Flexible scour protection around cylindrical piles

Abstract

This research focuses on the behaviour of a granular scour protection around piles when the bed is subjected to bed degradation. Furthermore, engineering guidelines are provided to account for bed degradation in the design of the initial scour protection. To address the behaviour of the protection, a physical model is used and to obtain the engineering guidelines, a theoretical approach is used which includes the phenomena that are observed during the physical model tests. As stated by many authors [Breusers et al., 1991; Chiew, 2004; Melville et al., 2000], a granular scour protection around a pile can fail due to several mechanisms. In clear-water conditions these mechanisms include shear failure, winnowing and edge scour and in live-bed conditions two other mechanisms can be added, namely bed degradation and bed form induced failure. This research focuses on the bed degradation mechanism, which can endanger the granular scour protection around a pile. Just like Van der Hoeven [2002] observed during his research on falling aprons, the stones spread evenly over the front and side slopes of the mound, which form after undermining of the protection has led to the ‘falling’ of the edge stones of the protection. This coverage of stones on the slopes of the mound effectively rearmoured the slopes and prevented further erosion. At the back of the mound this rearmouring is not observed, because the high turbulence levels, introduced by both the mound and the pile, caused stones to become unstable in this area. Furthermore, at the front and side slopes, where the slope of the mound was totally covered by the stones of the protection, the slope angles were constant and comparable to the slope angles that Van der Hoeven found in his research, namely 1:2,0. However, a remarkable difference between the observations of Van der Hoeven and the observations of the current research concerns the layer thickness of the protection on the slope. Van der Hoeven observed that the layer thickness after launching of the apron was only 1*df,50 thick, while the current observations show a decreasing layer thickness from the top of the slope to the toe of the mound. The theoretical approach that is used to develop a design formula focuses on the side slopes, where the flow is parallel to the interface between stones and base material. A balance between the volume of the initial protection and the required volume of stones after bed degradation has led to the formulation of the internal slope angle, ?. This angle describes the assumed linearly decreasing thickness along the slope, starting with a certain required thickness at the top, Df,A, to a thickness of only 1*df,50 at the bottom. Because the hydraulic load decreases with increasing distance from the pile and the thickness of the initial protection is based on the maximum load near the pile, the assumption that Df,A is equal to the initial thickness of the protection, leads to a conservative design formula. This approach is however taken in the development of the simplified design formula, which results in a linear relation between the bed degradation and the extra extent of the initial protection that should be constructed to deal with that bed degradation.