The collapse of a granular column in a viscous liquid is experimentally investigated. The morphology of the deposits is shown to be mainly controlled by the initial volume fraction of the granular mass and not by the aspect ratio of the column, an observation which differs from dry granular collapse. Two different regimes are identified corresponding to initially loose and dense packings. Loose packings give rise to thin and long deposits, the dynamics being fast. A positive liquid pressure is measured below the column. For dense packings, the runout distance is twice less, the flow is slow, and a negative pore pressure is measured during the flow. These observations suggest that the dynamics of the granular collapse in a fluid is strongly affected by the dilatancy or contractancy behavior of the granular medium.

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