Kanmon Undersea Railway Tunnel & the Invasion of Japan

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Mil-tech Bard
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Kanmon Undersea Railway Tunnel & the Invasion of Japan

Post by Mil-tech Bard » 11 Feb 2013 23:27

In April 1945 General MacArthur's planners placed the Kanmon Undersaea Railway Tunnel between Honshu and Kyushu on the priority target list.

In August 1945 it was still untouched.

This topic is to discuss the specifications and importance of this electrical railway tunnel for Japanese logistics prior to the end of WW2 and the American command failure that left it intact.

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Re: Kanmon Undersea Railway Tunnel & the Invasion of Japan

Post by phylo_roadking » 12 Feb 2013 01:11

Untouched as in un-ventured....or were attempts made to attack it? I know an OSS attack being planned by August 1945...

P.S. did the Americans know there was a second parallel tunnel completed in 1944, permitting two-way traffic?
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Re: Kanmon Undersea Railway Tunnel & the Invasion of Japan

Post by Mil-tech Bard » 12 Feb 2013 04:01

The Kanmon Railway Tunnel -- often called "Kammon" in various WW2 and
post WW2 documents/histories -- was the key piece of transportation
infrastructure for Operation Olympic which was left untouched at the
end of WW2.

The Shimonoseki straits were mined in March 1945, cutting off large
ship traffic between Honshu and Kyushu.

The Tunnel was included in an April 1945 SWPA/FEAF high priority
transportation campaign target (See Transport plan on pg 115 of MacArthur Reports).

Between April and August 1945 the Japanese Army built up from 300,000
troops to 900,000 troops. The majority of this build up happened in
June, July and August 1945

The Kanmon Railway tunnel was reserved as a target for the Javaman
remotely piloted boat-bomb that would have been controlled by a 20th
Air Force plane, with four 85-foot Javaman to be delivered by
submarine or Fast Destroyer transport (APD) within one-night cruising
range of an 85-foot air rescue boat tarted up as a fishing boat.

A cursory review of the physical structure of the Kanmon Railway
Tunnel makes the idea that Javaman could actually pull off the closure
of the tunnels highly doubtful. The problem is that both approaches to it
were long -- the northern one required going past fortified islands and the
Southern approach had the Kyushu coast as the left flank for several
hundred miles. You were not going to get APD's close enough past Kamikazes
and an 85 ft boat was too hefty to move more than one at a time on a Fleet Boat.

This Javaman project kept the Kanmon Railway Tunnel off the US Navy carrier,
20th Air Force and FEAF target lists until June 1945.

After the Javaman program was cancelled, the Kanmon tunnel was
assigned to the 20th Air Force bombers, which firebombed both Moji and
Shimonoseki, which were either end of the electric railway tunnels.

Firebombing was ineffective in destroying the rail lines, tunnel
entrances or the electrical power plant supplying the tunnel electric
engines.

After August 7, 1945, the Kanmon Railway tunnel was reassigned to the
FEAF, which made one attempt to bomb the tunnel that was thwarted by
bad weather.

The US Strategic Bombing Survey cut off all information on Japanese
railway transportation statistics past 30 June 1945, so those fact was
not visible in a narrative form I just related.

This is what one of the most researched USSBS reports said in two very
small and disconnected foot notes.

THE UNITED STATES STRATEGIC BOMBING SURVEY

The Effects OF Strategic Bombing ON JAPAN'S War Economy

Appendix ABC

OVER-ALL ECONOMIC EFFECTS DIVISION
December 1946


Page 39
"* One mission scheduled against the Kammon Tunnel entrance was frustrated
by foul weather and this particular target was later specifically assigned
to other forces. There remained, however, at least four vital rail yards in
the area, Moji. Shimonoseki, Ilatabu, and Tobata, as well as bridges and
yards on the Sanyo-, Kagoshima, and Chikuho lines which were essential
to the movement of Kyushu and Ube district coal."
and

Page 62
"' Even if it had proved impossible to do serious damage to the Kammon
tunnel which had been designated as a target for special weapons. Responsibility
for its destruction had been shifted to the B-29's."
Point in fact, the transportation campaign the Japanese War USSBS
reports harp on were almost completely unnecessary.

The Imperial Japanese Army had pushed so many train engines and
rolling stock through the Kanmon tunnel to Kyushu that the overall
Japanese railway system was operating at 30% capacity at surrender.

The majority of the equipment for those 600,000 extra troops was still
on rail cars in tunnels, burnt out factory area sidings in Northern
Kyushu or in caves adjacent to rail heads at war's end.

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Re: Kanmon Undersea Railway Tunnel & the Invasion of Japan

Post by phylo_roadking » 12 Feb 2013 20:28

IIRC the tunnel itself was a minimum of ten metres below the sea floor...which in turn was a minimum of eighteen metres deep? How was the remote controlled boat bomb option supposed to deal with the cushioning effect of over 50 feet of water? Even if it was carrying four tons of HE? http://www.nextreads.com/display2.aspx? ... 45565&FC=1
One of the most forward thinking projects undertaken by Lovell’s team was Javaman, a remote-controlled weapon consisting of a boat packed with four tons of explosives. Using early television technology, a camera mounted on the boat’s bow broadcast images to a plane circling fifty miles away where a crew member watching a monitor guided the boat to its destination, then triggered the explosives by remote control. Despite encouraging tests, the project was eventually dropped. According to Lovell, the Navy abandoned the idea because it judged the explosive load as too dangerous to carry either by ship or submarine.
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Re: Kanmon Undersea Railway Tunnel & the Invasion of Japan

Post by Mil-tech Bard » 15 Feb 2013 03:46

There were to be four boats in the attack with 50,000 lb of explosive in each boat.

See:
The Army Air Forces in World War II
The Pacific -- Matterhorn to Nagisaki June 1944 to August 1945

Chapter 22

page 699

Reorganization for Victory

In defense of FEAF, it must be noted that its air units were still feeling out Japanese resistance when the war unexpectedly ended. A systematic USASTAF-FEAF transportation assault had been planned, and the first USASTAF operation of the series took place on the last day of the war.134 It was recognized that the Kammon Tunnel between Kyushu and Honshu was the most important single transportation target in Japan,135 but the target was being saved until four disguised air rescue boats, remotely controlled and each loaded with 25 tons of high explosives, could be run into the west entrance of the tunnel and detonated. This project was canceled as the war ended.136 FEAF had also intended to emphasize napalm attacks, of proved worth in the Philippine campaigns, but there was time only for the preliminary tests. The Japanese already were licked. As Whitehead summed it up, the enemy "could decide that enough Nips had been killed or he could commit national suicide. He chose the former."137


133. USSBS, The Effects of Strategic Bombing on Japan's War Economy, pp. 61-65.

134. Ibid., p. 44.

135. Msg. WAR-65143, AFIJT to CINCSWPA, 7 Apr. 1945.

136. Msg. CX-30940, CINCAFPAC to Guam, 5 Aug. 1945; msg. A-80388, CG FEAF to WD, 7 Aug. 1945; msg. CX34703, CINCAFPAC to WD, 18 Aug. 1945.

137. Ltr., Whitehead to Chennault, 21 Aug 1945.

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Re: Kanmon Undersea Railway Tunnel & the Invasion of Japan

Post by ChristopherPerrien » 15 Feb 2013 13:25

I think Grand Slams could have taken it out.

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Re: Kanmon Undersea Railway Tunnel & the Invasion of Japan

Post by phylo_roadking » 15 Feb 2013 18:03

but the target was being saved until four disguised air rescue boats, remotely controlled and each loaded with 25 tons of high explosives,
Op, I find that amount somewhat questionable - do you know what 25 tons of explosive looks like???
...the Navy abandoned the idea because it judged the explosive load as too dangerous to carry either by ship or submarine.
Christopher...
I think Grand Slams could have taken it out.
IF there was some way of ensuring they exploded on the bottom of the channel ;)

One aspect of the construction of the tunnel to remember is...apart from the cushioning effect of the water above it...is that it was steel-lined - with bolted-together steel sections - because of the porous nature of the rock of the sea bed there ;) Whatever is used to attack the tunnel not only has to overcome the cushioning effect of the water - it has to crush a self-supporting steel tube inside the rock.
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Re: Kanmon Undersea Railway Tunnel & the Invasion of Japan

Post by ChristopherPerrien » 15 Feb 2013 22:00

Grand slams by design are made for this sort of target. Delayed action fusing and armor tipped so they could penetrate a 100 hundred feet of more and with a blast effect where they did not have to hit a target to destroy it. That was why they were called "earthquake bombs". IIRC , one target they destroyed was an italian RR tunnel through a mountain of granite . The bombs penetrated 80 feet before exploding and brought the tunnel down by the shockwave.

50 ft of water and fifty feet of earth ain't going to alter what a grand slam would do as compared to 100ft of dirt or rock, and even a steel line tunnel would not hold together from a near miss. Grand slams were know to miss by hundred's of feet and still "shake" and collapse major structures both on ground level and far below.

I can only figure it was a lack of cooperation/communication by the Pacific USAAF with the British to perhaps procure some and a lack of vision by USAAF generals as to even considering Grand Slams for this target.

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Re: Kanmon Undersea Railway Tunnel & the Invasion of Japan

Post by phylo_roadking » 15 Feb 2013 22:33

Grand slams by design are made for this sort of target.
No, they weren't actually designed to penetrate rock or concrete - tho' that's what they were occasionally used for - they were actually to undermine a concrete construction by causing it to collapse into the cavern excavated by the subsurface explosion...in "soft" earth. Used against "hard" targets it had a tendency to break up.
Delayed action fusing and armor tipped so they could penetrate a 100 hundred feet of more and with a blast effect where they did not have to hit a target to destroy it.


A 100 feet or more (actually 40 metres, or 130 feet) of earth ;) Not rock.
That was why they were called "earthquake bombs". IIRC , one target they destroyed was an italian RR tunnel through a mountain of granite . The bombs penetrated 80 feet before exploding and brought the tunnel down by the shockwave.
The sorties involving the 42 Grand Slams used are known, and an Italian railway tunnel isn't among them.

You're possibly thinking of the Saumur railway tunnel in the Loire, that was attacked with Tall Boys...but this was closed by one penetrating and going off IN the actual tunnel.
50 ft of water and fifty feet of earth ain't going to alter what a grand slam would do as compared to 100ft of dirt or rock,
But that's what the Kanmon tunnel was in - rock. Not earth.
and even a steel line tunnel would not hold together from a near miss. Grand slams were know to miss by hundred's of feet and still "shake" and collapse major structures both on ground level and far below.
It depends on what the Grand Slam dropped into, and what the nature of the target actually was.
I can only figure it was a lack of cooperation/communication by the Pacific USAAF with the British to perhaps procure some and a lack of vision by USAAF generals as to even considering Grand Slams for this target.
There was no USAAF aircraft in the Pacific theatre that could have dropped it IIRC except for possibly the B-29, you'd have to check bombbay dimensions. It could carry the U.S. equivalent of Tall Boy, the T-10....but I don't know about Grand Slam.
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Re: Kanmon Undersea Railway Tunnel & the Invasion of Japan

Post by Mil-tech Bard » 17 Feb 2013 05:18

The trains through the Kanmon Undersea Railway Tunnel were electrical because there was not enough air for a steam train to make it through.

Destroying the power plant would for the line would have closed the tunnel.

The power plant was untouched at war's end.

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Re: Kanmon Undersea Railway Tunnel & the Invasion of Japan

Post by Mil-tech Bard » 17 Feb 2013 05:22

The B-29 was trialed with the Grand Slam in Feb 1945.

Carrying the Grand Slam and having greater drag from the open bomb bay after its use cut range 300 miles, AKA a Marianas or Guam based B-29 would have to recover on Iwo Jima or Okinawa to use it.

The 20th Air Force didn't convert any to that configuration by war's end.
Last edited by Mil-tech Bard on 17 Feb 2013 16:36, edited 1 time in total.

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Re: Kanmon Undersea Railway Tunnel & the Invasion of Japan

Post by Mil-tech Bard » 17 Feb 2013 16:33

Attached is a cutaway target assessment of the Kanmon Undersea Tunnel complex from the Joint Target Group, Washington D.C. dated 16 June 1945.

Please note the target vulnerability assessment bar under the tunnel depth profile in middle and upper right.

The "Western Tunnel Entrance" was 1/4 mile -- several 100 meters -- inland and Javaman was a remote controlled speedboat. There was no way four converted PT-boat sized small craft could carry enough explosives to seal the entrance on land without carrying a nuclear payload.

The only portion of the Kanmon Undersea Railway Tunnel complex Javaman had a chance of success (See section seven) at the western end of the Kanmon Tunnel was with the Railway viaduct in the channel in the upper left of the photo, at "Target 772".

Even that was very low probability of success. There were Japanese automatic anti-aircraft weapons covering the approaches to the bridge in that area.
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Re: Kanmon Undersea Railway Tunnel & the Invasion of Japan

Post by phylo_roadking » 17 Feb 2013 17:26

Please note the target vulnerability assessment bar under the tunnel depth profile in middle and upper right.
IF...I presume...placed in exactly the right place on top of the tunnel? :wink:
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Re: Kanmon Undersea Railway Tunnel & the Invasion of Japan

Post by Mil-tech Bard » 17 Feb 2013 18:37

Consider 300 B-29, with 20x1000-lb bombs each (6000 ea. 1000-lb bombs), and a .0002 hit percentage.

Multiple .0002 X 6000 = 1.2 hits.

No such attack was made.

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Re: Kanmon Undersea Railway Tunnel & the Invasion of Japan

Post by phylo_roadking » 17 Feb 2013 18:57

Consider 300 B-29, with 20x1000-lb bombs each (6000 ea. 1000-lb bombs), and a .0002 hit percentage.

Multiple .0002 X 6000 = 1.2 hits.

No such attack was made.
Please note the target vulnerability assessment bar under the tunnel depth profile in middle and upper right.
Look at it more carefully - over 2/3s of the underwater length is marked as "invulnerable". The last third is marked as "vulnerable" only because of what's over the tunnel - sandy clay.

It's an assessment based only on known soil and rock types the tunnel went through - doesn't say anything about proximity of impact, height to be delivered at etc.,...

And of course - any of the eastern part of the tunnel destroyed by bombing on land - could be far more easily cleared/repaired than any part of the unserwater section ;) The effort might not actually "destroy" the target, just incapacitate it for a time.
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