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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

Evaluation of Long-Term Durability of Friction-Enhancing Asphalt Mat
taken from Caisson-type Breakwaters on-site
Masafumi Miyata1)2), Masaki Mimura3), Kouzo Nakagawa3), Yousuke Yamagishi4)
Keiichi Ikenaga4), Hisakazu Nonaka5), Seiji Umebara5), Masahiro Sato6), and Masahiro Takenobu1)
1) National Institute for Land Infrastructure and Management, MLIT, Yokosuka, Kanagawa, JAPANIntegrated Port Logistic Chair, Graduate School of Management, Kyoto University, Kyoto, Kyoto, JAPAN3) Kinki Regional Development Bureau, Ministry of Land, Infrastructure, Transport and Tourism(MLIT), Kobe, Hyogo, JAPAN4) Harbor Airport Bureau, Wakayama Prefecture, Wakayama, Wakayama, Japan
5)
Kochi Civil Engineering Office, Kochi Prefecture, Kochi, Kochi, Japan
6) Research Department, Costal Development Institute of Technology(CDIT), Minato-ku, Tokyo, Japan2)

sampling of the in-situ pieces was conducted in the three ports as shown inFigure 2(Wakayama Port: 57 years elapsing, Naoetsu Port: 50 years
elapsing, and Kochi Port: 29 years elapsing).

ABSTRACTA friction-enhancing asphalt mat is a material that is laid under thebottom of caisson-type breakwaters, and contributes to reducing the costand period for constructing breakwaters by increasing the sliding
resistance of the caisson. In Japan, asphalt mats have been applied atmany caisson-type breakwaters since 1963. This paper shows the resultof confirming the long-term durability of the asphalt mat using twomethods. The first method was to confirm the long-term durability of aspecimen installed under the sea for a long period of time, while the othermethod was to confirm the long-term durability of an in-situ piece
sampled directly from an asphalt mat laid immediately under the caisson
in an actual sea area. As a result, it was found that the physical properties,
mechanical properties and friction coefficients of both the test piece andthe actual mat piece showed no secular change, and the frictionenhancing asphalt mat had long-term durability of over 50 years underthe sea.

Figure 1. Schematic diagram showing effect of laying frictionenhancing asphalt mat on the bottom of caisson-type breakwater
0500km

KEY WORDS: friction-enhancing asphalt mat; long-term durability;
coefficient of friction; exposure in seawater; caisson-type breakwater

INTRODUCTION

Naoetsu Port
50 years past

When a friction-enhancing mat(asphalt, rubber) is laid immediately
under a caisson-type breakwater, the sliding resistance of the caisson willincrease. As a result, the required caisson width will be reduced, and thecost and period for constructing breakwaters may be reduced(Figure 1).
In Japan, an asphalt mat(hereafter called “AS mat”) as a frictionenhancing mat was introduced for a caisson-type breakwater in theWakayama Port in 1963(Kagawa.,1964). Since then, the AS mat has
been applied at a large number of breakwaters. Before the AS mat wasintroduced, various indoor and outdoor tests were conducted, and thecomposition of the AS mat and the friction coefficient applied between
the caisson bottom and the foundation rubbles were determined.
However, as for the long-term durability of the mat, it was considered asnecessary to constantly check it.
Addressing this issue, this paper shows the results of two tests that havebeen conducted so far in order to evaluate the long-term durability of theAS mat. The first one is a test that has continued to be conducted using atest piece for 50 years in the Wakayama Port(Kataoka et al.,1983;Hamadaet al.,2001;Adachi et al.,2020) . The test piece has been placed in a concrete
box with a lid and stored there under the sea near the breakwater. In thesecond test, in-situ pieces sampled directly from an AS mat laid on thebottom of a caisson-type breakwater in the actual sea area were used. The

Wakayama-shimotsu Port
57 years past

Kochi Port
29 years past

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Figure 2. Diagram of positions of ports where asphalt mats arecollected from breakwater on-site, and major ports where asphalt
mat is applied(●: port where actual pieces are collected, ●:major port where asphalt mat is adopted)

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