Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

Numerical Investigation of Filter Unit Deformation on Rubble Mound under Wave Action
Aye Nyein Mon, Shin-ichi Aoki, Susumu Araki
Department of Civil Engineering, Osaka University
Suita, Osaka, Japan

ABSTRACT

Mizutani et al., (2007) carried out laboratory experiments using S-type
FUs on uniform slopes of rubble mound to study wave force acting on
the FU near the shoreline and to estimate the stability coefficient of theFUs. This research highlighted that tangential and normal forces acting
on the FU are proportional to tangential velocity square, and the FUs are
more stable than natural armor stones. It was also noted from Saito’s(2017) hydraulic model experiments that S-type FU has better stability
than N-type FU on the surface of rubble mound.

Filter Unit (FU) is a synthetic fiber net bag wrapping rubble stones. In
this study, a two-dimensional numerical model to represent deformationof FUs is developed using circular finite-sized particles. The model
includes two particle types: virtual rubble and virtual fiber, and the fiber
particles are arranged like a string wrapping the rubbles. The
deformation of the model on rubble structure under wave action iscomputed by a combination use of two numerical models: CADMASSURF and Discrete Element Method (DEM). The model input
parameters are optimized for reproducing the flexible deformation ofexperimental FU models.

Mon et al., (2020) considered various S-type FU structures, having
different volume ratios (fiber bag/filling stones) and different inside
constraining rope lengths, and studied their stability and damage
mechanism on a fixed rubble mound breakwater experimentally.

KEY WORDS: Filter Unit; Deformation; Numerical Model; Rubble
Mound; CADMAS-SURF; DEM; Bonded-particle Model.

In the present study, the deformation of FU models on rubble moundstructure due to regular waves is studied numerically using two
dimensional CADMAS-SURF (CS2D) and Discrete Element Method(DEM/2D): CS2D for predicting the wave motion in and around themodels and DEM for estimating the models movement by the wavemotion. Numerical wave data from CS2D is validated with that of theexperiments conducted by the authors, and the deformation patterns of
the numerical models are investigated in detail during the wave attack.

INTRODUCTION
Filter Unit (FU) is a synthetic fiber mesh bag of rubble stones. The FUs
could be used in the protective cover layer of rubble mound structure
exposed to ocean wave action, especially when large natural rocks are
not available and concrete armor units are very expensive to cast. A
number of experimental studies on the stability of the FU as an armorunit covering rubble mound breakwater under wave attack have been
carried out. One of the significant characteristics of FU is that it isdeformable, which is known to be related to the stability of the FU.
However, how the FU deforms on rubble mound has not been examined
numerically yet.

Objectives and Scope
The main objectives of this research are 1) to develop a discrete element
model for simulating the deformation of FU armored rubble moundstructure due to wave motion, 2) examine the simulation model’s particle
interaction behaviors and movement, and 3) discuss the newly developedmodel in accord with some observations of physical experiments.

Akiyama et al., (2001) explained the construction method of FUs, andperformed hydraulic model tests with FUs on a mild slope revetment in
the place of armor stones, then showed that the FUs have high flexibility
and hydraulic stability in comparison to the armor stones.

In order to achieve these objectives, the input model parameters for DEM
calculation which significantly affect the calculated results are required,
and these parameters are determined either using analytical assumption
or inverse calibration methods (Sadek, 2013). In the former method, theinput parameters are estimated from material properties such as Young’s
modulus and Poisson’s ratio, and in the inverse calibration method, theparameters are found out by matching the calculated results with

Kubota et al., (2007) reported based on both small-scale and large-scale
tests that the stability and durability of FU on rubble mound could beimproved by using a rope to connect the top and bottom from inside ofthe bag. The modified unit is referred to as the restricted type (S-typeFU) and the conventional unit as the normal type (N-type FU) (Fig. 3).

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