Rocking of single layer armour units

Rocking revisited 3

Master thesis (2019)

Authors

H.P.G.M. Caldera Civil Engineering & Geosciences

Contributors

Bas Hofland Hydraulic Structures and Flood Risk - CEG (supervisor 1)
W.S.J. Uijttewaal Environmental Fluid Mechanics - CEG (supervisor 2)
A. Antonini Coastal Engineering - CEG (supervisor 2)
M. Muttray Delta Marine Consultants (supervisor 2)
Cock van der Lem Royal HaskoningDHV (supervisor 2)
Marcel van Gent Deltares (supervisor 2)
Faculty
Civil Engineering & Geosciences
Copyright: © 2019 Ganga Caldera
Translation IMU Rotation Rocking Single layer armour units Instrumented units Impact velocity
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Published Date

18-07-2019

Language

English

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Faculty

Civil Engineering & Geosciences

Abstract

After the failure of several large breakwaters in late 1970’s and early 1980’s where the role of rocking and breakage of armour became apparent, the importance of understanding this rocking phenomenon is given a great emphasis. Single layer randomly placed armour units are widely used in breakwater designs as they are more economical due to the less volume requirement. But these single layer units have a much more “brittle” behaviour and strength of the units becomes a critical factor. As the stresses developed after the impact is difficult to measure directly in the scale models, currently percentage rocking is used as a design criterion. But a clear relationship between this quantity and breakage has not been identified. Even though the single layer armour unit types are widely used the knowledge on the rocking behaviour of this type of units are limited. Therefore, to get a better insight into the rocking behaviour of single layer armour units a previously proposed technique of instrumented unit with embedded sensor was adopted. This technique was found to be promising in detecting the rocking motion in standalone mode. However, one issue of that technique was relatively lower sampling frequency. Because of that, enough data could not be captured to resolve the rocking motion in time very accurately.

Therefore, in this report the measurements with standalone IMU sensor embedded in 3D printed model armour unit has been developed further. Firstly , different techniques were adopted to get the optimum sampling frequency and sampling frequency was increased from 25 Hz to 100 Hz.

This report includes further details on the processing method and obtained results after performing the 2D physical model testings. It is the first time that stand alone rocking motions are reported for single layer armour units. By increasing the sampling frequency from25 Hz to 100 Hz, rocking events can now be resolved in time by 5-10 measurements points. Gyroscope data and accelerometer data were combined to separate the linear acceleration from raw accelerometer data. Upward and downward rotational motions were distinguished, and impact velocities were calculated based on both accelerometer and gyroscope. By analyzing the data it was observed that the angle of rotation during the rocking event is small and usually unit returns to its original position after a full rotation.