Chemical And Mechanical Behaviour Of GMZ Bentonite And Its Predictions

Many countries are planning to dispose high-level radioactive nuclear wasters in sealed repositories located deeply in geology system.As one of engineered barriers,highly compacted expansive clays are considered for isolating the nuclear wastes at great depth.Bentonite is used as buffer materials or backfill materials because of its low hydraulic conductivity,high swelling property and good self-sealing capacities,etc.In China,Gaomiaozi(GMZ)bentonite has been chosen as a potential buffer material for deeply buried geological disposal system to isolate the high-level radioactive wastes(HLW).The Beishan area located in Gansu Province has been identified as the most suitable area for a national repository for radioactive waste.Since the groundwater in this region contains the total dissolved solid(TDS)with cation Na+ and anions Cl-and S042-which will affect hydro-mechanical behaviour of bentonites,NaCl-Na2SO4 was chosen as a dissolved solid in the solution saturating GMZ bentonite.In this paper,A series of laboratory tests(including swelling pressure test,swelling deformation test,compression test and direct shear test)on two GMZ sodium bentonites(called as GMZ001 and GMZ07 which is exploited at different time)and GMZ calcium bentonite(Ca-bentonite)are performed at different initial water contents and initial densities.These issues are the important technical problems to be resolved both in theory and in engineering application for the deep nuclear wastes disposal system.As a result,two methods(called macro and micro predictions)to predict the swelling pressure and deformation are given from the dry density of the buffer materials and the ion concentration of salt solution.The main conclusions are shown as follows:1.Swelling pressure test,swelling deformation test and compression test are are performed on two GMZ sodium bentonites(Na-bentonite).The test results showed that GMZ001 has larger swelling pressure than GMZ07 at the same void ratio and is harder to be compressed than GMZ07.On the other hand,the microstructural features of two GMZ Na-bentonites in saturated state were investigated by combining the mercury intrusion porosimetry(MIP)tests and scanning electronic microscope(SEM)observations.The test results reveal that GMZ001 has less inter-aggregate porosity than GMZ07 and the aggregates of GMZ001 characterized by stronger hydration compares to GMZ07.The relationship between the swelling pressure and void ratio depends mainly on their montmorillonite contents followed by the influence of grain size.2.The swelling and compression tests on GMZ07 specimens with different initial water contents were performed.The test results show that the swelling pressure and swelling deformation almost unchanges with increasing the initial water contents and decreases when the initial water content is beyond 20%which is closed to the optimum water contents(19.3%)for compacted GMZ07.Moreover,the mercury intrusion porosimetry(MIP)on GMZ07 specimens with different initial water contents were performed for investegating the microstructure.The MIP results show that the increase in the initial water content leads to a micropore increase and a macropore decrease.The results also show that the micro-pore structure remained almost unchanged and starts to change only when the intra-aggregate pore approaches saturated.The above results revealed that the swelling pressure is unchanged for the initial water content range from 5 to 20%due to the specimens with similar microstructures.3.One-dimensional swelling deformation tests and swelling pressure tests were carried out on two GMZ Na-bentonites and GMZ Ca-bentonite saturated by the salt solutions with different TDS concentrations and distilled water.Based on the concept of the montmorillonite void ratio,the deformation characteristics of GMZ bentonites at saturation were consistently identified in the salt solution and distilled water circumstances,respectively.The results show that both the relationships of the montmorillonite void ratio versus the swelling pressure and the dry density versus the swelling pressure are the straight lines and the lines are parallel each other in the double logarithmic coordinates when the concentration of TDS is 0,0.3,1.0 and 12.3g/l,and the swelling pressure in the logarithm changes linearly with the TDS concentration.4.A simple new method with the different decay factors was proposed to describe the variation in the montmorillonite void ratio in the different salt solutions under the same vertical pressure.This method was used to predict the swelling pressures for the given final dry density,decay factor and TDS concentration.In addition,a microscopic prediction model was presented.The swelling pressures predicted by the proposed macroscopic method show a good agreement with those obtained by the microscopic model.5.Results of the direct shear tests on compacted GMZ07 indicate that shear strength has a noticeable improvement with increasing the salt solution concentration.The test results reveal that friction angle increases greatly with the salt solution concentration but the cohesion intercept doesn't.Results of compression tests performed with different salt solution concentrations show almost the same compressibility for compacted GMZ07 and its mixture with sand.At last,the micro-fabric changes associated with variations in the salt solution concentration have been photographed by the SEM.The photographs show that the increase of salt solution concentration would change the roughness on the aggregate surface,which will produce greater friction among aggregates.