Field Measurements to Investigate Submerged Slope Failures

Many flood defences in The Netherlands have been disapproved for flow slides of the Holocene subsoil. Traditionally these flow slides are assumed to be induced by static liquefaction. Only in recent times it has been recognized that flow slides may also concern breach flows, which do not necessarily require loosely packed sand. For both static liquefaction and breach flow the inaccuracy of the currently applied methods to determine in situ density lead to high computed probabilities of failure, which is one of the main problems in the safety assessment of flow slides. In order to reduce this uncertainty, based on a literature study a number of methods were selected and applied on four test locations: two sites where flow slides occurred and two sites where no flow slides occurred, but for which high probabilities on flow slides were calculated based on current Dutch assessment rules for liquefaction and breach flow. For these sites CPT's and electrical resistivity cone tests available from earlier investigations, were extended with seismic CPT's and interpreted for relative density and state parameter. The results of this study lead to the conclusion that some of the historical flow slides in The Netherlands may have been the result of static liquefaction in loosely packed sand. For many other slopes, however, it is more reasonable to assume that the failures must have been breach flows in medium or densely packed layers.