(Yicai Global) July 10 -- Qingdao Metro Group has ordered work stopped on all subway lines under construction after two accidents in 40 days claimed the lives of six workers.

One died last week due to a tunnel cave-in on Line One in the coastal Chinese city, and a similar accident at another project in late May resulted in five other deaths, The Paper reported, citing a statement from Qingdao Metro. The subway operator is still investigating the cause of the accidents, a spokesperson told the media today.

As of last December, Qingdao Metro had four lines with a total length of 172 kilometers. Six are under construction. The city will have a network of eight lines with a total length of 332 kilometers by 2021, according to public data. The 60-kilomter-long Line One has China's deepest and longest underwater tunnel with complex geology.

"The collapses are mostly related to geological conditions, which have no connection to jerry-building or outsourcing," the spokesperson said.

In late June, a subcontractor made allegations about illegal outsourcing and second-rate construction work on Line One that could have potential repercussions for safety. State-owned engineering contractor China Gezhouba Group has refuted claims that it illegally outsourced power supply construction on the line.

The Wuhan-based firm acknowledged that its subsidiary, China Gezhouba Group Electric Power, had managed the project poorly and pledged in a statement today that it would rebuild the power supply and investigate the staff involved.

The accident fully exposed the weak link in the control and management of the project, the company said in a statement after May's accident, adding that Qingdao Metro had decided to suspend all construction work and rectify any errors. But the move did not prevent the Line One mishap on July 4.