J. Cent. South Univ. (2020) 27: 2003-2016

DOI: https://doi.org/10.1007/s11771-020-4426-7

Mechanical properties of fiber and cement reinforced heavy metal-contaminated soils as roadbed filling

HUANG Yu-cheng(黄钰程)¹, CHEN Ji(陈季)¹, TIAN Ang-ran(田盎然)¹,

WU Hui-long(吴会龙)³, ZHANG Yu-qing(张裕卿)^{2, 4}, TANG Qiang(唐强)^{1, 2}

1. School of Rail Transportation, Soochow University, Suzhou 215131, China;

2. National Engineering Laboratory of Highway Maintenance Technology, Changsha University of Science & Technology, Changsha 410114, China;

3. Suzhou Rail Transit Group Co., Ltd., Suzhou 215004, China;

4. Aston Institute of Materials Research, Engineering Systems & Management Group, Aston University,Birmingham B4 7ET, UK

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract: The treatment of contaminated soil is a crucial issue in geotechnical and environmental engineering. This study proposes to incorporate appropriate polypropylene fibers and cements as an effective method to treat heavy metal contaminated soil (HMCS). The objective of this paper is to investigate the effects of fiber content, fiber length, cement content, curing time, heavy metal types and concentration on the mechanical properties of soils. To this end, a series of direct shear test, unconfined compression strength (UCS) test, dry-wet cycle and freeze-thaw cycle test are performed. The results confirm that the appropriate reinforcement of polypropylene fibers and cement is an effective way to recycle HMCS as substitutable fillers in roadbed, which exhibits benefits in environment and economy development.

Key words: roadbed filling; unconfined compression strength; shear strength; cement and fiber reinforcement; heavy metal contaminated soil

Cite this article as: HUANG Yu-cheng, CHEN Ji, TIAN Ang-ran, WU Hui-long, ZHANG Yu-qing, TANG Qiang. Mechanical properties of fiber and cement reinforced heavy metal-contaminated soils as roadbed filling [J]. Journal of Central South University, 2020, 27(7): 2003-2016. DOI: https://doi.org/10.1007/s11771-020-4426-7.

1 Introduction

With the advancements of industrial and agricultural modernization, the chemical, metallurgical, petroleum, transportation, fertilizer and other industries develop rapidly. Owing to the lack of effective environmental protection methods, deficient technical treatment processes and the doubtful industrial wastes disposals, a great deal of soil is contaminated [1-5]. Soil contamination attracts worldwide concerns as soil is a major natural resource relevant to each country’s environment and prosperity. It has been considered that soil contamination is one of the foremost factors threating the global ecosystem.

An increasing amount of urban and agricultural soils are contaminated by heavy metals during the past few decades. According to the Bulletin on the Investigation of Contaminated Soil Conditions in China, 16.1% of the soil test sites are contaminated, among which the majority are polluted by heavy meatal including cadmium, arsenic, lead, zinc, copper, etc. Heavy metals in soil are toxic, durable, and non-bio-degradable, inducing considerable impacts on the natural ecosystem [6-8]. Meanwhile, with the urbanization and increasing infrastructure constructions, high demands of stable and eco-friendly lands with suitable geotechnical properties call for the developments of effective enhancement techniques of contaminated soil. Thus, a wide variety of technologies are developed for the contaminated soil remediation. Among them, Portland cement- based solidification/stabilization(S/S) is an effective and widely used restoration technology for HMCS treatment which stabilizes the heavy metals by mixing the binders into the contaminated soils and strengthens soils via both physical and chemical methods. It encloses the contaminated soils, and decreases the mobility, bio-availability, and leachability of hazardous elements (e.g., soluble Pb) [9-13]. However, cement-based S/S method is doubtful for significant increase in brittleness of soil [14-16].

Besides, it is found that adding fibers is also a reliable and effective method to improve the mechanical properties of soil, in which the randomly distributed fibers are mixed uniformly with soil to form a stable matrix. The previous studies pointed out that the addition of fibers enhances the strength and decrease the stiffness of soil. More importantly, as compared to soil alone or cement stabilized soil, the soil with fiber reinforcement exhibits higher ductility and less loss of post-peak residual strength [17-19]. Based on the aforementioned conclusions, the incorporation of both cement and fibers as additives to treat HMCS can not only improve the strength and decrease the brittleness, but also enhance the ductile and engineering properties of the soil matrix. To this end, the HMCS with incorporation of Portland cement and fibers as partial alternative materials in roadbed filling projects is beneficial for waste recycling and cost saving.

This paper aims to investigate the combined effects (including polypropylene fiber content, fiber length, cement content, curing time, heavy metal ion concentration and types) on the mechanical properties of polypropylene fibers and cement reinforced heavy metal-contaminated soil (PFCMS). For this purpose, a series of unconfined compression strength test and direct shear tests, as well as dry-wet and freeze-thaw cycles are conducted on soil specimens, to figure out the appropriate incorporation of fiber and cement to treat HMCS for roadbed filling applications.

2 Materials and experiments

2.1 Materials

The tested soil utilized in this paper is laboratory prepared by mixing commercial kaolin and sand at ratio of 3:7 by mass. The sands and kaolin for the experiments are illustrated in Figure 1, and Table 1 presents the corresponding physical properties. Both commercial kaolin and sands are oven dried for 24 h at temperature of 105 °C [20, 21]. Then the sands are screened via 1 mm sieve prior to using. The maximum dry density and optimum water content are achieved in accordance with the compaction test (JTG E40-2007) to 1.98 g/cm³ and 12%, respectively (as illustrated in Figure 2). Considering that it is difficult to achieve the maximum dry density indoors, the actual dry density of the soil specimens is compacted at 95% of the maximum dry density.

Figure 1 Sands (a) and kaolin (b) for the tests

Table 1 Physical properties of sands and kaolin

Figure 2 Compaction test curve for mixture of sands and kaolin

The common-used Portland cement is applied in this paper as solidification binder. Table 2 depicts the main physical properties of the cement. Also, the cement is oven-dried at 105 °C for at least 24 h before use, and then screened using a 3 mm sieve to remove magazines and caking parts, so as to be mixed with soil more evenly.

According to the previous studies, a wide variety of fibers are applied into soil for reinforcement purpose, among which the polypropylene fiber is the most widely used. Consequently, the polypropylene fiber is utilized in this paper to investigate the effects of fibers [22-24]. The fiber lengths of 6, 12 and 19 mm and the fiber contents of 0.05%, 0.1%, 0.2% and 0.4% by weight are prepared for test matrix to assess the impacts of the fiber length and ratio on the properties of the PFCMS. Besides, the ordinary photograph and scanning electron microscopy (SEM) observation on the surface of the polypropylene fibers used in this experiment are shown in Figure 3. The surfaces of polypropylene fiber are smooth and the textures are not evident, which is concluded that the reinforcement of soil by incorporating polypropylene fiber alone is limited due to the absence of boning materials.

Table 2 Properties of cement

Pb, Cu and Zn are selected as the objective heavy metals as they are commonly encountered in the contaminated soils, especially in China. Thus, the predetermined volume of lead chloride (PbCl₂), copper chloride dihydrate (CuCl₂·2H₂O) and zinc chloride (ZnCl₂) are prepared for using. In this study, the contents of heavy metal ions in soil are chosen as 1000, 5000 and 10000 mg/kg, i.e., 0.1%, 0.5% and 1% of dry soil by weight, respectively.

2.2 Specimens preparations

First of all, the kaolin, sand and cement after sieving are dried at the oven (105 °C) for at least 24 h and then cooled to room temperature. To reach the target concentration of heavy metal (0.1%, 0.5% and 1% by weight), the corresponding certain dosage of heavy metal chloride is dissolved into a small amount of deionized water. Then, the kaolin, sands, cement, fibers and the heavy metal solution are mixed adequately, to ensure that the fibers are distributed evenly and oriented randomly within the soil. Finally, the mixtures are compacted within a mold of 100 mm-length and 50 mm-inner diameter. Simultaneously, to ensure the uniformity, the samples are compacted with five layers and the hammer is dropped 12 times at 200 mm height of each layer. The height of each layer is approximately 1/5 of the specimen height. In fact, the test matrix (as presented in Table 3) is designed to assess the combined effects of fiber length, fiber content, cement content, curing time, heavy metal concentration and types [25]. Fiber length is controlled at three levels (6, 12, 19 mm, denoted as P6, P12, P19). The fiber content is controlled at five levels (0, 0.05%, 0.1%, 0.2%, 0.4%, denoted as P0, P0.05, P0.1, P0.2, P0.4). Cement content is controlled at three levels (5%, 7.5%, 10%, denoted as C5, C7.5, C10). Curing time is controlled at three levels (7 d, 28 d, 90 d). Three kinds of heavy metals are utilized (Pb²⁺, Cu²⁺, Zn²⁺). Heavy metal concentration is controlled at three levels (0.1%, 0.5%, 1%, denoted as Pb²⁺, Cu²⁺ or Zn²⁺ 0.1, 0.5, 1). In general, the test matrix has 42 combination conditions in total and each condition has 3 replicates. For comparison purposes, the raw soil, the polypropylene fibers reinforced soil (PFS) and polypropylene fibers reinforced cement stabilized soil (PFCS) are also used to manufacture specimens for the subsequent tests.

Figure 3 Polypropylene fibers:

2.3 Unconfined compressive strength test

The test is conducted according to the roadway geotechnical specification JTG E40-2007 using the YSH-2 compactor. During the test, the compression speed is maintained at 1 mm/min. The major purpose of performing this test is to measure the unconfined compressive strength (UCS) and failure strain under different combined conditions [26, 27]. During the test, the failure stress equals the peak compressive strength of the strain–stress curve and the failure strain corresponds to the strain at failure stress. More importantly, failure strain is considered as a critical index in the evaluation of the brittleness of a solid body.

2.4 Direct shear test

This test procedure also refers to the Roadway Geotechnical Specification (JTG E40-2007). The test is performed with ZJ strain-controlled direct shear apparatus designed by Nanjing Soil Instrument, with the shear rate of 2.4 mm/min. The soil specimen is tested under four levels of vertical pressure (100, 200, 300 and 400 kPa), respectively.

Table 3 Test matrix

The main purpose of this test is to measure the shear strength index under different conditions, including diverse internal friction angles and cohesions.

2.5 Dry-wet cycle and freeze-thaw cycle tests

One dry-wet cycle comprises of oven-drying for 24 h at temperature of (45±3) °C, placing in room temperature (20 °C) for 1 h, then saturating in water and being in the curing box for 23 h, and finally placing in room temperature for hours until the weight is constant. The specimens are subjected to up to 5 dry-wet cycles, then conducted by UCS tests to assess the effect of dry-wet cycles on the mechanical properties of soil matrix.

One freeze-thaw cycle comprises of placing in the freezer at temperature of -20 °C for 12 h, then thawing in oven at temperature of 20 °C. The specimens are subjected to up to 7 freeze-thaw cycles, then conducted by UCS tests to assess the effect of freeze-thaw cycles on the mechanical properties of soil matrix.

3 Results and discussion

To explore the combined effects of fiber reinforcement and chemical solidification/ stabilization, the widely used Portland cement and polypropylene fibers with good qualities are selected as reinforcement materials. The engineering properties of PFCMS are investigated in terms of the UCS test, direct shear test, dry-wet cycle and freeze-thaw cycle test. The following parts analyze and discuss the effects of various factors (including polypropylene fiber content, fiber length, cement content, curing time, heavy metal ion concentration and types) on the strength and deformation characteristics of PFCMS.

3.1 Effect of polypropylene fiber content

As illustrated in Figure 4, with the rising of polypropylene fiber content, the UCS and cohesion of PFS, PFCS and PFCMS increase first and then decrease. The UCS of PFS exhibits maximum strength of 0.54 MPa at 0.2% fiber content, which is consistent with Welker and Josten’s findings [28], whereas the maximum UCS of PFCS and PFCMS reaches 2.21 and 2.02 MPa at fiber content of 0.1%. It is also noted that the maximum cohesions are observed when the polypropylene content is 0.1% for all the three soil samples.

Figure 4 Effect of polypropylene fibers content on UCS (a), failure strain (b), cohension (c) and internal angle (d)

Besides, when fiber content is less than 0.2%, no significant change is observed in the failure strain of PFS, PFCS and PFCMS. However, when the fiber content is more than 0.2%, the failure strain of the three materials rises substantially. This can be explained by that the extra amount of fibers notably enhance the ductility of the specimens. Moreover, the internal frictions are not extensively influenced by the change of fiber content.

It is clear that the soil incorporated with polypropylene fibers is capable to enhance the mechanical (tensile, flexural and shear) strength, in addition to provide rigidity, due to the frictions coupled with interlockings between fibers and soil particles. Moreover, the randomly distributed fibers mobilize the adhesive bonding between fiber and soil, which increase the soil structural integrity and hold the soil together by forming a structural mesh which utilize the composite strength and the interaction of the fibers [29, 30]. However, the excessive amount of fibers will alleviate the strength capacity of soil because it is difficult to mix the fibers and soil particles homogeneously and adequately, which induces the clumping and balling of fibers where the potential failure areas are located.

3.2 Effect of polypropylene fiber length

As can be seen in Figure 5, the maximum USC, internal friction angle and cohesion are observed at the fiber length of 12 mm. The soil specimens with longer fibers have the greater friction and interlocking forces between fibers and soil particles, because the longer fibers require larger fiber-soil interfaces. Nevertheless, the excessive long fibers mitigate the strength of specimens as well. The probable reason is that with the same fiber content by weight, a less amount of fibers with longer length is required to be incorporated into the specimens, which is not capable to form the effective “structure mesh” to ensure the integrity and strength of the specimens. Also, it is difficult to distribute the longer fibers evenly and orientated that might decrease the strength. It can be concluded that the optimum length of polypropylene fiber for reinforcement is 12 mm. Moreover, as the fiber length increases from 6 mm to 19 mm, the failure strain of PFCS and PFCMS raises and then reduces slightly, whereas the failure strain of PFS drops rapidly then decreases slowly to a certain value. It indicates that without the incorporation of cement, the increase of fiber length from 6 mm to 12 mm significantly increases the brittleness. However, further increase of the fiber length from 12 to 19 mm slightly increases the brittleness of soil sample. When the cement is incorporated, the fiber length has little effect on the failure strain, which is mainly determined by the cement reinforcement.

Figure 5 Effect of polypropylene fibers length on UCS (a), failure strain (b), cohension (c) and internal angle (d)

As compared among the three types of soil (shown in Figures 4 and 5), with the same fiber length and content, the USC, cohesion and internal friction of PFCS are the largest, followed by PFCMS and PFS. Nonetheless, the adverse rank is observed in failure strain. It demonstrates that the incorporation of cement increases both the compressive and shear strength, at the meanwhile, making the specimen more brittle. Also, it has been found that the presence of Pb²⁺ brings about the significant impact on the engineering properties of specimens, which is probably because the Pb²⁺ somehow hinders the hydration of cement, causing certain damage to soil structure.

3.3 Effect of cement content

The cohesion, internal friction angle and UCS of PFCS and PFCMS increase when the cement content rises from 5% to 10%, and their differences between PFCS and PFCMS are not evident (Figure 6), which implies that the Pb²⁺ has a slight negative impact on the strength of soil. The main reason is that higher cement content induces more soil-cement reactions, generating primary and secondary cementitious reactions [31-34]. The primary cementitious materials, comprising of calcium aluminates (C₃AH_x, C₄AH_x), calcium silicates (C₂SH_x, C₃S₂H_x) hydrated, and hydrated lime Ca(OH)₂, are formed via hydration reaction [35, 36]. The formations of hydrated calcium aluminate and calcium silicate are caused by the secondary pozzolanic reaction between hydrated lime, silica and alumina in clay minerals [37]. This reaction between cement and soil can clearly explain the enhancement of stabilized soil strength. In addition, the hydroxides with larger surface area are filled in the soil voids, increasing the compressibility of the soil which indicates higher UCS. At the same time, the hydroxides also bond fiber and soil particles more closely, which increase the cohesion and internal friction angle of soil implying larger shear strength.

Figure 6 Effect of cement content on UCS (a), failure strain (b), cohension (c) and internal angle (d)

It is noteworthy that as the cement content increases, the failure strain of PFCMS raises proportionally, while no profound change is observed in failure strain of PFCS. For PFCS, it is probably because the extra amount of cement and corresponding extra hydroxides have no influence on the brittleness of soil matrix. When the cement content is 5%, the failure strain of PFCMS is less than PFCS due to the presence of Pb²⁺ which is consistent with the findings in Figures 4 and 5. However, with the increasing of cement content, more hydroxides are generated to absorb or encapsulate the Pb²⁺ resulting in the less negative effect of Pb²⁺ on the failure strain.

3.4 Effect of cement curing time

Curing time is also a significant factor contributing to the strength of cement solidified soil. As excepted and shown in Figure 7, the UCS, failure strain, cohesion and internal friction angle of PFCS and PFCMS increase with the rising of curing time from 7 d to 90 d. Because the hydration reaction of cement continues in the later period of curing, and more hydroxides are generated to increase the compressive and shear strength of soil.

Figure 7 Effect of curing time on UCS (a), failure strain (b), cohension (c) and internal angle (d)

3.5 Effect of heavy meatal ion concentration

Figure 8 demonstrates the impact of heavy metal concentration on the UCS, failure strain and shear strength of PFCMS. In general, the existence of Pb²⁺ weakens the strength of soil, and higher concentration of Pb²⁺ weakens the strength more. On the other hand, the UCS and cohesion of PFCMS do not change significantly when the concentration of Pb²⁺ is less than 0.5%, but decline extensively when the Pb²⁺ concentration is higher than 0.5%. The failure strain sharply increases when the concentration rises from 0.5% to 1%. This is because the high concentration of Pb²⁺ somehow impedes the hydration reaction of cement, and weakens the soil properties, which is consistent with the findings of CHEN [37].

3.6 Effect of heavy meatal types

As illustrated in Figure 9, with the same concentration of 0.5%, the UCS, cohesion and internal friction angle of Pb²⁺ contaminated PFCMS are notably greater than that of Cu²⁺ and Zn²⁺ contaminated PFCMS, because the solubility of Pb²⁺ is less than that of Zn²⁺ and Cu²⁺. Furthermore, the failure strain of Pb²⁺ contaminated PFCMS is the least, compared with Zn²⁺ and Cu²⁺ contaminated PFCMS. The reason is the low solubility of the Pb²⁺ resulting in the high stiffness and brittleness of the specimen.

3.7 Influences of dry-wet cycles and freeze-thaw cycles

With the increasing number of dry-wet cycles, the UCS of PFCMS decreases dramatically at first then tends to be stable (Figure 10). The possible reason is that the soil is undergoing shrinkage or wet swelling deformation. The corresponding stress in the soil induces micro-cracks at the weak portion, and increasing the number of dry-wet cycles will lead to the enlargement of cracks in the soil. But also, the chloride is running off during the dry-wet cycles process, causing the weakening of cementation between the soil particles. After subjected to a certain number of cycles, a certain number of cracks occur within the specimen, meanwhile the initiation and propagation of cracks slow down. The polypropylene fibers serve as reinforcing bars to strengthen the specimens and remain to a relevant stable condition. As demonstrated in Figure 10, the axial stress-strain curves are also depicted, the peak stress declines as the number of dry-wet cycle increases. The specimens retain a certain post-peak strength rather than brittle failure mainly due to the reinforcement of polypropylene fibers. But in general, the wet-dry cycle damages the structure of the PFCMS to a large extent, and reduce the strength of the soil, which must be taken into account in the design and use.

Figure 8 Effect of heavy metal concentration on UCS (a), failure strain (b), cohension (c) and internal angle (d)

Figure 9 Effect of heavy metal types on UCS (a), failure strain (b), cohension (c) and internal angle (d)

It can be noted that the UCS increases then decreases to a stable value with the rising number of freeze-thaw cycles (Figure 11). The possible reason is that during the first frozen cycle, the volume of specimen expands due to the inside frozen water, making the polypropylene fibers start to strengthen the structure. After being subjected to several freeze-thaw cycles, it induces some damages within the specimens and the strength tends to be stable. Similar to the influence of dry-wet cycles, all the specimens exhibit strain softening and retain a certain residual strength, which also results from the existence of polypropylene fibers. Also, the peak stress declines with the increasing number of freeze-thaw cycle.

4 Conclusions

Based upon a series of experimental results regarding the combined effects on the mechanical properties of PFCMS, the following conclusions can be drawn:

1) The soil matrix behaves best UCS and shear strength when the optimum content and length polypropylene fiber is incorporated. Excessive higher content and overlong fibers have negative effects on the mechanical properties of soil matrix.

2) The cohesion, internal friction angle and UCS of cement solidified soil increase when the cement content and curing time increase. Due to the complex effect of fiber and the existence of heavy metal ions, higher cement content and longer curing time may reduce the brittleness of specimens. The possible reason is that with the increasing of cement content, the more hydroxides are generated to absorb or encapsulate the Pb²⁺ resulting in the less negative effect of Pb²⁺ on the failure strain.

Figure 10 UCS (a) and axial stress-strain curves (b) under different number of dry-wet cycles

Figure 11 UCS (a) and axial stress-strain curves (b) under different number of freeze-thaw cycles

3) The presence of Pb²⁺ weakens the UCS and cohesion of soil specimens, and higher concentration of Pb²⁺ weakens the strength more. However, the internal friction angle is slightly impacted by the concentration of Pb²⁺. On the other hand, the failure strain of PFCMS does not change significantly when the concentration of Pb²⁺ is less than 0.5%, but increases extensively when the concentration of Pb²⁺ is higher than 1%.

4) The strength of Pb²⁺ contaminated soil with cement and polypropylene fiber S/S remediation is higher than that of Cu²⁺ and Zn²⁺. Because compared with Cu²⁺ and Zn²⁺, the solubility of Pb²⁺ is relevantly lower, resulting in the more cement hydration products in Pb²⁺ contaminated PFCMS.

5) The UCS and cohesion of soil specimens exhibit similar trends when the factors change, whereas the failure strain exhibits the adverse trend. Moreover, the internal friction angle is slightly influenced by the combined factors except for the cement content.

6) The wet-dry and freeze-thaw cycle damage the structure of the PFCMS to a large extent, and reduce the strength of the soil, which must be taken into account in the design and use.

7) The appropriate treatment with optimum content and length of fibers and cement can considerably enhance the engineering properties of HMCS. In terms of the mechanical properties, PFCMS are validated as an applicable substitutable filler in roadbed. However, the long-term engineering performances and environmental impacts of PFCMS are required to be further evaluated.

References

[1] LIU Y, HUANG Y, SUN W, NAIR H, LANE D S, WANG L. Effect of coarse aggregate morphology on the mechanical properties of stone matrix asphalt [J]. Construction and Building Materials, 2017, 152: 48-56. DOI: 10.1016/ j.conbuildmat. 2017.06.062.

[2] SHEN J, WAN L, ZUO J. Non-linear shear strength model for coal rocks [J]. Rock Mechanics and Rock Engineering, 2019, 52: 4123-4132. DOI: 10.1007/s00603-019-01775-y.

[3] WANG Yu-ke, GAO Yu-feng, GUO Lin, YANG Zhong-xuan. Influence of intermediate principal stress coefficient and principal stress direction on drained behavior of natural soft clay [J]. ASCE International Journal of Geomechanics, 2018, 18(1): 04017128. DOI: 10.1061/(ASCE)GM.1943-5622. 0001042.

[4] TANG Q, KATSUMI T, INUI T, LI Z Z. Membrane behavior of bentonite amended compacted clay [J]. Soils and Foundations (JGS), 2014, 54(3): 329-344. DOI: 10.1016/ j.sandf.2014.04.019.

[5] TANG Q, KIM H J, ENDO K, KATSUMI T, INUI T. Size effect on lysimeter test evaluating the properties of construction and demolition waste leachate [J]. Soils and Foundations (JGS), 2015, 55(4): 720-736. DOI: 10.1016/ j.sandf.2015.06.005.

[6] HUANG Y, GUAN Y, WANG L, ZHOU J, GE Z, HOU Y. Characterization of mortar fracture based on three point bending test and XFEM [J]. International Journal of Pavement Research and Technology, 2018, 11: 146-152. DOI: 10.1016/j.ijprt.2017.09.005.

[7] TANG Q, GU F, ZHANG Y, ZHANG Y Q, MO J L. Impact of biological clogging on the barrier performance of landfill liners [J]. Journal of Environmental Management, 2018, 222: 44-53. DOI: 10.1016/j.jenvman.2018.05.039.

[8] BIAN X, CUI Y J, LI X Z. Voids effect on the swelling behaviour of compacted bentonite [J]. Géotechnique, 2018, 17: 283. DOI: 10.1680/jgeot.17.p.283.

[9] ZHANG Jun-hui, GU Fan, ZHANG Yu-qing. Use of building-related construction and demolition istes in highway embankment: Laboratory and field evaluations [J]. Journal of Cleaner Production, 2019, 230: 1051-1060. DOI: 10.1016/ j.jclepro.2019.05.182.

[10] HUANG Yu-cheng, CHEN Ji, SHI Shen-jie, LI Bin, MO Jia-lin, TANG Qiang. Mechanical properties of municipal solid iste incinerator (MSWI) bottom ash as alternatives of subgrade materials [J]. Advances in Civil Engineering, 2020: Article ID 9254516. DOI: 10.1155/2020/9254516.

[11] YILMAZ O, UNLU K, COKCA E. Solidification/ stabilization of hazardous istes containing metals and organic contaminants [J]. Journal of Environmental Engineering, 2003, 129(4): 366-376. DOI: 10.1061/(ASCE)0733- 9372(2003)129:4(366).

[12] SATO A, KAISAKI S, HATA T, HAYASHI T. Possibility for solidification of peaty soil by using microbes [J]. Int J Geomate, 2016, 10: 2071-2076. DOI: 10.21660/2016.22. 5353.

[13] CELIK E, NALBANTOGLU Z. Effects of ground granulated blastfurnace slag (GGBS) on the swelling properties of lime-stabilized sulfate-bearing soils [J]. Eng Geol, 2013, 163: 20-25. DOI: 10.1016/j.enggeo.2013.05. 016.

[14] IORLIAM A Y, AGBEDE I O, JOEL M. Effect of bamboo leaf ash on cement stabilization of Makurdi shale for use as flexible pavement construction material [J]. Am J Sci Ind Res, 2012, 3: 166-174. DOI: 10.5251/ajsir.2013.3.3.166.174.

[15] FAGONE M, LOCCARINI F, RANOCCHIAI G. Strength evaluation of jute fabric for the reinforcement of rammed earth structures [J]. Compos Part B, 2017, 113: 1-13. DOI: 10.5251/ajsir.2012.3.3.166.174.

[16] GU F, MA W, WEST R, TAYLOR A, ZHANG Y. Structural performance and sustainability assessment of cold central- plant and in-place recycled asphalt pavements: A case study [J]. Journal of Cleaner Production, 2019, 208: 1513-1523. DOI: 10.1016/j.jclepro.2018.10.222.

[17] OKUBO K, FUJII T, YAMAMOTO Y. Development of bamboo-based polymer composites and their mechanical properties [J]. Compos Part A, 2004, 35: 377-383. DOI: 10.1016/j.compositesa.2003.09.017.

[18] TANG Q, SHI P X, ZHANG Y, LIU W, CHEN L. Strength and deformation properties of fiber and cement reinforced heavy metal-contaminated synthetic soils [J]. Advances in Materials Science and Engineering, 2019: Article ID 5746315. DOI: 10.1155/2019/5746315.

[19] MIAO Y, HUANG Y, ZHANG Q, WANG L. Effect of temperature on resilient modulus and shear strength of unbound granular materials containing fine RAP [J]. Construction and Building Materials, 2016, 124: 1132-1141. DOI: 10.1016/j.conbuildmat.2016.08.137.

[20] ZHANG Jun-hui, GU Fan, ZHANG Yu-qing. Use of building-related construction and demolition istes in highway embankment: Laboratory and field evaluations [J]. Journal of Cleaner Production, 2019, 230: 1051-1060. DOI: 10.1016/ j.jclepro.2019, 5: 182.

[21] TANG Q, KIM H J, ENDO K, KATSUMI T, INUI T. Size effect on lysimeter test evaluating the properties of construction and demolition waste leachate [J]. Soils and Foundations (JGS), 2015, 55(4): 720-736. DOI: 10.1016/ j.sandf.2015.06.005.

[22] DU Y J, WEI M L, REDDY K R, LIU Z P, JIN F. Effect of acid rain pH on leaching behavior of cement stabilized lead-contaminated soil [J]. Journal of Hazardous Materials, 2014, 271: 131-140. DOI: 10.1016/j.jhazmat.2014.02.002.

[23] ZHANG J H, PENG J H, LIU W Z, LU W H. Predicting resilient modulus of fine-grained subgrade soils considering relative compaction and matric suction [J]. Road Materials and Pavement Design, 2019, 146: 110-124. DOI: 10.1080/ 14680629.2019.1651756.

[24] ZENG Ling, XIAO Liu-yi, ZHANG Jun-hui, FU Hong-yuan. The role of nanotechnology in subgrade and pavement engineering: A review [J]. Journal of Nanoscience and Nanotechnology, 2020, 20: 4607-4618. DOI: 10.1166/jnn. 2020.18491.

[25] ZHANG Jun-hui, DING Le, LI Feng, PENG Jun-hui. Recycled aggregates from construction and demolition wastes as alternative filling materials for highway subgrades in China [J]. Journal of Cleaner Production, 2020: 120223. DOI: 10.1016/j.jclepro.

[26] LI J, ZHANG J H, QIAN G P, ZHENG J L, ZHANG Y Q. Three-dimensional simulation of aggregate and asphalt mixture using parameterized shape and size gradation [J]. Journal of Materials in Civil Engineering, 2019, 31(3): 04019004. DOI: 10.1061/(ASCE)MT.1943-5533.0002623.

[27] GU F, ZHANG Y, DRODDY C, LUO R, LYTTON R. Development of a new mechanistic empirical rutting model for unbound granular material [J]. Journal of Materials in Civil Engineering, 2016, 28(8): 04016051. DOI: 10.1061/ (ASCE)MT.1943-5533.0001555.

[28] TANG Q, GU F, GAO Y F, INUI T, KATSUMI T. Desorption characteristics of Cr(III), Mn(II) and Ni(II) in contaminated soil using citric acid and citric acid containing istewater [J]. Soils and Foundations (JGS), 2018, 58: 50-64. DOI: 10.1016/j.sandf.2017.12.001.

[29] HUANG Y, WANG L, XIONG H. Evaluation of pavement response and performance under different scales of APT facilities [J]. Road Materials and Pavement Design, 2017, 18(sup3): 159-169. DOI: 10.1016/j.sandf.2017.12.001.

[30] HUANG Y, JI L, WEN R, ZHANG M. A methodology of implementing target mixing ratio for asphalt mixture [J]. Frontiers of Structural and Civil Engineering, 2017, 11: 314. DOI: 10.1007/s11709-017-0405-y.

[31] TANG Q, WANG H Y, TANG X W, WANG Y. Removal of aqueous Ni(II) with carbonized leaf powder: Kinetic and equilibrium studies [J]. Journal of Central South University, 2016, 23: 778-786. DOI: 10.1007/s11771-016-3123-z.

[32] ZHANG Y, TANG Q, CHEN S, GU F, LI Z Z. Heavy metal adsorption of a novel membrane material derived from senescent leaves: Kinetics, equilibrium and thermodynamic studies [J]. Membrane Water Treatment, 2018, 9(2): 95-104. DOI: 10.12989/mwt.2018.9.2.095.

[33] TANG Q, TANG X W, LI Z Z, CHEN Y M, KOU N Y, SUN Z F. Adsorption and desorption behaviour of Pb(II) on a natural kaolin: Equilibrium, kinetic and thermodynamic studies [J]. Journal of Chemical Technology and Biotechnology, 2009, 84: 1371-1380. DOI: 10.1002/ jctb.2192.

[34] TANG Q, TANG X W, HU M M, LI Z Z, CHEN Y M, LOU P. Removal of Cd(II) from aqueous solution with activated firmiana simplex leaf: Behaviors and affecting factors [J]. Journal of Hazardous Materials, 2010, 179: 95-103. DOI: 10.1016/j.jhazmat.2010.02.062.

[35] CHEN L, LIU S, DU Y. Unconfined compressive strength properties of cement solidified/stabilized lead-contaminated soils [J]. Chinese Journal of Geotechnical Engineering, 2010, 32(12): 1898-1903.

[36] WHIFFIN V S, van PAASSEN L A, HARKES M P. Microbial carbonate precipitation as a soil improvement technique [J]. Geomicrobiol J, 2007, 24: 417-423. DOI: 10.1080/01490450701436505.

[37] CHEN S X, LI F B, KONG L W. Engineering behaviors of weak expansive soil and its treatment measures for roadbed filling [J]. Rock and Soil Mechanics, 2006, 27(3): 353-359. DOI: 10.1016/S1872- 1508(06)60035-1.

(Edited by HE Yun-bin)

中文导读

纤维水泥土作为路基填料的力学性能

摘要：污染土的处理是岩土工程和环境工程中的一个关键问题。本研究提出采用适当的聚丙烯纤维及水泥作为处理重金属污染土壤的有效方法。研究了纤维含量、纤维长度、水泥含量、养护时间、重金属种类和浓度对土壤力学性质的影响。为此，本研究进行了一系列的直剪试验、无侧限抗压强度试验、干湿循环试验和冻融循环试验。试验结果表明，聚丙烯纤维和水泥的适当配筋是回收重金属污染土壤作为路基替代填料的有效途径，具有良好的环境效益和经济效益。

关键词：路基填料；无侧限抗压强度；抗剪强度；水泥和纤维加固；重金属污染土壤

Foundation item: Projects(51778386, 51708377, 51608059) supported by the National Natural Science Foundation of China; Project(BK20170339) supported by Natural Science Foundation of Jiangsu Province, China; Project(17KJB560008) supported by Natural Science Fund for Colleges and Universities in Jiangsu Province, China; Projects(KFJ170106, KFJ180105) supported by Open Fund of National Engineering Laboratory of Highway Maintenance Technology (Changsha University of Science & Technology), China; Projects(2016ZD18, 2017ZD002) supported by Jiangsu Provincial Department of Housing, Urban-Rural Development, China

Received date: 2020-03-25; Accepted date: 2020-06-04

Corresponding author: TANG Qiang, PhD, Professor; Tel: +86-18362676527; E-mail: tangqiang@suda.edu.cn; ORCID: 0000-0003- 2751-2455