Placed Riprap Deformation Related to Axial Load at Toe Support: Physical Modelling

Abstract

Rockfill dams are hydraulic structures of major importance. They can be exposed to extreme flood events, in turn leading to overtopping. These phenomena erode and affect structural and geotechnical integrity, which in turn can cause dam breach. Ripraps are broadly used for rockfill dam protection against such erosion processes. For steep slopes, as the one considered in this study (S = 1:1.5, vertical: horizontal), understanding the riprap behavior during overtopping is an important issue to improve dam design and reinforcement techniques. In this work, datasets are obtained from five experimental models of placed riprap built on a rock filter layer in a flume, at the hydraulic laboratory of the Norwegian University of Science and Technology, Trondheim. The riprap stones were placed in an interlocking pattern with a metallic support at the toe. The models were subjected to successive and incremental overtopping discharges until their complete failure. A laser traverse system was used to measure the coordinates (3D) of individual marked riprap stones between each discharge increase. Six load cells located at the toe measured the imposed loads during the entire procedure. From the total load values, two different types of load contributions could be distinguished: the self-weight of the stones and the hydraulic load depending on the discharge level of the overflow. This article highlights the strong relation in each of the five tests between riprap stone displacements, axial reaction load values measured at the toe and overtopping discharges. Moreover, as demonstrated in previous works, a buckling deformation of the riprap layer was observed and described. The results demonstrate that as the hydraulic load induces 2D deformations of the riprap, a larger part of the riprap weight is supported at the toe. Thus, the measured axial load during overtopping arises both from the hydraulic load and from the load imparted due to the compaction of the riprap layer. This compaction effect induces an even greater load than the one imposed due to the hydraulic contribution. The results from this study are finally put into perspective with the Norwegian Water Resources and Energy Directorate recommendations for full scale dams and suggest the great resistance of supported riprap at the toe.