Geographical explanation

According to experts directing the rescue work in field, the location of the cave-in used to be a pond 50 years ago. The backfill earth on the surface had never been tightly compacted and a layer of fine sand lay immediately below the backfill earth, whose density cannot sustain a tunnel.

"I think to absolutely avoid a disaster of this nature in subway construction remains an engineering difficulty worldwide," said Ma Yinsheng, a senior research fellow on geological disasters with Chinese Academy of Geological Sciences. He said in the pre-construction geological surveys for subway construction, the standard practice is to drill holes every dozens of meters to see if the geological texture is solid enough for sustaining tunnels. Under such circumstances, if there was a pond of less than 10 meters in diameter backfilled with earth, the odds are high that it won't be detected, which could be a hidden danger in the construction of tunnels.

Ma said besides the loosened backfill earth, another possible cause for this accident is the existence of a geographical crack, which his research team suspected during a geological study of Beijing city proper several years ago.

Ma said although Beijing was situated on a soft geological stratum, the construction of the planned subway network should have no technical difficulty from the perspective of geology. He said the most useful measure to avoid such a tragedy in the long run is to establish geological archives for every city by preserving all its geological data. This will ensure the geological situation of a region 100 years old could serve as the reference for future construction.

Yet Ma said the city geographical archives is still a new concept worldwide and has not been thoroughly established by any city in China. Immediately after China's successful bid for hosting the 2008 Olympic Games in 2001, he handed over a suggestion to the Beijing Municipal Government to collect the geological data during the city's unprecedented campaign of subway construction. "This offers a rare chance for us to gain knowledge of Beijing's geological information, which could serve as the technical reference for future projects," said Ma, who still has no idea whether his suggestion has been accepted.

Chronology of Subway Cave-in (Up to 9 a.m. April 6) 9:30 a.m. March 28 Construction site for the No.10 Subway Line in Haidiannanlu (Road) between the city's northern Third Ring road and Fourth Ring road experiences a cave-in, which traps six workers. The collapsed tunnel covers an area of about 20 square meters and is about 11 meters underground. Shortly after the accident, project managers order workers not to talk to media or police and confiscates their cell phones. 2:55 p.m. March 28 First-aid center of police in Anyang, Henan Province receives an emergency call from the relative of one of the trapped workers. The information is leaked via a cell phone of a worker at the accident site that was not confiscated. 4:39 p.m. March 28 After getting the report from Henan police, Beijing Municipal Government confirms authenticity of the information and location of the accident and activates an emergency response plan by forming a rescue command center of officials from different departments. Evening March 28 Beijing Mayor Wang Qishan and Minister of the State Administration of Work Safety Li Yizhong arrive at the scene to give field direction to the rescue work. 7:30 p.m. March 29 Beijing Municipal Government hosts a press conference on the accident, naming the contractor, subcontractor and supervisor of the project. 10:25 a.m. March 30 The first trapped worker is located. He shows no signs of life. 6 p.m. March 30 Rescuers recover the first body after more than 50 hours of excavation. The victim is confirmed to be 20-year-old Li Peng from Henan Province. March 31 The bodies of another three workers are located. They are crushed by collapsed concrete. The space underground for rescuers to move is narrow. 6-8 a.m. April 1 The bodies of 37-year-old Mu Chaoqing and 37-year-old Liao Zhengjun are extracted. 1 p.m. April 1 The body of 35-year-old Zhou Congyun is extracted.