Structural Analysis of Polyurethane Bonded Aggregate on Block Revetments

Abstract

For centuries dikes in the Netherlands have been protected against wave attack by revetments constructed of pitched blocks. Due to new insights into the behaviour of pitched blocks and increased hydraulic boundary conditions, a significant part of those dikes does not meet the current prescribed standards and safety norms anymore. As a result, large parts have to be reinforced in the coming years. The main failure mechanism of a placed block revetment is uplifting due to water overpressures. This happens when the upward pressures, caused by a hydraulic head difference over the revetment, exceed the dead weight of the structure. The concrete elements are lifted up and the waves subsequently induce erosion of the dike body. Therefore, the renovation of these revetments is currently done by adding more weight to the structure by covering the block revetment with large rocks, or even completely replacing the elements by bigger concrete blocks. In view of the large renovation areas, replacing the concrete elements is a time consuming process and will require considerable financial efforts. Therefore, innovative refurbishment techniques are desired and researched. A relatively new type of revetment is the polyurethane bonded aggregate (PBA) which consists of aggregate glued by the adhesive polyurethane (PU) and is currently marketed under the brand name Elastocoast. This study focuses on the use of PBA in strengthening pitched stone revetments. In this report an effort is made to describe the mechanical behaviour of refurbished block revetments under wave loading. A PBA refurbishment layer adds coherence to the revetment and will prevent the blocks from uplifting. Another notable advantage is its high permeability. Firstly, an analytical model was elaborated to gain a first general insight into the structural behaviour of a composite PBA/block revetment. In the analytical model, the interaction and stress distribution between the revetment and subsoil were modelled as an elastically supported beam (Winkler model). Furthermore the structure was modelled with a finite element package. Although this research is based on multiple assumptions, it is possible to formulate some qualitative statements that resulted from this study. • The findings suggest that a rigid connection between the PBA layer and the existing block revetment is most effective in reducing the bending stresses in the PBA layer. • The results of this study support furthermore the idea that the composite PBA/block revetment could be schematized as an Euler Bernoulli bending beam. • The findings also indicate that the current design method is conservative. In this design method it is assumed that only the PBA cover layer contributes to the flexural strength of the structure. The results of this thesis indicate that this design approach is conservative and therefore resulting in thick PBA layers. • Lastly, two conventional refurbishment techniques (Open Stone Asphalt (OSA) and hydraulic asphalt concrete) were compared with the PBA. Calculations were performed what the most effective approach would be. On the one hand, an impermeable cover layer constructed by asphalt concrete resulting in higher water pressures but increasing its dead weight (asphalt concrete), or on the other hand, applying a permeable refurbishment layer (PBA and OSA) which is less heavy but resulting in lower water overpressures. The results suggest that it is more effective to use a less heavier but more permeable material as a refurbishment than the other way around. It must be noted that this greatly depends of its permeability. If its leakage length is increased by clogging or when applying the refurbished material, it results in a significant increase of the maximum bending stresses. In this case the refurbishment techniques with asphalt concrete becomes more effective since it is heavier.