Although numerous studies focus on the mechanical behaviour and instability of tailings dam materials, a framework that adequately describes all the processes involved in their failure and fluidization is still needed. Therefore, we used a series of instrumented flume tests, with pore pressure transducers, micro-seismic accelerometers, internal displacement transducers and laser scanning, to investigate the mechanisms of tailings-dam failure and fluidization. The artificial rainfall induced water infiltration was adopted to induce the failure. The test results showed that continuous rainfall expanded macro-voids and led to particle rearrangement and local collapse, producing a gradual buildup of pore pressure. Furthermore, the increasing porosity enhanced seepage forces and thus reduced the shear strength of the tailing's materials so that the dam models were close to liquefaction. However, a dam model with a higher hydraulic conductivity would not be totally damaged, even if failure and fluidization occurred. The amount of antecedent rainfall likely significantly contributed to dam failure because it raised the moisture content of the tailing's materials, which promoted internal erosion; thus, the rainfall might have been sufficient to generate local fluidization. Rainfall decreased the peak pore pressures required to initiate failure and shortened the time to failure but hardly influenced the fluidization movement distance. •We use a series of instrumented flume tests to investigate the mechanisms of tailings-dam failure and fluidization.•We find that the relative density of tailing material can largely influence the failure pattern of the tailing slope.