Unexpected failure of subaqueous excavated slopes in the form of flow slides is considered in this graduate study. Excavated slopes are not the same as slopes constructed by means of hydraulic fill. Several recent flow slide failures in sand borrow pits, together with qualms in engineering practice about not knowing exactly what fundamental mechanism causes flow slide failure, gave cause for this study. The knowledge that flow slide failure may be caused by either static liquefaction of (part of) a sloping body (also called spontaneous collapse), or by the development of active banks, is taken as a point of departure. Liquefaction due to dynamic loading will be left out of consideration. Special attention will be paid to the geological circumstances of The Netherlands. Herewith, the central problem of this flow slide study becomes: "How, from an engineering point of view, can we understand and predict flow slide failure of excavated (or being excavated) slopes, better?". To be able to answer the question at issue, we need to understand the fundamental mechanisms, leading to flow slide failure. Then, the soil properties may be related to the mechanisms and specific circumstances may be taken into account. The main aim of this graduate study is to obtain insight in different mechanisms, possibly triggering flow slide failure and causing its retrogressive nature. Since the extent of a flow slide can be very large, and seems hard to predict, the prediction of the potential for failure will be the pith of the study, not the consequences of failure (Volume 1: Literature Review). The possible ways to model failure potential are investigated and their prediction values are evaluated on the basis of case histories (Volume 3: Case History Study / Appendix). With the insights obtained, an effort has been made to formulate a practical guideline for flow slide analyses, which should lead to improvements in engineering practice (this Volume 2: Directive for Engineering Practice).