Technical report on sand boils (piping)

Piping and hydraulic fracturing (heave) are phenomena which can threaten the stability of flood defences. These phenomena can arise when, in the case of large-scale hydraulic head, particles of soil in layers of earth which are susceptible to erosion are transported underneath the flood defence by the seepage flow, as a consequence of which erosion channels are created under the flood defence (piping), or mutual effective stress is lost in the soil (heave) in the upward seepage flow behind the hydraulic structure or cut-off wall. Within the scope of designing new flood defences or reinforcing or monitoring existing ones, safeguards against the occurrence of these phenomena must be verified. Various calculation rules are available to verify such safeguards, varying from simple empirical rules for the first (reinforcement) design for a flood defence or safety monitoring of an existing flood defence, to advanced calculation models to design or test more precisely. The recommended approach and calculation rules in these publications are not state-of-the-art, bearing in mind the developments in this field in the past few years. New developments are recorded in research reports which are often insufficiently accessible for day-to-day design and test practice. All in all, the knowledge available for practical application is fragmented.TAW/EN