil 1; b) soil 2; c) soil 3; d) soil 4.

SEM can capture a large number of digital images for use in analyzing the mechanism of BCP stabilization at the particle level. Figure 4 shows the morphologies of groups of the four types of soil, with each set containing untreated soil and specimens treated with 12% BCP A, all with a 7-day curing period under OMC. The untreated soil images show clear particle surfaces and boundaries and porous structures under 500 times magnifications. The image of soil show that the grains were coated with dark-colored material, and these grains were bonded closely together with fewer pores to produce a stronger soil-additive structure. These images provide visual evidence that BCPs co- perform the function of cementing bonded soil grains together [30-31]. 4. Conclusion

This study investigated the shear strength of soils stabilized by a BCP containing lignin. A laboratory experimental test program was organized to compare the shear strength of four different soil types treated with a BCP. In this experimental program, moisture content and curing period were used as variables for evaluating the effects on performance specimens treated with BCPs, especially with respect to direct shear strength. Freeze-thaw testing was carried out to investigate the benefits of BCPs in improving durability. We found that moisture content has a significant effect on the BCP treatment and significantly influences soil strength capacity. The shear strength results were compared, and the highest value was obtained in OMC-4% for both pure soil and soil treated with BCPs. More flocculated structures were present under dry side conditions, which increased the internal friction in soil. Generally, for pure soil and BCP-treated soil, lower moisture content contributes to higher strength. Shear strength values increased up to two times for all soil types. It also achieved a significant reduction in mass loss during thermal cycles and moderate improvement in soil resistance to moisture degradation. Using different moisture contents, we observed highest shear strength value for BCP mixtures, which was obtained at an early age. The results of these study indicated that BCP A is a promising additive for soil stabilization. The SEM and XRD analyses revealed the primary underlying mechanisms of BCP A to be coating and binding soil grains to form strong soil structures. An increase in curing time also increased performance with respect to durability and moisture susceptibility for samples treated with BCP A. 207

Acknowledgements

The author gratefully acknowledges the Iowa Highway Research Board and Iowa State University for supporting this study. This study also was supported by The Scientific and Technological Research Council of Turkey (TUBITAK-1059B191301249) and Research Foundation of Selcuk University.

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