A 23.5-ton section of new mortar collapsed in a tunnel in Kimitsu, Chiba Prefecture, on Wednesday, but no one was injured, the prefectural government said.
A traffic controller noticed the mortar falling over a 20-meter stretch of the 91.3-meter tunnel at around 8:10 a.m. The aging tunnel had been closed overnight for repairs but was reopened to traffic at 5:30 a.m.
"I don't know what caused the accident. I will look into it thoroughly," said Takayasu Akamatsu of Miyamotogumi Co., the Hyogo Prefecture-based construction company that built the tunnel.
The maintenance work had been underway since June following the fatal December 2012 collapse of the Sasago Tunnel in Yamanashi Prefecture that killed nine people. The mortar was installed about a month ago.
On Thursday, Chiba Gov. Kensaku Morita said he has requested help on the investigation from central government experts. The government was expected to send officials from the National Institute for Land and Infrastructure Management to the site later in the day.
"As an operator, I offer my apologies," said Morita. "We will investigate the cause of the incident, considering various possibilities."