Multi-scale Study On The Basic Mechanisms Of High Stress Mechanical Properties For Saturated Montmorillonite

Insight into the high stress mechanical properties of saturated clay is of greatimportance in the fields of deep mine construction and nuclear waster disposalengineering, and these properties have been revealed in general in the past, but not thegoverning mechanisms. On the other hand, the structure of clay presents inmulti-scale, which means that the description of its properties from a single scale willbe unilateral. So in this dissertation, by employing computer simulations andtheoretical analysis, a systematic studies are conducted to investigate the basicmechanisms of high stress mechanical properties for saturated montmorillonite.First, we investigate the microstructure and nano-scale mechanical properties forhydrated Na-, Ca-and Cs-montmorillonite under ambient and basin condition bymolecular dynamic simulation.The simulation results show that the bonding degree of the interlayer water issignificantly higher than free water, and ion hydration coordination is also remarkablyhigher than corresponding bulk case. By further examine the characteristics of boundwater under basin condition, we find that there is almost no difference for thestructure of bound water between basin and ambient condition within2km burialdepth, and the main affecting factor is temperature. On these basis, we quantitative getthe stiffness matrix of hydration montmorillonite together, the tensile and compressivestrength perpendicular to the clay lamellae is significantly lower than the in planeatom bond strength, showing a clear anisotropy. And interlayer bound water has shearstrength to some degree, which is governed by its high viscosity.Second, mesoscopic structure and mechanical response of hydratedmontmorillonite are simulated by means of dissipative particle dynamics simulation.The interaction parameters of the mesoscopic model are estimated by mapping thecorresponding no bonded energy values obtained from atomistic molecular dynamicsimulations, and the mesoscopic simulation results show that the water structure isslightly different from free water for the78.4%water content montmorillonite system,but its diffusion coefficient and viscosity parameter indicate that under this conditionthe interlay water performance mainly as free water, and the mesoscopic K0compression simulation reveals that with the vertical strain increasing, the verticalstress increases non-liner parabolic, while the lateral stress increases linear, therebymaking the mesoscopic K0nonlinear. Third, By taking into account the short-range hydration repulsion, we modify thetraditional Gouy-Chapman diffusion double layer theory and analysis the high stresscompressibility of saturated clay by this new theory in macroscopic.This new theory is modified by additional considering the dielectric saturation andion volume exclusion effects, compared to the others model the calculated dielectriccoefficient between particle plates changes more smoother, and the potential valueunder small plates spacing is higher than the tradition double layer theory, whichmeans that the short-range hydration repulsion plays a role. In addition, thequantitative calculation result of van der Waals shows that the attraction betweenplates should be taken into account under high stress compression. Then, wecomprehensive calculate the repulsion and attraction forces between particle platesand get the theory compressive curve for saturated under high stress. Finally, thefactors that affect the high stress compressibility are analysised together.Finally, based on the micro-and meso-simulations and macroscopic theory results,we comprehensive analysis the basic mechanisms that govern the high stresscompression and shear mechanical properties for saturated montmorillonite.The theory results show that the bilinear characteristic of high stress compressioncurves for initially saturated montmorillonite can be reproduced well by the modifiedtheory, and the interlayer bound water will only partially dehydrated under thecondition of maximum overburden pressure of40MPa. Further, the correspondingplates distance for interlayer water characteristic changing from free water to boundwater can be got for mesoscopic and microscopic molecular simulation, and thepressure range that calculated from the macroscopic theory corresponding with theseplate spacings are in good agreement with test turning pressure. Which revealed thatunder high stress compression the plate distance for montmorillonite particle willcompress into basic crystal spacing, and the water characteristic will change from freewater to bound water, that the partially dehydrated of bound water govern the basicmechanism of high stress compression for saturated montmorillonite, and thedifference between various compensative cation montmorillonite lies in the differenceof ion hydration ability. And there is transformation process form mineral friction tointerlayer water shear with the stress increasing, which controls the basic mechanismof high shear.