Fujita Corporation

Fujita Corporation (Headquarter: Shibuya-ku, Tokyo, President: Yoji Okumura) completed the blasting demolition of the watercourse*2 on its right bank as a part of the demolition*1 of the Arase Dam. We reduced environmental impact by implementing a blasting method using 3 detonators (the first time this has been applied to dam removal in Japan). We plan to use this knowledge in the remainder of the project.

[Blasting Demolition Process]

The timing of this demolition work was limited to the dry season (November to February) when water levels in the river are at their lowest. Therefore, the blasting method was chosen in order to shorten the project duration and reduce the budget. In the area around the site there are a brick tunnel of the JR Hisatsu-line and a number of residential houses. Under such circumstance, the concrete removal must be done using a controlled blasting method with reduced noise and vibration..

We measured the noise and vibration level at the opening of the JR tunnel and the residential area, in order to keep within legal limits during the removal work. In addition, we utilized 3 detonators: DS electronic detonator, MS electronic detonator, and the new electron detonator (eDevII *3), to control the vibration and noise. We completed the demolition of 9,500m3 right bank water route in about 3 months.

The Effect of Environmental Impact Reduction

We began blast tests using the DS electronic detonator (inter-blast interval of 250ms), which is normally used for blasting works. However, this lead to a discovery in this case it is not possible to completely separate the concrete body, since the gas that emerges at the time of blasting escapes from the construction joints before the next blast occurs, due to the large interval (250ms) between blasting stages.

To overcome this issue, we utilized an MS electronic detonator which has just 10% (25ms) of DS detonator's lag, so the following blast can occur before the gas escapes. This method enabled us to cleanly dismantle the structure.

As the demolition moves forward, the entire dam decreases in weight and loses the binding power of foundation rock, resulting in easier transmission of vibration. This increases the vibration level in the residential area, and risks exceeding legal limits. Therefore we decided to implement additional measurements of blast vibration.

The most efficient method to reduce the vibration caused by blasting is to lower the amount of explosive used per blasting stage. We applied the latest electron detonator (eDevII) for the first time to the concrete demolition - the detector enables blast of one blast-hole per blasting stage*4.

We can set the inter-blast delay freely with this electron detonator (eDevII), so we selected the minimum possible interval that prevents vibration converging. This reduces the blasting time as a whole, even though the blast occurs one blast-hole per blasting stage, and results in reduced impact on surrounding environment. Fujita has been practicing one blast-hole per stage blasting with the inter-blast intervals of 15 to 200ms and recognized that the vibration can converge even with the second distance of 15ms. With this method, we could lower the vibration level from 75dB with electronic detonator to 59bB at one hole per stage blasting with the inter-blast interval of 15ms.

What's Next

After the completion of this demolition, we diverted Kuma River into the demolished zone, which concluded the 3rd year of the 6 year project. Next, we will apply this experience onto the demolition of left bank gatepost scheduled after November 2015, and continue to promote reduced environmental impact and aim for the year 2017 completion.

*1 Arase Dam removal construction: Arase Dam is located at 19.9km in from the mouth of Kuma River. It is a hydroelectric dam. Its construction was completed on March of 1955. It stopped power generation at the end of its approved water usage on March 31st, 2010. The river structures: Arase Dam, the water intake facility and the floodway are currently being demolished.

*2 Watercourse: The part where the river ran before the dam was constructed.

*3 eDevII

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Fujita Corporation
Public Relations Office
Tel. +81-3-3402-1911
Fax. +81-3-3404-8477
Email. info@fujita.co.jp