Particle size distribution is one of the key factors affecting the dam outburst.In order to investigate the influences of particle size distribution of dam on dam outburst features

the average particle size of D50 are introduced as the main index

with different mean particle size of soil as the research object

throuhg the development of experimental dam outburs features under the different gradation conditions

analysis of the dam breaching character under three different conditions（the first condition:D50=5.4mm;the second condition:D50=6.8mm;the third condition:D50=9.2mm）

the preliminary conclusions are as follows:（1）The test results show that the dam under the third condition（D50=9.2mm）the outburst duration is 2.5times that of the first condition（D50=5.4mm）

while the peak flow rate is reduced by about 60%.This indicaes that the average particle size of the dam has a significant influence on the characteristics of the dam outburst

the larger the average particle size

the greater anti-erosion ability of the dam will be

the slower the break development

the smaller the outburst peak flow volume is the time to reach peak flow volume is also lagging behind;（2）the average particle size and permeability coefficient can be proposed to be a quadratic function

with the average particle size increaing.The permeability coefficient is also increasing;based on characteristics of pore water pressure

the breaking process is divided into three stages:upstream water level increase stage

water overtopping infiltration stage.dam outburs stage.