Much research has focused on the micro-mechanics of sand particles. The single particle uniaxial compression test is a common way to study breakage behaviour. However, there is still little agreement on particle breakage criteria and the mechanisms of breakage remain uncertain, partly because of the often rapid brittle failure of sand particles. In this study, a series of single particle uniaxial compression tests on different kinds of sand particles were carried out, using a high-speed microscope camera to capture the processes of breakage. This enabled a maximum of 2000 frames to be obtained per second to identify clearly the failure processes and crack propagation. Four failure modes have been proposed based on the rapidity of failure and the size and number of particle fragments created during the breakage: splitting, explosive, explosive-splitting and chipping. The relationship between the particle strength and the breakage mode has then been explored, investigating also whether immersion would affect the breakage mode and strength. The morphologies of the sand particles including local roundness, particle size, aspect ratio, regularity and two-dimensional sphericity were measured to investigate their influence. With assistance of the high-speed microscope camera, the crack initiation locations were identified and the frequency of different locations obtained, comparing the results with existing particle micro-mechanical theories.

3Particle crushing has been recognised to be of key importance for many engineering 4 applications. In soil mechanics, this phenomenon has become crucial in defining a complete 5 framework able to describe the mechanical behaviour of sands. In this study, the effect of 6 multiple discrete contacts on the breakage of a grain was investigated, crushing coarse grains 7 of a quartz sand and a crushed limestone sand between a number of support particles, thereby 8 varying the number of contacts, i.e. the coordination number. The stress at failure was 9 calculated when the particle broke, which was through a number of distinct modes, by 10 chipping, splitting or fragmenting which were observed with the use of high speed 11 microscope camera. The Weibull criterion was applied to calculate the probability of 12 surviving grain crushing and the fracture modes were observed for each configuration of the 13 supporting particles. The data showed that in addition to the number of the contacts the nature 14 of those contacts, controlled by the particle morphology and mineralogy, play a significant 15 role in determining the strength of a particle. The sphericity affected the strength for the 16 softer limestone while the local roundness at the contacts was important for the harder quartz 17 sand. Catastrophic explosive failure was more often observed in particles with harder 18 contacts while softer contacts tended to mould relative to their neighbouring particles 19 inducing a more frequent ductile mode of crushing. 20 21 22

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