Placement and structural strength of Xbloc and other single layer armour units

Abstract

The Xbloc, a new type of breakwater armour unit has been developed by Delta Marine Consultants. The structural strength of single layer armour units and the ease of placement have been studied extensively in the course of the Xbloc development. Prototype drop tests have been conducted with Xbloc armour units and compared with results from previous studies with Accropode and Coreloc®. The static and dynamic structural response of Xbloc, Accropode and Coreloc® has been investigated with three-dimensional finite element models. It was found that the structural strength of Accropode and Xbloc are of the same order of magnitude while the Coreloc® is more fragile. The placement of Xbloc armour units has been studied systematically including sling placement, the sensitivity to settlements and the variation of interlocking between individual armour units. Three additional design parameters are proposed in order to quantify the quality and the ease of placement and to assess the risks of improper placement.

Introduction

Single layer concrete armour units represent the state of the art for the protection of larger breakwaters. These units are characterised by a large interlocking capacity and random placement. The most commonly applied single layer armour units are Accropode™ (more than 120 breakwaters since 1980) and Coreloc® (16 applications since 1996). The Xbloc has been developed by Delta Marine Consultants and has been presented to the public in 2003 (Figure 1). The first Xbloc application was completed early in 2004, the armour unit production for the second project commenced in 2004 and the start of the breakwater construction is expected for 2005. The performance of single layer armour units has been studied extensively in the course of the Xbloc development (2001 – 2003). The current design practise for breakwater armour layers is mainly focused on hydraulic stability while structural stability and construction aspects play a secondary role. Cases of damage to breakwaters with single layer armouring (Table 1 and Figure 2), indicate a need for a more comprehensive armour design including structural strength and placement of armour units. While the hydraulic stability of breakwater armour units can be determined (in hydraulic model tests) and quantified (by a stability number), no appropriate criteria are available for the structural strength of armour units and for the ease and quality of placement. As a consequence these aspects are frequently neglected by designers. In this paper the results of a structural strength and placement analysis, which is based on comparative testing with various types of single layer armour units, will be presented and discussed.

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Placement-Structural-Strength-Xbloc-Breakwaters-London-2005.pdf