Physical And Dynamic Properties Of Chemically Modified Bentonite Under Freeze-Thaw Cycles

With the development of urbanization and industrialization in China,the amount of garbage removal and transportation is increasing year by year,and the environmental problems are becoming increasingly prominent.At present,the main treatment method of municipal solid waste in China is landfill treatment,and the leachate produced in the process will cause serious harm to the cushion and the surrounding environment.Therefore,the selection of cushion material and its mechanical properties become particularly important.Bentonite is often chosen as the first choice of barrier material because of its strong adsorption capacity,ion exchange capacity and low permeability.In addition,as a large country of frozen soil,there must be many landfills built in permafrost regions in China.The freeze-thaw cycle in seasonal frozen regions will cause the deterioration of cushion soil,but its deterioration mechanism and mechanical response characteristics are still unclear.In this paper,the physical and mechanical properties of landfill cushion in seasonal frozen area are studied by considering the interaction of chemical,thermal and mechanical fields,which is of great practical significance for the long-term stability and safe service of landfill.This paper is based on the National Major Scientific Research Instrument Development Project(41627801),the State Key Laboratory of Frozen Soil Engineering(SKLFSE201601)and the Special Project Fund of Taishan Scholars of Shandong Province,China(2015-212).The physical and dynamic characteristics of the soil before and after the modification were studied by X-ray diffraction test,free expansion rate test,limit moisture content test,CT scan test and MTs dynamic triaxial test.The main research results are as follows:(1)The modified bentonite can be obtained by direct mixing and electrochemical method.Compared with the mixing method,the electrochemical method improves the migration efficiency of ions under the action of external electric field,which makes the replacement effect of counterions in the modified bentonite by electrolysis method better than that by mixing method.(2)The effect of chemical modification on bentonite is closely related to the cation valence and concentration of salt solution.The higher the ion valence state or the higher the concentration of salt solution,the more significant the reduction of the displacement effect and free expansion rate of the counterbalance ions.The higher the valence state of the cations,the more charged,the ion exchange between a single high-priced cation and several monovalent Na~+can be achieved,which reduces the number of cations in the diffusion layer.The higher the concentration of salt solution,the higher the cation content,the more cation can be exchanged among layers.The higher the concentration of salt solution,the smaller the thickness of the double layer,the larger the gravity between particles and the lower repulsion force.(3)The freeze-thaw cycle has a significant effect on the microstructure of modified bentonite.CT scanning results show that cracks appear in the inner and outer walls of the samples after freeze-thaw,and the length,width and number of cracks increase with the increase of freeze-thaw cycles.(4)The freeze-thaw cycles have a significant effect on the dynamic characteristics of bentonite before and after modification,especially in the first two cycles.The dynamic triaxial tests of bentonite before and after 0-14 freeze-thaw cycles were carried out by dynamic triaxial equipment.It was found that when the shear strain was constant,with the increase of freeze-thaw cycles,the dynamic shear modulus of two kinds of bentonite decreased first and then increased,while the damping ratio showed the opposite trend:the damping ratio of sodium bentonite increased first and then decreased,while that of potassium bentonite was on the contrary.When the freeze-thaw cycles are fixed,the dynamic shear modulus of the two bentonites decreases with the increase of dynamic shear strain,and the damping ratio increases with the increase of shear strain.In contrast,the effect of freeze-thaw cycles on the dynamic parameters of K-bentonite is less than that of Na-bentonite.(5)The experimental data of dynamic shear modulus and damping ratio are fitted by davidenkov model and the damping ratio model proposed by Chen Guoxing.S~2 is above 0.9,and the fitting effect is better.