This paper describes an image-based discrete element method (image-based DEM) simulation using irregularlyshaped grains under high confining stress. A series of one-dimensional compression simulations using specimens with various spring constants was performed to optimize the spring constant. As a result, a compression curve of specimen with 105 N/m spring constant was in good agreement with that obtained in past experiment within about 20MPa normal compression stress where the effect of grain crushing is thought to be still small. Since the imagebased DEM cannot express the rearrangement of small grains due to grain crushing, the compression curves of specimen disagreed with that of the experiment after 20MPa normal compression stress. Triaxial compressive properties (e.g., stress-strain relation, dilation curve and relation between angle of shear resistance and confining stress) in the image-based DEM simulations under high confining stress using specimen with 105 N/m spring constant were in good accordance with those of past experiments. Additionally the friction behavior of an interface between soil and rough surface structure was analyzed by the image-based DEM. The relation between friction angle and confining stress obtained in the simulations was almost identical to that obtained in past tests conducted by pushing out a steel rod embedded in sand.
To design underground structures and estimate their stability, it is essential to understand mechanical behaviors of rock and applicability of constitutive equations not only in compression but also in tension. In this study, the non-linear visco-elastic constitutive equations proposed by the authors in compression were applied to the behaviors in uniaxial tension. The deformation characteristics and mechanism in uniaxial compression and tension under dry and wet conditions were investigated by comparison of the mechanical properties and the constants in the equations. It was found that three non-linear visco-elastic constitutive equations can reproduce the stress-strain curves in uniaxial tension for Sanjome andesite in dry and wet, Kawazu tuff in dry and wet, and Inada granite in dry condition. Time-dependent behaviors of Sanjome andesite in dry condition such as loading-rate dependency and creep can be also reproduced with the same equations. Because the calculated results of unloading curves were not consistent with the testing ones, the new constitutive equation combining the previous ones was proposed. The testing results showed that strength degradation by water is larger in tension than in compression and that initial Young's modulus in tension is nearly equal to the slope of unloading curves in the pre-failure region in compression. Relationship of the constants in the constitutive equations under various testing conditions was discussed. The exponents representing the stress dependency of strain rate in transient creep were calculated with the constants in tension, and found to be consistent in the three constitutive equations under dry and wet conditions.
We investigated co-precipitation using magnesium (Mg) salt as a cost efficient method of removing B from wastewater. To clarify the mechanism of co-precipitation and the effect of precipitation rate against B sorption mechanism, we conducted co-precipitation experiments changing reaction time at different temperatures. For these experiments, we carried out sorption isotherm formation and XRD analysis to clarify how co-precipitation of B with magnesium hydroxide (Mg (OH) 2) occurred. The sorption isotherm of co-precipitation was a BET type, while the XRD peak shift occurred as the initial B/Mg molar ratio increased. These results suggested that the mechanism of B uptake was a combination of surface precipitation and surface complexation, and the later mechanism became more apparent as the initial B/Mg molar ratio increased. XRD spectrum of co-precipitated residues was relatively similar to that of hydromagnesite, which indicated that structure of surface precipitation is similar to that of hydromagnesite. Co-precipitation experiments for 6 hours revealed that Mg precipitation rate was faster at the higher temperature, while sorption density became worse as the temperature increased. At 40℃ and 60℃, XRD peak shift did not occur when the initial B/Mg molar ratio was0.063, which suggests that fast precipitation rate disturbed production of surface precipitation. In addition, the initial B/Mg molar ratio in which B sorption mechanism changed from formation of surface compexation to production of surface precipitation became larger as Mg precipitation rate increased.