A series of high-pressure compression tests with various stress paths under monotonic and cyclic loadings were performed to examine the compression and particle crushing characteristics of silica sand in a dense state. Different stress paths were particularly designated to individually investigate the influences of mean and shear stress on particle crushing. Test results show that the degree of particle crushing increases with the stress level and is markedly affected by the stress paths. For each compression test following a designated stress path, an increase in the cyclic loading number also enhances the degree of particle crushing. Particle morphology is affected by the accompanying occurrence and evolution of grain damage. The relative breakage expresses a linear relationship with the maximum volumetric strain during monotonic and repeated compression loadings. A good correlation between the relative breakage and plastic work per unit volume is obtained for silica sand at various mean stresses regardless of the stress history. However, this correlation is affected by cyclic loading number because of different crushing mechanisms. During the entire cyclic compression process, the increasing rate of relative breakage is lower than the plastic work increment for dense silica sand as the cyclic loading progresses.

Clinker ash is a byproduct obtained from coal-fired electrical generators. It is a type of recycled granular material with a complex particle shape and large surface roughness. Particle shape descriptors and surface fractal dimensions are effective parameters to quantify sand grain morphologies. A simple image analysis approach is employed to determine the aspect ratio, sphericity, roundness, and convexity of 12 types of clinker ash collected from thermal power plants distributed on the western island of Japan. Compared to the majority of sands with other mineralogical components, clinker ash particles have a smaller aspect ratio, sphericity, and roundness. Under similar test conditions, clinker ash exhibits higher shear resistance than natural sands at the critical state. Critical state friction angles tend to decrease with increasing aspect ratio, sphericity, roundness, and convexity. A decreased aspect ratio, roundness, and convexity enhance the slope and intercept of critical state lines in the void ratio and mean stress plane. The mean regularity, which is the average of four particle shape descriptors, is regarded as an effective index to characterize the shear strength of clinker ash at the critical state. The surface fractal dimension could also be decided from digital image analysis. The results also show that the critical state friction angle is well correlated with the surface fractal dimension for different types of clinker ash. The implication is that a rougher surface texture hinders the rotation, frustration, and interlocking of particles at the critical state, resulting in higher shear resistance.

The particle shape of coral sand is a crucial factor that affects its accumulation characteristics. Two-dimensional particle images of coral sand with different particle sizes were obtained through optical imaging, and the basic size parameters of particles were measured by digital image processing. The particle shape parameters were created, and on this basis, the variation of shape parameters with size, the distribution characteristics, and the sensitivity of shape parameters were analyzed by mathematical statistics and the fractal theory. In addition, a comparative analysis was conducted for the particle shape and bulk density of coral sand and quartz sand with the same particle size. The results show that (1) for coral sand with particle size ranging from 0.5 to 5.0 mm, as the particle size augments, its overall profile coefficient grows, while the flatness, angularity, and roughness diminish and the particle shape deviates more from the regular circle. (2) The shape of coral sand particles exhibits good fractal characteristics, and the particle shape gets more complex as the particle size grows as evidenced by the fact that the fractal dimension enlarges. (3) All the shape parameters obey a skewed distribution. Concerning the sensitivity to the change in particle shape, the flatness occupies the first place, the overall profile coefficient and angularity come second, and the roughness ranks third, accordingly. It is suggested that flatness should be preferred as the evaluation parameter of the particle shape. (4) Compared with that of quartz sand, the particle shape of coral sand is more irregular, and the intergranular pores are larger under the same accumulation conditions, which is the primary reason why the specific gravity of coral sand is greater than that of quartz sand while the bulk density is smaller than that of quartz sand.

A series of undrained monotonic and cyclic triaxial tests were performed on silica sand at two initial densities and different confining pressures from 0.1 to 5 MPa to investigate their shear response and crushing behaviour. The influence of particle crushing on the undrained shear strength and pore-water pressure was examined. To clarify the evolution of particle crushing, undrained monotonic and cyclic tests were terminated at several distinctive stages and sieving analysis tests were subsequently performed on the tested specimens. In the undrained monotonic test, specimens exhibited remarkable dilation behaviour and experienced no apparent particle crushing at low confining pressures. An increase in the mean stress suppressed the dilatancy due to a faster increase of the pore-water pressure, giving rise to the occurrence of particle crushing. In the undrained cyclic test, a higher confining pressure and cyclic stress ratio resulted in a much higher relative breakage. At a specific cyclic stress ratio, the relative breakage increased as the cyclic loading progressed. The confining pressure and shear strain amplitude played a significant role in controlling the evolution of particle breakage. The correlation between the relative breakage and plastic work for specimens under isotropic consolidation, undrained monotonic, and cyclic loadings has been validated experimentally.