ABSTRACT

A friction-enhancing asphalt mat is a material that is laid under the bottom of caisson-type breakwaters, and contributes to reducing the cost and period for constructing breakwaters by increasing the sliding resistance of the caisson. In Japan, asphalt mats have been applied at many caisson-type breakwaters since 1963. This paper shows the result of confirming the long-term durability of the asphalt mat using two methods. The first method was to confirm the long-term durability of a specimen installed under the sea for a long period of time, while the other method 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 and the actual mat piece showed no secular change, and the friction-enhancing asphalt mat had long-term durability of over 50 years under the sea.

INTRODUCTION

When a friction-enhancing mat(asphalt, rubber) is laid immediately under a caisson-type breakwater, the sliding resistance of the caisson will increase. As a result, the required caisson width will be reduced, and the cost and period for constructing breakwaters may be reduced(Figure 1). In Japan, an asphalt mat(hereafter called "AS mat") as a friction-enhancing mat was introduced for a caisson-type breakwater in the Wakayama Port in 1963(Kagawa.,1964). Since then, the AS mat has been applied at a large number of breakwaters. Before the AS mat was introduced, various indoor and outdoor tests were conducted, and the composition 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 as necessary to constantly check it.

Addressing this issue, this paper shows the results of two tests that have been conducted so far in order to evaluate the long-term durability of the AS mat. The first one is a test that has continued to be conducted using a test piece for 50 years in the Wakayama Port(Kataoka et al.,1983;Hamada et 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 the second test, in-situ pieces sampled directly from an AS mat laid on the bottom of a caisson-type breakwater in the actual sea area were used. The sampling of the in-situ pieces was conducted in the three ports as shown in Figure 2(Wakayama Port: 57 years elapsing, Naoetsu Port: 50 years elapsing, and Kochi Port: 29 years elapsing).

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