Failure Knowledge Database / 100 Selected Cases

Gas Explosion at a Subway Construction Site
Apr. 8th, 1970, Osaka city
KOBAYASHI, Hideo (Tokyo Institute of Technology)(Summary)
On April 8th, 1970, a huge gas explosion occurred at the site of construction for elongation of Osaka
city subway line No.2 (Tanimachi line). The gas was leaked out of a detached joint for a drainer on themedium pressure piping, which was under construction for suspension. The gas was filled in the tunnel,
ignited, and exploded violently creating a fire pillar of over 10m in height, and causing cover plates over toscatter over an area 200m in length (Figures 1, 2, and 3).

The explosion resulted in a terrible accident(so-called Tenroku gas explosion accident), with 79 persons dead, 420 persons injured and 495 houses andbuildings burned and destroyed. Almost all of the victims were bystanders having no direct relation to theconstruction work. The causes of the accident were believed to be a decrease of the joint strength by theweight of passing traffic vehicles together with the influences of the construction work itself. Furthermore,
no measures were taken against drawing off the joints. Because most of victims were bystanders, thisaccident had a large social influence, and it left various precepts as a municipal disaster that was producedby the tremendous boom of municipal development in an age of high economic growth.

1. Event
At the site of construction for elongation of Osaka city subway line No.2 (Tanimachi line), thesuspending work had been conducted in the underground for the exposed pipes of medium and low
pressure gas (Figures 4 and 5). A joint of the drainer for the medium pressure pipe became detached, andthat caused gas to blow out, ignite, and then explode violently (Figure 6).
Results of fault tree analysis are described below.
Figure 7 - Fault tree diagram based on morphology, mechanism, and process of the destruction.
The possible causes of the gas leak are considered to be rupture, draw out, crack, or opening of the pipe.
Results of investigation after the accident showed that a joint of the drainer for the medium pressure pipe
was drawn down, and that was assumed to cause the leak of gas.
The same type of the drawn joint had an initial strength that was rated against an inner pressure of about
8 tons, however, the detached joint could not resist an inner pressure of even 1 ton. The cause of thischange in the strength of the joint could be one or more of the following; (1) the strength of the joint haddecreased, (2) no measure was taken to prevent the drawing off the joints, and (3) the construction work on
the day of the accident formed a suspending condition of the pipe. These complex conditions caused thedrawing down of the joint of the pipe.
Cause (1) of the decrease in strength of the joint was considered to be the result of the following threefactors, initial defects, gradual degradation over time by the traffic load, and influences from theconstruction work of the subway. The possibility of initial defects that were formed when the joint buried1

Failure Knowledge Database / 100 Selected Cases

under the ground, was estimated by considering the working situation and procedure of the burying process
as well as the burying conditions at that time. The gradual degradation of the joint strength was considered
to be important because of the shallow depth at which he medium pressure gas pipe was buried that caused
it to suffer from vibration and subsidence by the passing traffic vehicles prior to the start of the construction
work on the subway. After beginning the subway construction work, repeated excavation and burying in thearea for the experimental drilling and covering was estimated to exert a larger influence than the trafficloads even though the period was much shorter.
The source of the ignition was regarded at first to be an emergency vehicle, which caught fire at the siteof the gas leakage. However, because the fire of the emergency vehicle continued for about ten minutes, theconcentration of the gas mixture in the upper part of the pit just below the fire was believed to have alreadyexceeded the limit of combustion, so that there was no fear of catching fire. Otherwise, the fire in theemergency vehicle should have occurred at the same time as the ignition of the explosion. The explosion
occurred at near the center where the emergency vehicle that had caught fire located. However, the ignition
point does not always coincide with the center of explosion because of a phenomenon by which the flame
caused by the ignition propagates in the mixed gas before the explosion. From the reasons mentioned above,it is difficult to consider that the emergency vehicle fire was the source of ignition. There were many otherpotential sources of ignition around the site. The ignition source was considered to be one of those otherpotential sources except the emergency vehicle.
Results of event tree analysis are described below.
Figure 8 - Event tree diagram for the explosion and the leak of gas by drawing off the joint.
The fastening strength of the gas pipe was decreased by an initial defect that had occurred immediately
after the burying work. Furthermore, the joint strength degraded gradually over time from vibration andsubsidence caused by the traffic loads, and finally the joint strength decreased rapidly as a result of theinfluences of the subway construction work. The pipe was exposed in hanging condition during theconstruction work. The decreased joint strength could not resist the inner pressure because there was no
prevention against drawing off. Then the joint was drawn off, and caused the gas leak. The explosion
occurred as a result of some unidentified ignition source.

2. Course
The medium pressure gas pipe, the joint of which was drawn off in the accident was installed in May
1957.
The construction work of the subway was conducted by the open-cut method, one of the methods for
digging tunnels. The procedure of the open-cut method is as follows. First, steel piles are hammered along
both sides to form walls. Then, the steel piles are bridged with H-shaped steel bars, upon which covering
plates are placed to minimize the traffic disturbance on the roads. Finally, the area between the walls ofsteel piles is excavated from the ground surface. In this construction work, a tunnel of 11m in width, 5m in
depth, and 225m in length was excavated along the north side of a road of 26m in width, and covered withcovering plates (2m in length, 75cm in width, 20cm in thickness, and 380kg by weight) which were made2Failure Knowledge Database / 100 Selected Casesof concreted steel frames.
The gas pipe, a total of 200 m in length, had two traversing points at both ends forming right angles
bends. On the morning of April 8, the soil under the north corner of the traversing point was removed
causing the entire length of the medium pressure gas pipe including the traversing points to be suspended,making it easy for pipe to move near the traversing point in a horizontal direction.
At about 5:15 pm, 27 workers under suspending work for the exposed pipes escaped from the tunnel tothe surface because they noticed the gas that blew out suddenly. At 5:20 pm, a patrol car of the gascompany, on the way back to the base by chance, also found the gas leak by chance, and took
communications by wireless with an emergency car and a repair car to call out. Fire trucks also arrived bydispatch from stations around the site. At that time, the gas blew out from cracks, 20m in length, between
the cover plates. Fire men requested all residents and bystanders in the area to evacuate and avoid using fire.
At 5:39 pm, when the emergency car of the gas company started its engine over the cover plates, the carcaught fire, and then the gas blowing from the cracks also ignited. The fire gained force gradually, and at
5:47 pm, a huge explosion with a fire pillar of over 10m in height occurred, causing the cover plates toscatter over an area 200m in length. The injured persons and the cover plates fell down to the ground, andthe fire pillar expanded to the surrounding houses on both sides.

3. Cause
The strength of the joint had decreased by multiple causes including initial defects, gradual degradation
with traffic loads, and influences from the construction work for subway. Construction work related
personnel should have estimated the decrease of the joint strength, even though the failure may not been
anticipated. Therefore, the basic cause of the accident was attributed to the lack of measures taken toprevent the drawing off of the joints.

4. Immediate Action
When a large gas leak happens, construction work related personnel must inform not only the gas
company, but also the fire station and the police station. Depending on the place and situation of the gas
leak, the personnel must announce to persons in a wide area that persons and cars are not to enter the site
and they must inform people around the site that the use of fire is prohibited as soon as possible.
Furthermore, they must order the neighboring residents to take refuge in a safety area.
The victims of this accident included a crowd of curious spectators for the first gas leakage, who did not
realize that they were standing on the cover plates. In order to avoid such a tragedy, persons around the sitemust be ordered to take refuge in a safety area.

5. CountermeasureTaking opportunity of the accident, the Gas Service Act, 77th and 78th articles, "Protection of gas pipe
exposed by excavating" was legislated. The act contained the following stipulations.
○ Confirm that sand and soil at both ends of an exposed pipe have no possibility of collapse.

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Failure Knowledge Database / 100 Selected Cases○ Take suitable measures to prevent leakage.
○ Take measures to absorb and to disperse the expansion and contraction of the pipe by the change oftemperature.
○ Take measures to shut off the gas in an emergency.

6. Knowledge○ Consider the possibility of defects:
This accident occurred because of no one had considered the possibility of a defect causing the decreaseof joint strength. Construction work related personnel should take actions based on consideration that oldfacilities used for a long time might have some number of defects. It should be remarked that routine
actions considered to be safe in ordinary circumstances might in fact be quite dangerous unknown toanyone.
○ Recognize the dreadfulness of urban civil engineering works.

7. SequelIn regard to the criminal responsibility of the accident, the accused side, that is, the constructor,
Osaka-city, and Osaka Gas Company, was tried with the procurator side in criminal court. The judicial
decision, given fourteen years later, judged a responsibility of negligence on the part of the accused side. At
that time, a series of judgments of "not guilty" had continued in the criminal trials on urban disasters. This
judgment changed the trend of criminal trial and became an epoch-making decision to recognize anew theimportance of safety.
The judgment concluded that the accident was the joint responsibility of the accused three juridical
persons for not applying an appropriate preventive draw off measure that would have been an effective
method for preventing the accident.

8. Information Source(1) On the Liaison Office for Accident of Gas Explosion at Osaka, May, 1970, Gas Section, Departmentof Public Utility service, MITI.
(2) Information about Cases of Serious Accidents, The Japan Gas Association.

9. Primary Scenario
01. Poor Value Perception
02. Poor Safety Awareness03. Lack of Safety Measure
04. Organizational Problems
05. Poor Management
06. Poor Management for Work
07. Production
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Failure Knowledge Database / 100 Selected Cases

08. Hardware Production
09. Construction Work for Subway
10. Open-cut method
11. Suspension of Gas Pipe
12. Regular Operation
13. Nonobservance of Procedure
14. No prevention against Drawing Off
15. Malfunction
16. Poor Hardware
17. Decrease of Joint Strength
18. Failure
19. Fracture/Damage
20. Drawn Off Joint for Drainer
21. Secondary Damage
22. External Damage
23. Leak of Gas
24. Explosion
25. Bodily Harm
26. Death
27. Bodily Harm
28. Injury
29. Damage to Society
30. Social System Failure
31. Burned Down of Houses

5Failure Knowledge Database / 100 Selected Cases

Fig. 1

Spot of gas explosion accident.

6Failure Knowledge Database / 100 Selected Cases

Fig. 2

Appearance of fire by gas explosion.

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Failure Knowledge Database / 100 Selected Cases

Sakaisuji-line

Kokubunji-2chome
Kokubunji

Tenroku-crossTanimachi-line
Miyakojima-doriStarting spotof accident

Tenjinbashi-suji-6chome St.

Accident
spot

Kanei-choFig. 3

Rough sketch of accident spot.

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Failure Knowledge Database / 100 Selected Cases

Drainer for medium
pressure gas

Drawing off partasre gssu ter)
erpeum r diamedi
Hanging boltr m inneofingm in
Pip 300m(asre gssu eter)
erw p r diamer log fo in innnip
Pi 0mm(50Drainer for highpressure gas

SleeveSleeveCable duct

Fig. 4

Appearance of piping for city gas.

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Failure Knowledge Database / 100 Selected Cases

Fig. 5

Imaging sketch of hanging situation.

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Failure Knowledge Database / 100 Selected Cases

Pressing ring

Fig. 6

Rubber LeadO-ring O-ring

Jute

Connecting method for piping.

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Failure Knowledge Database / 100 Selected Cases

Gas explosion
or GateIgnitionsouse

Gas leakageor Gate

OthersFired caror Gate

BurstCrack,
Opening mouth

Drawing offand Gate

No prevention
against drawing off

Decrease of
joint strength

Hangingin spaceand Gate

Initial defectFig. 7

Degradation on time by traffic load

Influence by construction
work for subway

Fault tree diagram based on morphology, mechanism, and process of the destruction.

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Failure Knowledge Database / 100 Selected Cases

Gas piping

Initial defectDegradation
on time bytraffic load

Influence byconstruction workfor subway

Hangingin space

No prevention
Ignition souseagainst drawing off
Yes

ResultExplosion

Yes
NoYes
NoYes
NoYes
NoYes

Decrease of
joint strength
Decrease of
joint strength
Decrease of
joint strength

No

Decrease of
joint strength

NoFig. 8

LeakageSoundness

Event tree diagram for the explosion and the leak of gas by drawing off the joint.

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