The load a tephra fall deposit applies to an underlying surface is a key factor controlling its potential to damage a wide range of assets including buildings, trees, crops and powerlines. Though it has long been recognised that loading can increase when deposits absorb rainfall, few efforts have been made to quantify likely load increases. This study builds on previous theoretical work, using an experimental approach to quantify change in load as a function of grainsize distribution, rainfall intensity and duration. A total of 20 laboratory experiments were carried out for ~ 10-cm thick, dry tephra deposits of varying grainsize and grading, taken to represent different eruptive scenarios (e.g. stable, waxing or waning plume). Tephra was deposited onto a 15° impermeable slope (representing a low pitch roof) and exposed to simulated heavy rainfalls of 35 and 70 mm h⁻¹ for durations of up to 2 h. Across all experiments, the maximum load increases ranged from 18 to 30%. Larger increases occurred in fine-grained to medium-grained deposits or in inversely graded deposits, as these retained water more efficiently. The lowest increases occurred in normally graded deposits as rain was unable to infiltrate to the deposit’s base. In deposits composed entirely of coarse tephra, high drainage rates meant the amount of water absorbed was controlled by the deposit’s capillary porosity, rather than its total porosity, resulting in load increases that were smaller than expected. These results suggest that, for low pitch roofs, the maximum deposit load increase due to rainfall is around 30%, significantly lower than the oft-referenced 100%. To complement our experimental results, field measurements of tephra thickness should be supplemented with tephra loading measurements, wherever possible, especially when measurements are made at or near the site of observed damage.

火山灰坠落沉积物施加于下垫面的负载是控制其损坏包括建筑物、树木、农作物和电力线在内的各种资产的可能性的关键因素。尽管人们早就认识到当沉积物吸收降雨时负荷会增加，但很少有人努力量化可能的负荷增加。这项研究建立在先前的理论工作的基础上，使用实验方法来量化作为粒度分布、降雨强度和持续时间的函数的负载变化。总共对 10 厘米厚、不同粒度和等级的干火山灰沉积物进行了 20 次实验室实验，以代表不同的喷发情景（例如稳定的、上蜡或减弱的羽流）。⁻¹持续时间长达 2 小时。在所有实验中，最大负载增加范围为 18% 到 30%。细粒到中粒矿床或反分级矿床的增加幅度更大，因为这些保留水的效率更高。由于雨水无法渗透到矿床底部，因此在正常分级矿床中的增幅最低。在完全由粗火山灰组成的矿床中，高排水率意味着吸收的水量由矿床的毛细孔隙率控制，而不是由其总孔隙率控制，导致负载增加小于预期。这些结果表明，对于低坡度屋顶，降雨导致的最大沉积物负载增加约为 30%，明显低于经常引用的 100%。为了补充我们的实验结果，