A monthslong investigation into the landslide that killed 52 people and injured 30 on an expressway in Guangdong province in May last year, has determined that the cause of the serious disaster was prolonged heavy rain combined with multiple factors.

The landslide occurred early on May 1 at the Chayang section of the Meizhou-Dabu Expressway, sending 23 vehicles tumbling and causing widespread devastation, the Emergency Management Department of Guangdong Province said in a report released on Wednesday.

Rainfall in April last year was unusually concentrated in most parts of Guangdong, with the Meilin weather station, about 700 meters from the disaster site, recording 740.5 millimeters for the month. It was the highest amount since records began at the weather station in January 2021, the report said.

The disaster site had a raised section of road alongside and above a steep embankment. Prolonged heavy rain caused underground water to accumulate, leading to rising water levels and increased pressure. The soil at the bottom of the embankment softened, reducing its holding strength.

The embankment's lower and middle sections, along with a retaining wall, suddenly deformed and slid, triggering the collapse of soil on the upper section, the report said.

The collapse occurred in poor visibility during light rain, while an unusual spike in traffic added to the danger. Some of the fallen vehicles caught fire, exacerbating the disaster's impact.

All 30 injured individuals have since been treated successfully, and efforts to mitigate losses have been praised, the report noted.

The eastern extension of the Meizhou-Dabu Expressway reopened in September, with traffic flow now comparable to levels before the incident.

Disciplinary authorities in Guangdong have punished 32 officials for dereliction of duty.

Wang Xiaotian, a former Party committee member and deputy director of the Guangdong Provincial Highway Administration Center, is under investigation for alleged violations of discipline and laws. Other officials have faced disciplinary actions or been admonished under Party and government regulations.

Related enterprises and individuals have received administrative penalties from regulatory authorities, the report said.