The mechanical responses of cemented soils intimately rely on the amount and nature of cementing agents. In this study, artificial cemented sands made with two different cementing agents, Portland cement (strong cementation) and gypsum (weak cementation), are selected to explore the underlying mechanisms of unique cemented-sand behavior from the particulate-scale point of view. The experimental evaluations by triaxial tests and numerical simulations using the discrete element method (DEM) are integrated for examinations. The experimental results manifest the strength enhancement, volumetric dilation, and shear banding associated failure mode can be observed in the Portland cement sand specimen; these features become more pronounced as increasing the cement content. Completely different...[ Read more ]

The mechanical responses of cemented soils intimately rely on the amount and nature of cementing agents. In this study, artificial cemented sands made with two different cementing agents, Portland cement (strong cementation) and gypsum (weak cementation), are selected to explore the underlying mechanisms of unique cemented-sand behavior from the particulate-scale point of view. The experimental evaluations by triaxial tests and numerical simulations using the discrete element method (DEM) are integrated for examinations. The experimental results manifest the strength enhancement, volumetric dilation, and shear banding associated failure mode can be observed in the Portland cement sand specimen; these features become more pronounced as increasing the cement content. Completely different responses are found in the gypsum-cemented sand specimens even with very similar initial void ratios. The volumetric contraction, less strength enhancement, and a bulging type of failure without visible shear bands are measured. The underlying mechanisms accounting for these various mechanical responses are revealed by the aid of DEM simulations with particular arrangements, i.e., using smaller cementing particles and flexible membrane boundaries. If the cementing agents are stiff and strong like the Portland cement, all the particles in the bonding network jointly share the load and many micro force-chains associated with cementation is created. Hence, a more stable and stronger force-chain network subjected to less force concentrations is resulted for higher strength. If the cementing agent is weak like gypsum, such an enhancement is less discernible. The intensive bond breakage, concentrated relative particle movement, column-like force chains (instead of a webbed pattern found in the intact bonding network), greater particle rotation, and higher local porosity concur inside the shear band of Portland cement sand specimens. The proper bonded cluster helps stabilize the particle arch and maintain large voids for subsequent volumetric dilation.

The peak- or ultimate-state strength parameters are more or less enhanced by cementation but the increment depends on the cementation characteristics and the applied confinement. For Portland cement sand, the volumetric response can be changed into dilation; the underlying mechanism is relevant to the rotation and/or displacement of particles and clusters which produce various packing combinations to form the large voids. Also, the peak strength does not concur with the maximum dilatancy, in contrast to the behavior of dense sand. The maximum dilatancy increases with cement content but is suppressed by confinement. The stress-dilatancy relationships of Portland cement sand are analyzed at different states together with the bonding breakage events obtained from the numerical simulation. Prior to yielding, the dilation is hindered and the energy is either stored or dissipated in bond breakages which attain the peak to initiate yielding. After that, the plastic strain accompanied by dilation appears and also accelerates. The peak strength is the net contribution from two competing but intimately related processes: bond breakages and the subsequent dilation. After the peak strength, the bond breakage begins to localize along the shear band. The strength loss caused by broken bonds is no longer compensated by the subsequent dilation at this stage; therefore the strength is lower than the peak value although the dilatancy is maximized. Ultimately, the mechanistic picture of Portland cement sand is depicted based on the stress-dilatancy relationships integrated with the energy dissipation owing to bonding breakages.