The site had already been evacuated due to safety concerns early Friday morning, before the collapse, and police had closed off a 1km stretch of road around the construction area.
A police official told local media that a site supervisor had reported cracks in an underground support column, which led to a joint response with city authorities.
“There were concerns over possible ground subsidence, so we blocked the road as a precaution,” the official told reporters.
Police said they would keep the road closed until the reinforcement work is finished and the site is deemed to be safe.
The incident follows a deadly collapse of a highway construction site at Anseong, south of Seoul, in February as well as a 20m sinkhole in Seoul that killed a motorcyclist in March.
