April 2, 2002

At the Tokyo Metropolitan Government's disaster countermeasure headquarters, to promote disaster recovery on Miyakejima Island, existing facilities are being refurbished one after another to enable personnel involved to be accommodated locally. Takenaka Corporation (Head office: Osaka, President: Toichi Takenaka) proposed a construction method that would enable facilities that are easy to maintain with a high degree of safety to be achieved with a short construction schedule and with low initial costs. Takenaka was responsible for the construction work in a total of four projects, starting with the first project which was the refurbishment of "Tokyo Metropolitan Government Miyake Branch Building No. 2 " and finishing with the refurbishment of the "Miyake Village Central Clinic" completed at the end of March. In this series of refurbishment projects, Takenaka installed elaborate air purification technology which does not allow the inflow of even the smallest quantities of dust or chemical substances, and room pressure control systems with a proven track record in air membrane structures.

To ensure the safety of workers involved in the restoration work on Miyakejima, a plan to refurbish the Tokyo Metropolitan Government Miyake Branch Building No. 2 located on the north of the island as an overnight stay facility that is resistant to volcanic gas was promoted. In carrying out the refurbishment, the following requirements had to be met:

1. Create an indoor environment capable of ensuring the safety of occupants even if the building is exposed to outside air containing sulfur dioxide and other volcanic gases.
2. When the impact of the gases is lessened and the environment becomes safe, it should be easily returned to its original facility.

In the refurbished facilities, in addition to improving air tightness, when drawing in air via the fans, dust and sulfur dioxide gas is removed by passing it through an air purification unit comprising dry chemical filters, and clean air is drawn into the indoor area. Additionally, pressure is applied on the air inside the building and controlled at a constant pressurized state to ensure that gas does not enter the building from small gaps in the walls. This keeps the level of sulfur dioxide inside the building below safety standard values (threshold limit value: 2 ppm max.). In normal air purification units the water scrubber method which purifies air using water as a media is generally used in conjunction with dry filters, but the water scrubber method requires specialized maintenance. To enable desulfurization units on Miyakejima to be maintained easily without the presence of specialized maintenance staff, dry filters which are easy to replace were adopted in all of the four refurbishment projects handled by Takenaka.

The Tokyo Metropolitan Government Miyake Branch Building No. 2, the first emergency countermeasure work with a view to overnight stays on Miyakejima, was completed in late April 2001, with night stays commencing on May 4, making it possible for personnel from the local disaster countermeasure headquarters to stay on Miyakejima permanently. After that, Takenaka handled the Miyake Village Office (opened on July 9), Miyakejima Police Station (opened on September 5), and the Central Clinic completed at the end of March.

SO2 (Sulfur Dioxide) Environmental Standards

One-hour value (average concentration over a one-hour period) 0.1 ppm max., daily average (average concentration over a one-day period) 0.04 ppm max.

Current situation on Miyakejima

Miyakejima is a circular volcanic island with a diameter of approximately eight kilometers, located approximately 180 kilometers to the south of Tokyo. Mount Oyama, with an altitude of 800 meters in the center of the island, erupts once every 20 years or so. From July to August 2000 the mountain erupted explosively, and an evacuation order was issued to all residents on September 2, making the island uninhabited. From September onwards major eruptions ceased to occur, and reconstruction work has continued centering on the disaster countermeasure headquarters, but large volumes of volcanic gases including the hazardous sulfur dioxide (SO2) continue to be discharged today, making it impossible for the island's residents to return.

Air shield technology used in the Tokyo Metropolitan Government Miyake Branch Building No. 2

1. Ensuring airtight buildings - Shutting out inflows of external air from gaps in building
To enable the building to return to its original facility when the impact of the gas lessens and the island becomes safe once more, transparent acrylic double-glazed windows using airtight materials were mounted inside existing windows, with the four sides of the windows sealed with rubber materials or seals. The entrance at the front on the first floor of the building uses double doors, gaps are minimized, and ceiling and floor are double-layered, ensuring the air tightness of the indoor area overall.

2. Volcanic gas removal function - Using easy-to-maintain filters to lower sulfur dioxide gas to below threshold limit values
Normally when removing gas dry chemical filters and water scrubber methods are often used in conjunction with one another, but considering that the management and maintenance of the Miyake Branch Building was to be carried out by the Miyake Branch, equipment comprising only dry filters, which are easy to use and easy to replace, was adopted. This is a technology unique to Takenaka, which was achieved through our know-how of air purification in clean rooms for semiconductor plants. The only maintenance involves removing filters whose useful life has expired and replacing them with new filters, a task that can be completed without difficulty by anyone. By passing through three different types of filter, air that is drawn in from outside by fans has sulfur dioxide gas, volcanic ash and other dirt and dust removed, maintaining the resultant air at safety standard values (threshold limit values: 2 ppm max.). In preparation against all eventualities such as breakdowns, two sets of filters and fans are installed as necessary, ensuring the systems function properly.

3. Sulfur dioxide gas entry control function - Takenaka the Dome Expert's pressurization technology that does not let in even minute quantities of gas
When purified air is fed into the airtight indoor area using a fan, the flow rate at which air is fed is controlled while monitoring the indoor pressure using a pressure sensor, and the pressure indoors is maintained at a level approximately 30 pa (equivalent to 1/10 the pressure difference of Tokyo Dome) higher than the pressure outside. This prevents outside air from entering the indoor area through miniscule gaps in the outside walls. This is a pressure equivalent to a force of three kilograms per one square meter being applied from inside the building to outside, which is not overcome even if a strong wind of seven meters per second blows against the building.

4. Monitoring and measuring gas concentrations indoors and outdoors - Indicating situations indoors and outdoors with measuring instruments and monitoring panel
Measuring instruments are used to automatically measure and record sulfur dioxide concentrations indoors and outdoors, and results are displayed in real time on a monitoring panel installed inside the building. In the unlikely event that sulfur dioxide concentration indoors exceeds safety standards, personnel are prompted to inspect fans and filters by warning alarms and lamps.

Since the volcanic eruptions occurred, reconstruction work and surveys have been carried out on Miyakejima mainly by the local disaster countermeasure headquarters, but due to the presence of hazardous sulfur dioxide gas, workers were unable to stay overnight on the island, instead travelling back and forth each day by boat from Kozushima Island. However, this mode of transport only allowed workers to work a maximum of four hours each day, and work was cancelled on days where weather conditions made it impossible for boats to leave port, so the provision of an accommodation facility to protect workers and surveying personnel from volcanic gas was an issue that needed to be addressed urgently.
The completion by Takenaka Corporation of the first clean house on Miyakejima - Tokyo Metropolitan Government Miyake Branch Building No. 2 - was the first step in allowing personnel to stay on the island permanently, and contributed to the centralization and improved efficiency of observation and reconstruction work. The number of personnel permanently stationed on the island has increased from the original 20 to around approximately 480, and work is underway to restore the island and enable residents to return to their original lives as they were before the eruption.