Making bridges less susceptible to tsunami damage
A superstructure washed-away
General view of the tsunami
experiment using a water channel
(Click to play video(ca. 5.9MB))
Close-up of the tsunami experiment
*Filmed with a high-sensitivity camera
(Click to play video(ca. 14.1MB))
The massive tsunami triggered by last years Great East Japan Earthquake caused damage over a vast area of Japans Pacific coast, ranging from Tohoku in the north to Kanto in the south. Road bridges were no exception, with bridge girders swept away by the tsunami in some cases. We now face the important challenge of designing bridge girders to withstand the force exerted by very large tsunamis as experienced this time, with a view to restoring the functions of bridges as quickly as possible.
As examples of the approach to structural planning of girders to withstand tsunamis, the basic rationale from the viewpoint of preventing disasters is to secure space beneath the girders against the height of the tsunami. Here, issues such as local disaster prevention plans and others concerning tsunamis must also be taken into account. In addition to this, from the viewpoint of mitigating disasters, it will be important to consider other approaches wherever necessary; for example, that of designing structures to make them less susceptible to the impact of tsunamis, or taking the position and structural format of piers into account, so that even if girders are washed away, they will be easier to restore. As the role to be played in this by CAESAR, a body specializing in the structure of bridges, we have now started to tackle particularly pressing research challenges, such as how girders should be structured to make them more capable of withstanding tsunamis, what methods are available as measures to mitigate disasters for existing bridges, and so on.
Focusing on bridges damaged in last years tsunami, we find that some bridges were washed away while others were not. Among bridges washed away, it was observed in some bridges that girders of some spans were turned upside down but girders of the other spans were not. This difference in the state of damage provides an important focal point for elucidating the force exerted on girders by tsunamis and its mechanism, and for devising countermeasures. That is, the impact exerted on girders by a tsunami is thought to depend not only on the conditions of the tsunami itself, but also on other factors including the structural conditions of the girders. Moreover, a bridges resistance to the impact of a tsunami is thought to be related not only to the structural conditions of the bearings that support the girders, but also to the width of the girders and the intervals between them, their length, and other factors.
Based on the mechanism when girders are subject to the impact of a tsunami, we plan to analyze differences between structural characteristics from numerous data on bridges that were washed away and others that were not washed away in this tsunami. In this way, we will proceed with our research in an attempt to find rational methods of design at the structural planning stage, and thus make bridges less susceptible to tsunamis.
At CAESAR, we are currently generating tsunamis in an experimental water channel and verifying the force applied to a 1/20-scale model reproducing a road bridge as accurately as possible. We would like to verify the force applied to girders by a tsunami and its mechanism, while comparing the results with the state of actual damage.
High-performance SMA: A multifunctional durable asphalt paving technology
Fig. 1 Concept of high-performance SMA
Photo 1 Comparison of high-performance
SMA and standard pavement
Photo 2 Frozen high-performance
Pavement is a road structure that supports the loads of moving vehicles and provides a smooth surface for comfortable driving, and various types with different functions have recently been developed. Among these is porous drainage pavement, which prevents splashing by allowing rainwater to quickly drain from the road surface and reduces car-tire noise. Its rough surface also provides an anti-skid effect in winter. However, as porous drainage pavement is disadvantageous in terms of durability, the development of a paving material resistant to damage caused by snow chains in winter is required for cold snowy regions.
SMA (stone mastic asphalt) is a paving material mainly consisting of relatively large pieces of crushed stone known as coarse aggregate that are interlocked to increase durability. Through joint research with the private sector, the Road Maintenance Research Team developed high-performance SMA ? a new paving material combining the functionality of drainage pavement and the durability of SMA pavement (Fig. 1).
High-performance SMA has two layers with different properties. The upper layer has a rough surface similar to that of drainage pavement, while the lower layer is dense and durable like SMA. It can be installed in a single step, and its rough surface reduces the likelihood of puddle formation on road surfaces (Photo 1). This results in a better driving environment in rainy conditions than with standard pavement. Even on slippery black-ice roads (which look wet but are actually covered with extremely thin sheet ice) in winter, the higher parts of the rough surface remain above thin-ice cover and help to prevent skidding (Photo 2).
The Road Maintenance Research Team continues to study the long-term durability, functional longevity and optimum design of mix for high-performance SMA to support its use on roads where drainage pavement is conventionally used, such as high-standard highways and road sections requiring noise control measures, in addition to roads/runways requiring anti-skid measures in winter.
(Contact: Road Maintenance Research Team ,CERI)