Failure criteria of unreinforced masonry walls of rural buildings under the impact of flash floods in mountainous regions

Damage to rural buildings in mountainous regions caused by flash floods accounts for a significant proportion of economic losses from disasters. The unreinforced masonry (URM) wall is the most vulnerable structural element of rural buildings exposed to flash floods. The failure of a URM wall indicates damage to rural buildings in flash floods. Based on the yield line theory of out-of-plane damage of URM walls and the virtual work method, brittle failure criteria for URM walls under the impact of flash floods were established. According to the field investigation data of the 26 June 2020 flash flood event in Damawu Gully and the corresponding simulation results of FLO-2D, the disaster-causing process was analysed, and the failure criteria were validated. Three building parameters were identified to influence the flood-resistance of URM walls, including the mortar grade, the span-to-height ratio of the wall, and the number of floors of the rural building. The results showed that the cause of the 26 June disaster was the diversion of a 50-year flash flood into the residential community on the alluvial fan. The affected buildings were constructed with hollow blocks and lacked flood-resistance reinforcement. The critical failure depth of a URM wall restrained at the top by ring beams (RBs) under hydrostatic load conditions is 1.17 to 1.20 times greater than that of a URM wall without RBs, and the difference is even more pronounced when lower-strength mortar is used. The flood-resistance of a URM wall constructed with Mb 7.5 mortar and restrained by RBs is almost as strong as that of a URM wall constructed with Mb 20 mortar and without RBs. The span-to-height ratio of a URM wall should not be greater than 1.875 in this case. However, the flood-resistance of a URM wall with RB restraint is almost independent of the span-to-height ratio. The brittle fracture energy of masonry mortar is more crucial to the flood-resistance of 4-edge restrained URM walls if L/Z > 1.875. The flood-resistance of the URM wall of the first storey increases linearly with the number of floors. Single-storey rural buildings should be given priority to the use of high-grade masonry mortar and high-density blocks to improve flood-resistance. The failure criteria and the influence laws of building parameters on the flood-resistance of URM walls can provide references for flash flood mitigation and flood-resistance reinforcement of rural buildings in mountainous regions of Southwest China.