Massive failure of submerged slopes form a major threat for many dikes along estuaries in the Netherlands, the subsoil of which consists of alternating layers of loosely packed and more densely packed sand. Static liquefaction in the loosely packed sand plays an important role. Insufficient knowledge is available to predict the flow of sand and the retrogression of the instability after initial liquefaction. Liquefaction flow slides have been studied during an extensive experimental research program in the period 1973 - 1977 on behalf of the design of the storm surge barrier in the Oosterschelde estuary. Results of this program are revisited to learn about the response of a loosely packed sand layer to a local instability. The program included more than a hundred of tests in a large and medium sized flume that were filled with sand. Each sand body had a horizontal surface and a slope as boundaries. A remarkable difference was observed between the tests in which retrogressive liquefaction flow slides did and those where such slides did not occur. This may be quite relevant for the understanding of flow slides in natural slopes with pockets of loosely packed sand. The test set-up, measurements and main results of the large scale tests are described in this paper.