A mountain stream used to flow near the site for the power transmission tower for Fukushima I Nuclear Power Plant. They dumped a lot of soil and filled the stream to create the embankment, and built the power transmission tower. They thought the water would naturally flow underground at the old surface level. Well it didn't.
From Jiji Tsushin (2/17/2012):
The power transmission tower for Reactors 5 and 6 at Fukushima I Nuclear Power Plant fell because the embankment collapsed from the intense and long shaking from the mega earthquake. In late 1960s the nearby mountain stream was filled but a drain pipe was not installed. TEPCO reported the findings to the Nuclear and Industrial Safety Agency on February 17.
Reactors 5 and 6 lost external power when the tower fell and the electricity supply was cut off when the power lines were severed. But one of the the emergency power generators worked, and the reactors were cooled. For Reactors 1 through 4, the other tower and the power lines remained intact, but the power distribution equipment and the emergency power generators were damaged by the tsunami, resulting in the complete loss of power for the cooling systems.
According to the report. the strongest shaking near the tower from the quake occurred at 2:48PM on March 11, 2011. At 2:49PM, the tower fell and the power lines were severed. According to the blueprint from 1966, there was a mountain stream flowing toward the site for the power transmission tower. After the embankment was built, the stream went underground. The underground stream then flowed at about 2 meters above the old surface, flowing right through the embankment.
TEPCO's Matsumoto explained in the press conference, "When the stream was filled, we assumed that the underground water would flow along the old surface level."
The general contractor who built Fukushima I Nuclear Power Plant is Kajima, who should have known better. The company was, and still is, the first and foremost general contractor in Japan in huge civil engineering projects. I would give them some slack though, as building something that would withstand Magnitude 9 earthquake may be near-impossible.
TEPCO's report to NISA is in Japanese only, and it is not known if NISA will ever provide the English translation. If you read Japanese, the report is here.
The report says the tower fell 30 seconds after the maximum acceleration.
From the report, it looks a large tree jammed into the tower:
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