流沙
当向上的动水力与土的浮容重相等时,土颗粒间的压力就等于零,土颗粒将处于悬浮状态而失去稳定,这种现象称为流砂现象。
A campus emergency ______ occur at any time of the day or night, weekend, or holiday, with little or no warning.
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