Shoddy construction of a subway line extension has caused the formation of large underground cavities in southeastern Seoul, the city government said Thursday.

Earlier this month, the city government discovered the seven hollows below an underpass in Seokchon-dong while inspecting the underground conditions following the appearance of sinkholes, sparking safety concerns.

Announcing the result of an inspection, the city government blamed Samsung C&T Co., the construction arm of Samsung Group in charge of building an extension of Seoul Subway Line 9, for the hollows.

"Samsung C&T dug out more soil than it had planned when building a tunnel underneath the Seokchon underpass," said Park Chang-geun, a civil engineering professor who led the municipal inspection of the cavities. 

Park said Samsung C&T excavated 14 percent more soil than planned from the beginning of construction in May 2013 until recently.

A specialized chemical used to cement soil surrounding the tunnel may have also failed to do its job, contributing to the excess digging, he added.

The construction of Lotte World Tower, which will be the world's sixth-largest tower when completed, near the underpass and a possible sewer collapse were ruled out as reasons for the formation of the hollows.

The Seoul Metropolitan Government unveiled several measures to prevent similar cavities and sinkholes, which are considered serious road hazards.

A total of 5,000 kilometers, or an annual average of 680 km, of old pipes will undergo safety inspections by 2021, with an additional 101.7 billion won ($100.3 million) being allocated to repair them.

Monitoring staff will be assigned to construction sites pumping out more than 100 tons of groundwater per day since overpumping is considered one of the many factors contributing to the creation of sinkholes. (Yonhap)