Study On Mechanical Properties And Theory Of Bentonite Under Alkaline Solution Erosion

At present,deep geological disposal is considered to be a more suitable way for the treatment of high-level radioactive waste.Bentonite has the characteristics of low permeability,good nuclide adsorption and large expansion self-healing function,so it is an ideal cushion material in deep geological disposal repository and has good engineering application value.However,the concrete,the main material of the surrounding rock lining of the disposal repository,will gradually age and decay in the operation stage,and interact with the infiltrated groundwater to form a high alkaline solution.When this kind of alkaline aqueous solution comes into contact with bentonite and interacts with bentonite,it will erode the montmorillonite minerals in bentonite,and then have a significant impact on the mechanical properties such as the deformation and strength of bentonite.In this paper,the swelling deformation test and direct shear test of compacted bentonite samples were carried out in different concentrations of alkaline solution.By using e-p′fractal relation and Mohr-Coulomb criterion,and comprehensively considering the effects of various factors on swelling properties and shear strength,the fractal calculation model of swelling deformation of compacted bentonite in alkaline solution was established,and the variation law of shear strength of bentonite was explored.It can provide an effective calculation method for the mechanical properties of bentonite in complex application environment.The specific research contents are as follows:（1）The swelling deformation tests of bentonite in distilled water and different concentrations of NaOH and KOH solutions were carried out.The experimental results show that the vertical load,the concentration of alkaline solution and the type of alkaline solution all affect the swelling property of bentonite.In the solution of NaOH and KOH,the dissolution of montmorillonite and the increasement of osmotic suction weaken the swelling property.By introducing the modified effective stress p′considering the effect of osmotic suction,it is found that the void ratio e of bentonite can be expressed as e=κp′^D_s-3 in the same concentration of NaOH and KOH solution,while with the increase of alkaline solution concentration,the dissolution of montmorillonite increases,and the swelling coefficientκdecreases.At the same time,the surface of montmorillonite particles becomes rougher and the surface fractal dimension increases due to the corrosion of alkaline solution to montmorillonite.At the same concentration,the inhibition of KOH solution on swelling is greater than that of NaOH solution.（2）The direct shear tests of bentonite after swelling/consolidation saturation in distilled water and different concentrations of NaOH and KOH solutions were carried out.The experimental results show that the specimen shows obvious strain softening characteristics under various conditions,and the shear strength increases gradually with the increase of alkaline solution concentration and vertical load,especially under high concentration and high load.Based on the fractal theory of bentonite surface,the effective stress p′with the effect of osmotic suction is adopted,and the test results are calculated with Mohr-Coulomb criterion.With the help of SEM image analysis,it is found that the increase of concentration in the same alkaline solution leads to the dissolution of particles and the increase of surface roughness,which leads to the increase of effective internal friction angle.Compared with NaOH solution,the effective cohesion and effective internal friction angle of KOH solution increase,mainly because K⁺enters the silicon-oxygen tetrahedral cavity to form a stable K-bond,which makes the particles connect closely and enhance the interaction force.（3）The direct shear test of compacted bentonite under constant volume after saturation in distilled water and different concentrations of NaOH and KOH solutions and at different temperatures was carried out.It was found that the shear strength increased with the increase of concentration and temperature.The relationship between shear strength and effective stress at different concentrations is basically kept in the same line,and the increase of concentration does not significantly change theτ_f-p’ relationship,which is mainly due to the poor water permeability of constant volume bentonite in high density state,less water absorption at saturation,little reaction between alkaline solution and sample,and little change in mineral composition.The increase of temperature increases the kinetic energy of water molecules and increases the shear strength and effective internal friction angle.Therefore,when bentonite is in the state of high pressure and constant volume,the adverse effects of alkaline solution can be alleviated by increasing its compactness.