Stability Assessment Considerations for Articulated Concrete Block Scour Mats Under Wave Attack.

This paper explores the complexity of evaluating the stability of submerged articulated concrete block mats used as seabed scour protection for the footings of monolithic maritime structures. A case study is presented where these mats were installed 50 years ago to prevent wave scour around large concrete caissons, forming the berth foundation of Berth 2 at Port of Hay Point, QLD. The paper illustrates the limitations of available techniques to assess the stability of mats in these situations and presents a customized physical modelling approach that was required to provide a reliable evaluation of the stability of the original mats under contemporary design wave conditions, as well as the performance and integration of upgrade mats to provide a more robust solution. To better understand the scale effects on prediction of scour mat stability, initial physical model tests were completed with uniform uni-directional flows in a 0.9 m wide flow flume at scales of 1:24 and 1:42. This provided the opportunity to review the stability of the mats under a series of controlled flow velocities simulated at the two different model scales. Results showed that the smaller the scale of the model, the lower the turbulence and threshold of mat stability. The tests also provided an indication of the extent of conservatism introduced by the scale modelling approach. The scour mats were then tested under wave-induced loading at a scale of 1:42 in a 3.0 m wide wave flume. Tests were completed with the original scour mats in both intact and degraded conditions, with results identifying that some of the original scour mats were unstable under the design wave conditions, even when accounting for the conservatism introduced by scale effects. Further model tests assisted in confirming a suitable mat design mass with which to upgrade the scour protection for the contemporary design wave conditions. [ABSTRACT FROM AUTHOR]