The influence of the under layer on the stability of single layer armour units

Abstract

This thesis tests the influence of the under layer profile on the stability of the armour layer of concrete interlocking armour units. The analysis is aimed to offer insight into the influence of different under layer profiles on the stability of the armour layer. Furthermore the thesis includes a test of the design guideline of the under layer for Xbloc armour units, which is developed by Delta Marine Consultants (DMC). DMC prescribes that the maximum vertical difference between the constructed and designed profile, i.e. the tolerance, may not exceed 0.5 times the nominal diameter D_n50 of the under layer rocks. This requirement holds in both the positive and negative perpendicular direction. Physical model tests have been carried out at the wave flume of DMC in Utrecht, The Netherlands. Different vertical deviations have been tested in combination with variable length scales of the deviations. Furthermore convex and concave shaped profiles are also tested to assess the influence of the direction of the deviation. The under layer profiles are measured with a laser device. A spherical foot staff is simulated numerically in order to represent prototype values of the tolerances. The difference between the laser and simulated sphere is comparable to that of the conventional staff and the sphere and therefore in line with the theoretical difference between a sphere and conventional staff. Tolerance levels are tested in the range between 1.0 and 3.5 D_n50. The tests show larger damage numbers with increased tolerance. This can be explained by introducing a transition from a steep slope to a milder slope, i.e. the step. Around the step the quality of interlocking is low and consequently this area is very vulnerable to damage. Furthermore deviated profiles have locally milder slopes which result in less interlocking as well. No clear influence of the length scales of the deviations is observed. Based on three tests with a convex profile it can be concluded that convex shaped profiles are more stable than concave shaped profiles. This can be explained by the absence of the step. Because convex profiles show a different behaviour, they are excluded from the trend line which describes the trend between tolerances and damage. An additional scenario with a narrower grading and larger D_n50 of under layer material is tested. The results show smaller settlement and larger damage of the armour layer, which are a logical consequence of the increased natural roughness. It can be concluded that both an increased additional and natural roughness result in more damage of the armour layer. It is therefore recommended not to increase the tolerance requirement for Xbloc armour layers. On the other hand, the experiments show that the lower bound of the tolerance in model situation is close to the prescribed tolerances. It is therefore concluded that sufficient efforts should be made to secure the quality of placement of the under layer. In order to determine the quality of placement of the under layer, a well carried out measurement campaign is recommended.