Abstract
Armor porosity significantly affects construction costs and hydraulic stability of mound breakwaters; however, most hydraulic stability formulas do not include armor porosity or packing density as an explicative variable. 2D hydraulic stability tests of conventional randomly-placed double-layer cube armors with different armor porosities are analyzed. The stability number showed a significant 1.2 power relationship with the packing density, similar to what has been found in the literature for other armor units; thus, the higher the porosity, the lower the hydraulic stability. To avoid uncontrolled model effects, the packing density should be routinely measured and reported in small-scale tests and monitored at prototype scale.
Highlights
When quarries are not able to provide stones of the adequate size and price, precast concrete armor units (CAUs) are required for the armor layer protecting large mound breakwaters
This paper focuses the attention on armor porosity, p, and the associated packing density, φ=n[1p]; special attention is given to conventional randomly-placed double-layer cube armors
This paper aims to explain the quantitative impact of packing density (φ=n[1-p]) on the hydraulic stability of randomlyplaced double-layer cube armors
Summary
When quarries are not able to provide stones of the adequate size and price, precast concrete armor units (CAUs) are required for the armor layer protecting large mound breakwaters. Hydraulic stability significantly decreases if packing density (dimensionless number of CAUs per unit surface) is lower than the recommended values. The stability coefficient (KD) introduced by Hudson (1959) and popularized later by USACE (1984) has been used for decades to compare the hydraulic stability of different armor units in randomly-placed double-layer armors (with recommended packing densities). The stability coefficient concept is so popular among practitioners that KD values are recommended by patent owners of -placed single-layer interlocking units, whose hydraulic performance does not follow the Hudson formula (interlocking units are usually less stable if they are placed on milder slopes). One of the key factors in assessing economic feasibility is the packing density, φ=n[1-p], which directly affects concrete consumption and logistic costs, as well as the hydraulic stability of the armor. For single-layer armors, packing density and placement technique are usually explicitly prescribed by patent-owners; for randomly-placed double-layer armors, packing density is usually implicitly defined by engineering manuals (e.g. USACE, 1984, and CIRIA et al, 2007)
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