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.　©　2019,　Science　Press.　All　right　reserved.