To understand the mechanism of the settlement caused by water and sand inrush in underground engineering

a set of visual test device that can change the blow mouth diameter and simulate gushing of water and sand is designed. This device can vary the condition of different thickness-to-span ratios and different contents of fine particles in sandy soil. When the thickness-to-span ratio is more than a certain value

regardless how much content of fine particles in sand

the soil will not settle. When the thickness-to-span ratio is within a certain range and the fine particles is less than a certain value（i.e. critical fine particle content）

the soil will settle; if the fine particle content is greater than the critical particle content

sand burst will occur in the soil. The critical fine particle content of quicksand increases parabolically with the thickness-span ratio at the initial stage

but remains at a level at the later stage. When the thickness-to-span ratio is less than a certain value

regardless how much content of fine particles in sand

quicksand will occur.The settlement induced by water gushing and sand gushing in circular mouths is circular in plane and quadric in profile. The settlement scope after quicksand is related to the shape damage angle of subsidence surface

which is roughly 9° larger than the natural angle of repose of the soil.