Cold Regions Science and Technology

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1 Cold Regions Science and Technology 62 (2010) Contents lists available at ScienceDirect Cold Regions Science and Technology journal homepage: Unconfined compressive strength and post-freeze thaw behavior of fine-grained soils treated with geofiber and synthetic fluid Hamza Gullu, Kenan Hazirbaba Department of Civil and Environmental Engineering, University of Alaska Fairbanks, Fairbanks, AK 99775, United States article info abstract Article history: Received 24 February 2010 Accepted 3 April 2010 Keywords: Silt Geofiber Synthetic fluid UCS Freezing and thawing This study focuses on a relatively new non-traditional stabilizer (synthetic fluid) used in conjunction with geofiber to improve the strength characteristics of a low-plasticity fine-grained soil. The investigation is based on unconfined compressive strength (UCS) tests. An efficient geofiber dosage was determined for the soil; treating it with geofiber only for the dosage rates varying from 0.2% to 1% by weight of dry soil. The individual contribution of the geofiber and synthetic fluid to the UCS gain was studied through testing each additive independently with the soil. Additionally, UCS tests were conducted on soil samples treated with geofiber and synthetic fluid together. All experiments were conducted for both unsoaked and soaked sample conditions. Strength developments were also investigated under freezing and thawing conditions. The treatment results are discussed in detail in terms of UCS and stress strain response of the UCS test. The results demonstrate that the use of geofiber with synthetic fluid provided the highest UCS improvement (170% relative gain) in unsoaked samples when compared with the other treatment configurations. On the other hand, the synthetic fluid, when used alone, caused a relative decrease of 21% in the UCS of untreated soil in soaked conditions. The use of geofiber with synthetic fluid performed better in terms of the UCS under freezing and thawing conditions, while the synthetic fluid alone under the same conditions performed inadequately. The stress strain responses of the soil treated with geofiber and synthetic fluid in terms of post-peak strength, strain hardening, and ductility were better than that of treated with synthetic fluid alone. Finally, the resilient modulus for the various treatment configurations was estimated from the UCS results. The findings indicate that the investigated soil stabilization technology appears to be promising for sites that can be represented by unsoaked conditions (i.e., where adequate drainage and unsaturated conditions can be ensured) Elsevier B.V. All rights reserved. 1. Introduction A non-traditional soil stabilization technology in which geofiber and synthetic fluid (a liquid stabilizer) are used to improve locally available fine-grained soils in Interior and Western Alaska was investigated through an extensive testing program. In the first phase of the investigation, the California Bearing Ratio (CBR) performance was the basis for evaluation and analyses. The results from the first phase of the research are presented in Hazirbaba and Gullu (in review). This paper is a follow-up effort to Hazirbaba and Gullu (in review) and presents the results from the second phase of the investigation. The primary objective of the research described in this paper was to investigate the freeze thaw strength and stress strain characteristics of fine-grained soils improved through the use of randomly-oriented discrete-polypropylene geofiber and synthetic fluid. Corresponding author. address: khazirbaba@alaska.edu (K. Hazirbaba). Fine-grained soils, especially encountered in Interior Alaska, are not desired as subgrade, subbase material or as a foundation supporting layer under buildings due to their frost-susceptible nature. They are prone to significant ice segregation with higher moisture conditions (Chamberlain, 1981). The use of geofiber and liquid stabilizers separately to improve various soils has been researched to some extent. However, the research on the combined use of the two additives for stabilizing and improving cold region soils, particularly fine-grained soils, is very limited (Hazirbaba and Gullu, in review; Hazirbaba and Connor, 2009). The majority of available literature on the use of geofiber deals with cohesionless or granular soils. Typically, adding geofiber to cohesionless or granular soils improves the shear modulus, liquefaction resistance and particle interlocking, and increases load bearing capacity (Freitag, 1986; Arteaga, 1989; Maher and Ho, 1994). It has been reported by various investigators that addition of geofiber to soil increases the peak strength (shear, compressive, and tensile) (Gray and Ohashi, 1983; Gray and Al- Refeai, 1986; Maher and Ho, 1994; Ranjan et al., 1996; Webster and Santoni, 1997). Previous studies showed that the improvement of the engineering properties with the inclusion of geofiber depends on X/$ see front matter 2010 Elsevier B.V. All rights reserved. doi: /j.coldregions

2 H. Gullu, K. Hazirbaba / Cold Regions Science and Technology 62 (2010) various parameters such as type, length, content, orientation and aspect ratio (length/diameter) of the geofiber, and natural soil properties. Al-Refeai (1991) found that for fine and medium sand no appreciable increase in the stiffness of the sand was gained by using fibers longer than 51 mm. Stabilization of sands with the geofiber contents greater than 2% by dry weight of soil presented no added benefit (Ranjan et al., 1996). A laboratory study by Ahlrich and Tidwell (1994) indicated that monofilament and fibrillated geofiber types were not effective in stabilizing a high-plasticity clay, while both geofiber types at 0.5% dosage rate enhanced the properties of a sandy soil. However, Kumar et al. (2006) reported that the unconfined compressive strength of clay and clay sand mixtures increased with the addition of geofiber. Tingle et al. (1999) recommended using a geofiber content between 0.6% and 1%, and they reported that a geofiber content of 0.8% is sufficient to ensure a strain hardening behavior. Maher and Gray (1990) noted that randomly-oriented geofiber has a primary advantage of the absence of potential planes of weakness that can develop parallel to oriented reinforcement. A comparative study by Lawton et al. (1993) revealed that geofiber reinforced soils require some amount of deformation before the strengthening benefits can be seen. Ranjan et al. (1996) studied the relationship between soil grain size and the geofiber-bond strength, and found that finer sand particles had significantly greater geofiberbond strengths than coarser grained soils. Kaniraj and Havanagi (2001) reported that the inclusion of geofiber increased the strength of cement-stabilized fly ash-soil samples and changed their brittle behavior to ductile behavior. As for the non-traditional fluid stabilizers, Scholen (1992) described five different groups: electrolytes, enzymes, mineral pitches, clay fillers, and acrylic polymers. Oldham et al. (1977) reported that polymer resin was more effective than asphalt, cement, and lime with sandy materials and provided the greatest increase in unconfined compressive strength. Rauch et al. (2002) studied the use of three liquid stabilizers; an ionic stabilizer or electrolyte, an enzyme, and a polymer product, with five high-plasticity clay soils to measure the improvement in soils in terms of reduced plasticity. They found that the only effective reduction in plasticity occurred with the ionic stabilizer in sodium montmorillonite. Santoni et al. (2002) performed tests on a silty-sand material with traditional (cement, lime, and asphalt emulsion) and non-traditional stabilizers (polymers and tree resin). The results indicated that the strength gain in the soil treated with non-traditional additives was much quicker than that treated with traditional stabilizers. Newman and Tingle (2004) used emulsion polymers for soil stabilization of airfields and found that all of the polymers increased the unconfined compressive strength after 28 days of cure time for both wet and dry conditions. The present research effort investigates the use of randomlyoriented discrete-polypropylene geofiber and synthetic fluid as an alternative non-traditional stabilization method with a fine-grained soil. In particular, the stress strain characteristics and freeze thaw performance of treated and untreated soil samples were studied for various contents of the additives through an extensive experimental program that consisted of unconfined compressive strength (UCS) tests and freeze thaw tests. Table 1 Index properties of the silty soil. Property Value Specific gravity 2.73 Liquid limit (%) 26 Plastic limit (%) 24 Plasticity index (%) 2 Maximum dry density (kg/m 3) 1713 Optimum moisture content (%) 12 USCS classification ML dry-unit weight of Fairbanks silt was found to be 1713 kg/m 3 at the optimum water content of 12%. The geofiber used was discrete 51-mm long and 2-mm wide tape type polypropylene geofiber. Polypropylene material was chosen based on its availability, resistance to ultraviolet degradation, chemical stability and reasonably high strength characteristics (Fletcher and Humphries, 1991). The index properties of the geofiber are listed in Table 2. The geofiber dosages investigated were: 0.2%, 0.375%, 0.5%, 0.625%, 0.8%, and 1% by dry weight of the soil sample. The dosage in this study was limited to 1% due to the greater costs of geofiber at higher dosages. The synthetic fluid used in this investigation is colorless (clear and bright) with a specific gravity of and a viscosity index of 70. The index properties of the synthetic fluid are given in Table Testing program and procedures The soil was tested in four treatment configurations: (1) in its natural state (no additives), (2) with geofiber, (3) with synthetic fluid, and (4) with geofiber and synthetic fluid together. Both unsoaked and soaked conditions were investigated. The target water content for the tests at natural soil moisture with no additives and for those with geofiber treatments was selected as 12%, which was the optimum moisture content of untreated soil determined by modified Proctor energy. As for the treatment with synthetic fluid only, and with geofiber and synthetic fluid together, the target water content was kept at 6% to: i) represent the in situ conditions, as the in situ water content for Fairbanks silt was measured to be about 6%, and ii) minimize the need for additional water in soil improvement especially for cold region applications. The synthetic fluid content in the treatments (configurations 3 and 4) was selected as 4% by dry weight of soil as recommended by Hazirbaba and Gullu (in review). The geofiber dosage was varied from 0.2% to 1.0% for the treatment that involved geofiber alone (configuration 2) and kept constant at 0.5% when used in combination with synthetic fluid (configuration 4). Addition of geofiber beyond 1% was not considered as dosages larger than 1% usually present an uneconomical mix (Fletcher and Humphries, 1991). 2. Experimental program 2.1. Material The soil used for this study is a fine-grained soil and referred to as Fairbanks silt. Basic soil index properties of the silt are given in Table 1. It is a low-plasticity silt and is classified as ML-type material according to the Unified Soil Classification System. The particle size distribution was determined by hydrometer analysis and is shown in Fig. 1. The mean size (D 50 ) was measured as 0.03 mm. The maximum Fig. 1. Grain size analysis of Fairbanks silt.

3 144 H. Gullu, K. Hazirbaba / Cold Regions Science and Technology 62 (2010) Table 2 Properties of geofiber. Property Tape Material Polypropylene, 99% Shape Flat-wide (2 mm wide) Color Black Moisture Nil Specific gravity 0.91 Carbon black content 0.5%, minimum Tensile strength (kpa) 206,843 Tensile elongation 20%, maximum Length used (mm) 51 All samples tested in this investigation were approximately 100 mm in diameter and 200 mm in height; a minimum of 2:1 height to diameter ratio was used. Soil samples were obtained by thoroughly mixing dry soil by hand with water and/or synthetic fluid first. This mixture was placed in a sealed plastic bag for about 18 h prior to compaction to achieve equilibrium of the free moisture. Next, geofiber at the desired amount was introduced to the mixture. Extreme care was taken during the mixing process to ensure a uniform mixture. Then, samples were compacted with modified Proctor energy conforming to the procedure of ASTM D1557. The compacted samples were extruded from the compaction mold using a hydraulic jack extrusion device and trimmed to desired dimensions (i.e., 100-mm diameter and 200-mm height). For the samples of unsoaked conditions, UCS tests were performed immediately after compaction. For soaking samples, following extrusion from the compaction mold, a rubber membrane was applied around the sample and it was placed in a PVC plastic mold that has a perforated base and a split lengthwise on side. Duct tape was applied around the mold against any lateral expansion, especially in case of soaking. Then the sample in mold was placed in a water bath in a controlled temperature environment and allowed to soak for 96 h. During the soaking, a surcharge stress of 3.5 kpa was applied to the sample using a steel weight. At the end of soaking, the sample was carefully removed from the plastic mold and the rubber membrane surrounding the sample was removed as well. Next, measurements of sample dimensions and weights were retaken, and the UCS test was performed. For freeze thaw tests the freezing and thawing cycles were applied to the sample while the sample was still within the PVC plastic mold. UCS tests were conducted following the procedure in ASTM D The compression loading machine used is a SoilTest G-900 Versa Loader with a maximum load capacity of 44.5 kn. A schematic drawing of the compression test set-up is shown in Fig. 2. The testing load was applied at a rate of mm/s, which corresponds to a strain rate of %/s for a 200 mm high specimen. The stress readings were digitally taken at 0.25 mm intervals. Loading of the sample continued up to an axial strain of 15% (up to mm) in case the sample included geofiber (i.e., samples treated with geofiber only and those treated with geofiber and synthetic fluid together). The loading of samples with no geofiber was continued until failure or the load value indicated a decrease with increasing strain. Thus, the UCS performance corresponds to the maximum (peak) stress attained, or to the stress at 15% axial strain; whichever was obtained first. If the Table 3 Properties of the synthetic fluid. Property Value Specific gravity Viscosity, cst at 40 C 10.7 Viscosity, cst at 100 C 2.6 Viscosity index 70 Color Clear, bright Flash point, C 175 Pour point, C 33 stress showed continuous increase with strain, which was the case in samples containing geofiber, the peak strength was taken as the stress at 15% axial strain Freezing and thawing tests Closed system freezing was applied for the freeze thaw tests. Closed system freezing in a soil is a condition in which no source of water is available during the freezing process beyond that originally in the voids of the soil at or near the zone of freezing; ice lenses may or may not form (Jones, 1987). Closed system freezing is appropriate when no significant change in the in situ water content is expected between winter and summer seasons. In addition, for low permeability soils, such as Fairbanks silt, the rate of frost penetration is generally expected to be greater than the rate of moisture transport, so that there is no sufficient time available during freezing to permit a continuous supply of water to reach the freezing front (Wong and Haug, 1991). Thus, a closed system freezing was adopted in this study. The testing samples were unidirectionally frozen from top down. This condition was approximated by applying insulation to the bottom as well as around the sample mold so that the freezing process could start from the open top of the sample. In order to reduce the frictional effect between the soil and inner wall of the mold, each sample was covered by a membrane. Additionally, a thin layer of vaseline was applied to the inner wall of the mold. For freeze thaw tests, soil samples were subjected to one freeze thaw cycle as the detrimental effects of freeze thaw cycles usually occur during the first cycle (Lee et al., 1995; Qi et al., 2008). Soil samples were subjected to freezing temperature of 20 C for 24 h in order to obtain a complete frost penetration. After freezing, soil samples were allowed to thaw at a temperature of 20 C for 24 h. This was done in a container allowing for a relative humidity of 100%. A surcharge stress of 3.5 kpa was applied on the samples during the freezing and thawing processes. Then, the samples were subjected to UCS testing. 3. Results and discussion 3.1. Improvements in UCS The effect of geofiber on UCS was investigated for a range of dosages between 0.2% and 1%. UCS test results for the treatments with geofiber only for both unsoaked and soaked conditions are given in Fig. 3. The UCS values for natural soil (i.e., compacted soil with zero geofiber content) were measured as 157 kpa and 86 kpa for unsoaked and soaked conditions, respectively. It is clear from Fig. 3 that up to 0.375% geofiber content there is no increase in UCS in unsoaked conditions while a slight increase occurs in soaked conditions. Beyond 0.375% geofiber content, the UCSs were significantly enhanced in both sample conditions. For the range of the geofiber content considered (0 to 1%), the highest strengths were obtained at 1% geofiber content, where the improvements by relative gain in UCS of natural soil are 440% and 569% for unsoaked and soaked conditions, respectively. An effective geofiber content based on the UCS test results may be determined. The effective geofiber content can be defined as the amount of geofiber that yields the minimum design requirement strength. ASTM D 4609 (Standard Guide for Evaluating Effectiveness of Admixtures for Soil Stabilization) suggests that if an increase in UCS of 345 kpa or more due to treatment occurs, or if no significant strength is lost due to soaking, then the treatment may be considered effective. Project specific minimum design requirements may also be considered when determining an effective geofiber content. For the purposes of this study and as recommended by Hazirbaba and Gullu (in review), 0.5% geofiber content was considered to be effective. In terms of consistency, the improved soil with 0.5% geofiber content for both unsoaked and soaked conditions corresponds to the range of hard to very hard according to Terzaghi et al. (1996).

4 H. Gullu, K. Hazirbaba / Cold Regions Science and Technology 62 (2010) Fig. 2. Schematic of the unconfined compression test set-up. The resultsof the treatments withsyntheticfluid alone, and synthetic fluid with geofiber are presented in Fig. 4 along with those obtained from natural soil and soil treated with geofiber only. The testing dosages of geofiber and synthetic fluid were 0.5 and 4%, respectively. It is clearly seen from Fig. 4 that unsoaked samples produced better strengths than the soaked samples. For the unsoaked conditions, the treatment with synthetic fluid alone provided 36% increase in the UCS of untreated soil while the treatment with geofiber and synthetic fluid together improved the UCS by 170%. The improvement with geofiber alone was found to be 142% in the UCS of untreated soil. The results of the treatments strongly suggest using the synthetic fluid with geofiber for applications that can be represented by unsoaked conditions. As for the soaked conditions, the treatment with synthetic fluid alone resulted in a decrease by 21% in the UCS of untreated soil while the treatment with synthetic fluid and geofiber together increased the UCS by 88%. It appears that the synthetic fluid is not an effective stabilizer in soaked conditions according to the ASTM D 4609 criterion where an effective stabilizer should cause no significant loss of strength upon soaking. Thus, it is not recommended to use the synthetic fluid in cases where soaked conditions prevail. It is evident that the best improvement in soaked conditions was obtained from the treatment with geofiber alone. Thus, the treatment with geofiber only can be considered as effective and suggested for applications where saturated soil conditions prevail or the presence of excessive water is expected. The experimental work revealed that the synthetic Fig. 3. UCS versus geofiber content for unsoaked and soaked conditions. Fig. 4. UCS of the synthetic fluid additions for unsoaked and soaked conditions. GF: geofiber, SF: synthetic fluid, W: target water content.

5 146 H. Gullu, K. Hazirbaba / Cold Regions Science and Technology 62 (2010) fluid performs much better as a stabilizer in unsoaked conditions. In practice, unsoaked conditions can be approximated by providing sufficient drainage during the service life of stabilization. The poor performance of synthetic fluid in soaked conditions may be mostly attributed to moisture susceptibility and the hydrophobic behavior of the fluid. Scholen (1995) reported that non-traditional stabilizers sensitive to moisture tend to cause weakness in the fabric structure leading to lower strengths. The behavior of the synthetic fluid in soaked conditions seems to be similar to that of some of the liquid stabilizers (acids and enzymes) tested with silty sands and clay soils (Santoni et al., 2002; Tingle and Santoni, 2003) where the liquid stabilizers either did not help improve the strength or caused a strength loss under soaked conditions. The UCS results of the treatment configurations subjected to freezing and thawing are presented in Fig. 5 for both unsoaked and soaked conditions. The UCS of the untreated soil in unsoaked conditions increased from 157 kpa to 271 kpa after freezing and thawing. This was somewhat surprising because the strength improved by about 73% upon a freeze thaw cycle and without any treatment. For soaked conditions, the UCS remained nearly unchanged (with and without freeze thaw) for the untreated soil. Although the freezing and thawing processes are generally expected to cause a decrease in strength, some researchers have reported increase in strength upon freeze thaw (Alkire and Jashimuddin, 1984; Ogata et al., 1985; Wang et al., 2007; Tsarapov, 2007; Qi et al., 2008), particularly for unsaturated soils. The change in strength due to freezing and thawing can be related to particle reorientation and moisture redistribution. The findings of this study are in agreement with those of Alkire and Jashimuddin (1984) where freezing and thawing are reported to cause decrease in strength for saturated conditions, and increase in strength for partially saturated conditions. The results in Fig. 5 further suggest that the use of geofiber alone and with synthetic fluid generally provides resistance against the strength loss that may occur due to freezing and thawing. The amount of resistance depends on: (i) the sample conditions (i.e., unsoaked, soaked) and, (ii) the treatment configuration. In unsoaked conditions, the use of geofiber alone, and with synthetic fluid resulted in 41% and 10% increase in the UCS, respectively, while treatment with synthetic fluid alone brought about a 25% decrease in the UCS. Thus, the use of geofiber alone or with synthetic fluid can be recommended as effective treatments against strength loss due to freeze thaw. Similar trends were also observed for soaked conditions. The effect of freeze thaw on strength of the treated samples may be investigated by comparing the results in Figs. 4 and 5. The UCS of unsoaked samples treated with geofiber alone appears to remain approximately the same for no freeze thaw and freeze thaw conditions while samples treated with synthetic fluid alone were found to experience a slight decrease (4%) in UCS strength due to freeze thaw. The unsoaked samples treated with geofiber and synthetic fluid together, when subjected to freeze thaw, yielded 30% less strength than that obtained in case use of no freeze thaw, however the UCS was still about 300 kpa. Similarly, the results from treated and soaked samples show a drop in the UCS upon freeze thaw regardless of the treatment type. The decrease in strength due to freeze thaw was found to be 4.4% for the treatment with synthetic fluid only, 14% for the treatment with geofiber only, about 20% for the treatment with geofiber and synthetic fluid together Failure planes and reinforcement mechanism In this section, the observations about the failure planes developed in the treated samples are presented and the reinforcement mechanism particularly due to the inclusion of geofiber is discussed. The material behavior of the treated compositions, brittle or ductile, can easily be realized from the failure planes. It has been recognized that a more ductile material will have higher resistance to cyclic load and less cracking potential (Li, 2005). During the tests in this study each soil sample was monitored throughout the test and the following observations were recorded regarding failure planes: (i) similar failure planes were observed for the same treatment configurations when comparing unsoaked and soaked samples suggesting that saturation of samples had little to no effect on the shape and orientation of the failure plane. In addition no significant change in failure planes was observed between pre- and post-freezing and thawing tests; (ii) untreated soil samples and samples treated with synthetic fluid alone showed brittle behavior and usually failed in shear on distinct diagonal planes. The angle between the diagonal shear failure planes and the major principal stress planes was measured to range between 58 and 67 ; (iii) samples that included geofiber alone or together with synthetic fluid were found to show brittle behavior and develop a distinct diagonal shear failure plane for geofiber content less than 0.5%, and exhibit a bulged or drum-shaped ductile failure with small localized shear failures around the sample at larger geofiber content than 0.5%. As an illustration of the failure planes, soaked samples that were treated with geofiber alone are shown in Fig. 6. Some of the samples (e.g., 0.8%-Test2) showed both diagonal and bulge-shaped failure together. Adding geofiber to the soil affects the direction of the failure surface as well as the shear zone because of the tensile forces developed in the geofiber under load. The inclusion of geofiber results in an increase in both the shear strength and ductility of the soil composition Stress strain behavior Fig. 5. UCS of the treatment configurations subjected to freezing and thawing. GF: geofiber, SF: synthetic fluid, W: target water content. Through studying the stress strain responses, a better understanding can be developed as to how adding geofiber and synthetic fluid affects soil behavior. Specifically, the effect of the additives on post-peak strength, strain hardening, and ductility and brittleness is discussed in this section. Fig. 7 presents the stress strain response from both unsoaked and soaked samples improved with geofiber only while Fig. 8 shows stress strain response of the samples that included synthetic fluid; with and without geofiber. In Fig. 9, the stress strain response from samples subjected to a freeze thaw cycle is presented. The stress strain responses presented in Figs. 7 9 show that the addition of geofiber (without or with synthetic fluid), in general, becomes effective in preventing or limiting the reduction in post-peak strength at geofiber content equal to or larger than 0.5% while at smaller geofiber content significant reduction in post-peak strength still occurs and no improvement or recovering trend is recorded up to 15% axial strain, most likely due to the less frictional and tensile forces developed at lower geofiber content. The drop in the post-peak

6 H. Gullu, K. Hazirbaba / Cold Regions Science and Technology 62 (2010) Fig. 6. Failure planes of soaked samples that included geofiber. strength is less pronounced in soaked samples. A strain hardening behavior was observed in samples containing equal to or larger than 0.8% geofiber. It is interesting to note that for soaked conditions, the samples improved with 0.5% geofiber content showed slight strainsoftening when not subjected to freeze thaw, and strain hardening behavior when subjected to freeze thaw. As for the effect of the synthetic fluid on post-peak strength, the addition of synthetic fluid alone does not provide any resistance against the post-peak strength loss (see Figs. 8 and 9). The stress strain curves are useful in assessing the ductility, which indicates the strain-energy-absorption capability. Simply, the area under a stress strain curve during loading defines the energyabsorption capacity (Mamlouk and Zaniewski, 2006). Through analyzing the stress strain curves the strain-energy-absorption capacity was evaluated and the results are presented in terms of normalized strain-energy capacity in Fig. 10. Normalizing the energy of the treated soil with that of untreated soil provides a relative and better assessment of ductility. The increase in the energy-absorption capacity is usually related to the increase in peak strength and improvement in post-peak response. Fig. 10a shows that as the geofiber content increases significant gain occurs in the energyabsorption capacity leading to more ductile behavior. Fig. 10b presents the normalized energy for all sample conditions considered (i.e., unsoaked, soaked, no freeze thaw, freeze thaw). As can be seen, the synthetic fluid alone did not appear to provide improvement in the energy-absorption capacity (the normalized energy for these samples is 1 or less) except when used in combination with geofiber. The energy-absorption capacity was found to slightly decrease by freeze thaw with the exception of the samples treated with 0.5% geofiber alone where a significant increase in energy occurred. Similar results with regard to the ductility can be obtained through calculating the brittleness index (I B )(Consoli et al., 2002), which is defined as I B = q f q ult 1 ð1þ

7 148 H. Gullu, K. Hazirbaba / Cold Regions Science and Technology 62 (2010) Fig. 9. Post-freeze thaw stress strain response of silt treated with synthetic fluid alone, and with geofiber and synthetic fluid together. GF: geofiber, SF: synthetic fluid, W: target water content. following the peak. In the case of strain hardening behavior, the peak stress at 15% axial strain was taken as ultimate stress. The calculated brittleness indices, which are presented in Fig. 11, confirm the Fig. 7. Stress strain response of soil treated with geofiber alone for various geofiber dosages. a) Unsoaked samples. b) Soaked samples. where q f and q ult are the failure and the ultimate deviatory stresses, respectively. As the index decreases towards zero, the failure behavior becomes increasingly ductile. For the analyses, the ultimate stress was determined from the stress strain data where the curves leveled out Fig. 8. Stress strain response of silt treated with synthetic fluid alone, and with geofiber and synthetic fluid together. GF: geofiber, SF: synthetic fluid, W: target water content. Fig. 10. The strain-energy-absorption capabilities of treatments. a) With respect to geofiber content. b) For the additions of geofiber, synthetic fluid for pre- and postfreezing and thawing. GF: geofiber, SF: synthetic fluid, W: target water content.

8 H. Gullu, K. Hazirbaba / Cold Regions Science and Technology 62 (2010) ductility behavior represented by the absorption energies (Fig. 10). The analyses of ductility and brittleness indicated the following: (i) generally the inclusion of geofiber with or without synthetic fluid leads to an increase in the ductility behavior (or decrease in brittleness) while the addition of the synthetic fluid without geofiber does not improve the ductility; (ii) when geofiber and synthetic fluid used together for improvement, significant contribution to energy absorption and ductile behavior can be obtained Estimation of resilient modulus Resilient modulus (M R ) is an important material property of subgrade soils in pavement design. Resilient modulus of a soil is the ratio of the amplitude of the repeated axial stress to the amplitude of the resultant recoverable axial strain. It is adequately determined by repeated load triaxial method where the stiffness of the material is estimated by applying a number of haversine load pulses. However, this testing method is usually a rather time consuming and requires elaborate laboratory testing facilities, thus it is expensive to use, particularly for the subgrade soils that require routine tests. The resilient modulus has been correlated to various soil strength parameters such as the unconfined compressive strength, and thus can be determined indirectly. Lee et al. (1995) proposed that the stress corresponding to axial 1% strain in the unconfined compression test (S U1.0% ) could be used to estimate the resilient modulus using the following regression-based model: thaw appears to have much less influence on resilient modulus than water content; unsoaked samples yielded much larger resilient modulus values than soaked samples for both no freeze thaw and freeze thaw conditions. Thus, regardless of the treatment configuration used, once samples are soaked (i.e., increased water content and near full saturation) a dramatic drop in resilient modulus should be expected. To avoid such decrease in the field, an effective drainage system should be recommended with the improved soil for field applications. 4. Conclusions and recommendation Based on the experimental research effort presented herein the following major conclusions regarding the stabilization of a lowplasticity fine-grained soil through the use of geofiber and synthetic fluid may be drawn: 1) The use of geofiber with synthetic fluid provided the largest UCS improvement in unsoaked samples when compared with the other treatment configurations (i.e. geofiber alone, synthetic fluid alone). The relative strength gain from this treatment was 170%. The synthetic fluid, when used alone, did not improve the UCS. 2) Based upon the UCS values of untreated silt, the geofiber alone and when used with synthetic fluid proved effective during freezing and thawing, however the use of synthetic fluid alone was found to be ineffective against the detrimental impacts of freeze thaw. M R = 606:6S U1:0% ð2þ where M R is in psi (1 psi=6.9 kpa). This relationship was specifically recommended for soils having S U1.0% less than 241 kpa where the regression analysis indicated an R 2 value of Using Eq. (2), resilient moduli for various sample conditions were calculated and the results are presented in Fig. 12. The figure shows that the unsoaked samples have greater resilient modulus than the soaked samples. The change in resilient modulus with increasing geofiber content is shown in Fig. 12a where unsoaked samples generally indicated improvement in resilient modulus up to 0.8% with a peak at 0.7% geofiber content. The resilient modulus value at 0.7% geofiber content is approximately twice that estimated from samples containing no geofiber. For soaked samples, significantly lower resilient modulus values were estimated and the geofiber content seems to have little to no effect on the resilient modulus property. The estimated resilient modulus values for different treatment configurations and freeze thaw are shown in Fig. 12b. In general, the freeze Fig. 11. Brittleness index of treatments (for the additions of geofiber, synthetic fluid for pre- and post-freezing and thawing). GF: geofiber, SF: synthetic fluid, W: target water content. Fig. 12. Resilient modulus of treatments. a) With respect to geofiber content. b) For the additions of geofiber, synthetic fluid for pre- and post-freezing and thawing. GF: geofiber, SF: synthetic fluid, W: target water content.

9 150 H. Gullu, K. Hazirbaba / Cold Regions Science and Technology 62 (2010) ) The samples treated with synthetic fluid alone usually failed in shear on diagonal planes. The samples that included geofiber alone or together with synthetic fluid failed in shear on diagonal shear failure planes at low geofiber dosages (less than 0.5%), while larger geofiber dosages resulted in bulge-shape failure patterns. 4) When considering post-peak strength, strain hardening and ductility, the stress strain behavior of the soil was found to be significantly influenced by the geofiber content. The synthetic fluid alone did not provide any resistance against post-peak strength loss. 5) Resilient moduli were estimated from the stress strain curves. Evaluation of the resilient moduli indicated that the addition of synthetic fluid alone or together with geofiber could improve the resilient modulus, which is significant for subgrade applications. Based on the estimated values, the use of synthetic fluid alone in unsoaked samples with no freezing and thawing indicated about 76% improvement in the resilient modulus. Freeze thaw appears to have little effect on the estimated resilient moduli. However, in general, the resilient modulus from soaked samples is dramatically lower than that from unsoaked samples. The conclusions presented above imply that geofiber and synthetic fluid together could be used to improve the strength characteristics of fine-grained soils. However, adequate drainage to limit excessive water and prevent saturated conditions will be required for successful application. Acknowledgements Financial support for this research was provided by U.S. DOT, Research and Innovative Technology Administration through Alaska University Transportation Center, and State of Alaska Department of Transportation & Public Facilities under Grant No. G This support is gratefully acknowledged. 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