Case Ditails

Scenario
Case Name Methane gas explosion hazard of an earth pressure type shield tunnel.
Pictograph
Date February 1, 1993
Place Huyuki town, Koto ward, the metropolis of Tokyo.
Location At a depth of 34m from the surface of the earth, near the face 1300m away from the bottom of a shaft.
Overview At a depth of 34m from the surface of the earth, 1300m away from the entrance of the tunnel, the explosion by the methane gas was caused when boring a shield tunnel. As a result, 4 workers died, and 1 worker got injured.
Incident Methane gas, which arose from the underground tunnel excavation, exploded by a fire source, and 4 workers working in the excavation died, and 1 worker got injured.
Sequence Since the evening at the day when the explosion occurred, a record of methane gas detector had moved, but no measures had been taken.
On that day when the explosion occurred, the methane gas alarm at the office outside the tunnel was issuing alarms, but the staff members were absent.
The person who measured the combustible gas was not nominated, and anyone did not make the person measure the concentration of the combustible gas in the place where it was possible for the combustible gas to arise or to accumulate, before the work was started every day.
An automatic alarm device was not in the tunnel interior.
Cause As it seemed that the stratum including the methane gas didn't exist, this construction had been carried out.
The existence of methane gas was not confirmed by geological survey.
The volume of the ventilation was small, and it could be not correspondent with the methane gas.
Since the methane gas sensor had been installed 90cm below the crown of the shield, a gush of the methane gas could not be detected early.
The alarm part of the stationary type detector had not been installed near the face where the alarm could inform the worker but at the office outside the tunnel .
No measures could be taken even though the dial of the stationary-type detector had moved and showed a gush of the methane gas.
The measurement of methane gas by the measuring instrument was not carried out.
Smoking in the tunnel had been tolerated.
Response 4 persons outside the tunnel heard the explosion sound of "kaboom", and they approached the face by battery train for the purpose of the exploration. They discovered 2 victims almost 1250m away from the bottom of a shaft. Only 1 survivor ( burnt ) was carried out of the tunnel, because 1 person had already died. While carrying him, they made an emergency call to an ambulance ( at 2:17, on February 2nd ).
The fire brigade arrived, and the rescue activity was started ( around 2:35 ).
2 corpses were removed ( around 5:15 ).
The rescue activity was stopped, because the surroundings became worse ( around 6 o'clock ).
Temperature of the tunnel : 40° concentrations of methane gas : 13% CO : 90ppm
The rescue and activity was resumed, because the surroundings of the tunnel improved ( around 14 o'clock ).
The last 2 corpses were carried out, and the rescue activity was finished ( around 17 o'clock ).
Countermeasures The geological investigation is made elaborately, and following the result, a scheme of construction is made.
Designating one person as the methane gas concentration measuring person, it makes him measure the level every day before the work starts.
The ventilation which can dilute methane gas is installed.
As the need arises, the explosion-proof electrical equipment is used.
An automatic alarm device is installed.
A detector sensor is installed near the crown of shield .
Correspondence of the alarm actuation has been decided beforehand, and it is effectively communicated to the party.
Knowledge Comment All over Japan, many stratum which contains methane gases such as oil field and gas field exist.
The deeper the place is, the more the danger of the methane gas increases.
Countermeasures of methane gas are sufficiently examined before the construction is undertaken.
Background It had been known from the ancient times that the stratum including methane gas exists in the underground of the construction place. However, the scheme of the construction was made based on the condition that the stratum including methane gas did not exist, from the result of the investigation boring before commencement of the construction.
As a ventilation for 1430m tunnel, φ 400mm nylon tarpaulin air pipe and axial flow blower of about 340mmAq blast pressure were used.
Stationary type methane gas detection machine which was the closest to the face had been installed at the edge of the screw conveyor of the shield, and it has been installed 90cm below the crown of the shield.
Since the alarm part of methane gas alarm device had been installed only in the office outside the tunnel, workers in the tunnel could not hear the alarm.
Electric installation in the tunnel was the usual type ( non-explosion-proof ), and workers had also been permitted to smoke.
Segment transportation hoist motor ( non-explosion-proof ) was in the position of 60cm dropping from the crown of the tunnel.
There was the gravel inflow to the tunnel 1week before the explosion.
Incidental Discussion Methane gas explosion hazard is the fearful disaster which is generated when people neglect the ventilation and the countermeasures which are necessary for explosion prevention, such as measurement, and not being off guard because of its rareness. The carelessness towards methane gas is taboo.
Scenario
Primary Scenario Insufficient Analysis or Research, Insufficient Environment Study, Insufficient Analysis or Research, Insufficient Environment Study, Organizational Problems, Poor Management, Carelessness, Insufficient Understanding, Poor Concept, Poor Strategy or Concept, Malicious Act, Rule Violation, Bad Event, Chemical Phenomenon, Failure, Fracture/Damage, Bodily Harm, Death, Loss to Organization, Economic Loss
Sources Newspaper article, the report of Ettchu-island gas explosion accident investigation committee.
Number of Deaths 4
Number of Injuries 1
Field Civil Engineering
Author KITAJIMA, Munekazu (TEKKEN CORPORATION)
Scenario