Study On Technology And Engineering Application Of Silt Solidified By Lignin

Silt subgrade treatment is a difficult technical problem usually encountered in the traffic engineering construction in southeast coastal areas of China. So exploring an economic and efficient silt treatment technology is one of the urgent needs to be solved in geotechnical engineering. Lignin is the main by-product of bioenergy and paper industry which has not been used efficiently for ground improvement. A new silt subgrade treatment technology based on industrial by-product of lignin is proposed in this dissertation, and the study of stabilization mechanism and engineering properties of lignin treated silt is intended to provide a scientific basis for the application of this new technology.Based on this background, with the financial support of Jiangsu Traffic Science Research Project under Grant No.2013Y04, the mechanism and engineering properties of lignin treated silt were studied by laboratory tests, theoretical analysis, and field tests. The main works and conclusions of this dissertation are shown as following:(1) The state-of-the-arts of the basic properties and resource utilization of lignin based by-product and the application of lignin in the treatment of soils, mechanical properties, stabilization mechanism, and the constitutive models of treated soils were reviewed. The problems in literature studies were discussed.(2) The variation in the physical and mechanical properties and durability of silt after treatment of lignin was systematically investigated. Investigations were carried out with respect to the effect of lignin content and curing time on the basic physical property indices and mechanical characteristics by laboratory tests. In addition, the improvement effects on silt by lignin and lime were compared and analyzed, furthermore, the feasibility and benefits of lignin treated silt technology were discussed. The results indicate that the strength of silt treated with lignin increases significantly as lignin content increases from 0%to 12%, but the value of strength decreases slightly when the lignin content exceeds 12%. The optimum content of lignin for silt in Jiangsu Province China is approximately 12%. The durability of lignin treated silt is better than that of lime treated silt, but the ability to reduce shrinkage of soil is worse than that of lime.(3) Based on the testing technique of bender element, the evolution of small-strain shear modulus Go for lignin and cement treated silt during curing time was studied. Moreover, the relationships between the small-strain shear modulus and unconfined compression strength of treated soils were analyzed. Normalized parameters were defined and introduced to develop a new method for nondestructive evaluating the strength of soils. The results indicate that for a given stabilizer, the time-dependent development of normalized small-strain stiffness of treated soils essentially remains the same regardless of the dosage. There is a unique relationship between normalized small-strain shear modulus and normalized unconfined compression strength of treated soils. The small-strain shear modulus can be used as an evaluation index for the development of strength for treated soils.(4) The thermal resistivity of natural soil and lignin treated silt was monitored by the non-steady-state thermal probe test technology. The effect of the factors, such as moisture content, dry density, particle size, and saturation on the thermal resistivity of natural soil was discussed. A general prediction model of thermal resistivity was established. For lignin treated silt, the evoluation of thermal resistivity of sample during curing period was investigated. The effects of lignin content and compaction moisture content on thermal conductivity were also discussed. The intrinsic correlations between thermal resistivity and mechanical indices were investigated. The results indicate that with an increase in moisture content, dry density, particle size, and saturation, the magnitude of thermal resistivity for natural soils decreased. There is a rapid increase in the thermal resistivity with a small reduction in moisture content, at moisture content less than the critical moisture content. The relations between thermal resistivity and influencing factors do not show a single correction. Thermal resistivity of lignin treated silt increases with an increase of lignin content and curing time, the values of thermal resistivity for specimens with different lignin contents become almost the same after 60 days of curing. The thermal resistivity differences at different moisture contents disappear when the curing time exceeds 28 days. Linear relationship is established between thermal resistivity and unit weight for lignin treated silt, and an increasing trend is observed between resistivity and unconfined compression strength, and resilient modulus.(5) The changes of soil microstructure and chemical composition of silt after treatment of lignin were analyzed by a series of micro-chemical analysis including SEM, MIP, pH value, electrical resistivity, XRD, EDS, and FTIR. The results indicate that a more dense and stable soil structure is formed and no new functional groups and clay minerals are produced after treatment comparing to natural silt. The mean crystalline sizes of main clay minerals are reduced to varying degrees. The thickness of double-layer reduces and a cementing material is produced after treatment by hydrolyzation, protonation, and electrostatic reaction. The pores are filled and particles become coated with this cementing material, which bonds them closely together to produce a stronger soil structure.(6) The energy required to direct shear during different stages were analyzed according to energy conservation. A theoretical model was proposed to evaluate the shear strength characteristics of treated silt. Moreover, based on the results of triaxial compression test, the variation characteristics of shear strength for treated silt at critical state were investigated, and a boundary surface plasticity model considering cementation was established. The results indicate that the cementing material produced by lignin could not only improve the mechanical properties of silt but also enhance the ability of plastic deformation. The energy required to shear for treated silt having cementation is mainly made up of the energy required to break inter-particle bonds, the energy required for a particle to roll, and the energy required for a particle to slide. Among these energies, the breaking bonding energy is the inherent feature of soil, and it is independent of vertical stress. Shear strength at critical state increases with increase of lignin content and the critical state line of treated silt shifts to the left side with respect to natural silt. Apparent cohesion can be used to indicate the effect of treatment by lignin on the strength of soil at the critical state. The proposed constitutive model including new parameters, such as new governing equation and destruction rate of bounding effect could successfully capture the characteristics of stress-strain for lignin treated silt.(7) Field tests were carried out in a construction section of Funing-Jianhu highway, Yancheng city to verify the performance of silt treated by lignin is better than that of lime. A construction technology of lignin treated subgrade silt reclamation was proposed in this study and the application effects of using lignin as subgrade silt stabilizer were systematically evaluated. The pavement performance of lignin and lime treated silt was also monitored. In addition, economic benefits and environmental impact of treated silt were studied. Finally, the treatment method of leaching liquor of subgrade soils was discussed by literature investigation. The results indicate that the strength growth of lignin treated silt is slower than that of lime treated soil at the beginning of curing time, but then the pavement performance indices of silt treated by 12%lignin is better than that of silt treated by 8%lime. The increase of moisture content has a negative effect on the development of treated soil strength. The material cost of lignin treated silt is less than that of lime treated silt, and the environmental quality of lignin treated silt satisfies the requirements of two grade quality standard.