In the morning of the 9th of October, also the last full day in Tokyo, we went to the company named Central Consultants. When we arrived at the company we were welcomed by 14 employees and one interpreter. Central Consultants is a company that was established in 1967 as a general engineering consultant financed by the Mitsubishi Group with expertise in the civil egineering factor. Their main clients are MLIT (Ministry of Land, Infrastructure and Transport) and the prefectures of Japan, but besides this Central Consultants shares their knowledge with other countries as 22% of their projects are overseas.
The excursion consisted of 4 presentations and Q&A sessions about the next subjects:
The construction of the Tokyo Gate Bridge
Liquefaction Mitigation Measures in Residential Areas
Maintenance on Japan’s Road Bridges
Intelligent Transport Systems
The Tokyo Gate Bridge is a truss bridge with a span of 440 meters in Tokyo Bay. The challenge in the construction of the Tokyo Gate Bridge was caused by the restricted air height because of the near lying Tokyo Airport and the needed length of the fairway off 300 meters. With both these restrictions and the fact it had to be earthquake proof and the shape had to pop out, a normal cable bridge was not possible and other solutions had to be found. Solutions as making the truss and the floor-system as one body, conducting a Seismic Isolation Technology and use Bridge High-Performance Steel were applied. The new technologies were experimented on by a professor from the University and he guided through the whole process. The structure was installed in 4 pieces on-site using 6 cranes able to support 3000 tons each. After the construction the Tokyo Gate Bridge is on the 9th place of truss bridge’s with the highest span length.
One of the short presentations of Central Consultants was about the liquefaction mitigation measures in residential areas. Firstly, a brief explanation was given of the phenomena ‘Liquefaction’, which is mainly caused by seismic waves of an earthquake. In the soil sand particles are bound together with water in the pores. Shaking of the earth will cause the binds to break, resulting in a separation between the sand particles and the water. The sand particles will sink and the ground water will concentrate in the upper part of the soil. Because of that the soil loses its strength and stiffness and buildings will collapse. Also the manholes and sewerage systems could float up due to the water.
After the Nigigata earthquake in 1964, liquefaction mitigation measures are being placed since then. At first the causes of liquefaction are investigated in order to come up with measures. Soft sand soils and a high groundwater level are characteristics for liquefaction prone areas. Measures such as sand compaction, soil improvement and groundwater level reduction are often used to make the soil resistant against liquefaction. Other possible measures are foundation piles supported by a strong soil layer and underground walls to reduce the seismic force conveyed to the soil.
For this specific project the ground water level reduction method is used to increase the resistance of a residential area to the same level of resistance as the surrounding areas. Manhole pumps and waterstop sheet piles are used in order to lower the groundwater level. However, there are some consequences of groundwater level reduction like ground subsidence which needs to be monitored. The amount of subsidence should be within a safe range, so it would not harm any buildings and the groundwater level should not cross the maximum groundwater level after extreme precipitation events for instance.
A strong relation is visible with the micro study: ‘Earthquake proof building measures’. In the preliminary report it became clear that soft soils have a positive relation to the destructiveness of an earthquake. The reason for that was an increase in amplitude of the seismic wave when it propagates through soft soils. High buildings are more sensitive for a lower frequency (goes hand in hand with a bigger amplitude most of the times), because of its natural frequency. After the presentation it became clear that soft soils are also more sensitive for liquefaction causing buildings to collapse.
The measures stated in the presentation to prevent liquefaction were partly based on changing the soil characteristics of soft soils into more suitable soils. These measures will both benefit the liquefaction resistance and limit the increase in amplitude of seismic waves. Suitable measures would be sand compaction, soil improvement and grain size replacement in order to make the soil better. Also the underground walls or piles reducing the seismic force conveyed to the soil would be a suitable measure to include in the micro study. After the presentation we gained a lot more insight in the soil characteristics and measures taken to improve soils, so buildings would be safe during heavy earthquakes which we will definitely use in our final report of ‘Earthquake proof building measures’.
Maintenance on Japanese road bridges is needed nowadays with the percentage of bridges that are 50 years or older increasing from 18% to 43% in the coming 10 years and the percentage increasing more to 67% in the coming 20 years. The types of damage these bridges mostly suffer from are Salt damage, Alkali-aggregate reaction and fatigue. The extra damage by fatigue is caused by the percentage of large vehicles on the road. In Japan this is 1/3, where it is only 1/5 in Europe and 1/20 in the United States. The way of maintenance is changed from corrective to preventative maintenance. Now every bridge is inspected every 5 years. With the implementation of systematic prevention maintenance the costs can be reduced and the life cycle of the bridge can be extended. This new way of maintenance is more in the line of the literature we have read during our preliminary research. Japanese have a big uncertainty avoidance, so that damages, accidents and so on can be reduced.
The last presentation from Central Consultants was about the Intelligent Transport System (ITS). This system offers fundamental solutions for creating ideal traffic conditions, reducing traffic accidents and congestion, and saving energy and protecting the environment. One part of ITS is the Traffic Signal Prediction System which can show you when the traffic light Will turn green and suggests a proper speed to pass through the traffic light without stopping. We asked if installing loops in the road was something they were thinking about, they answered that this could not be done because the traffic lights are controlled by the police, so this was not an option.