Quarrying led to bridge collapse

After a study conducted by a nine-member investigation team headed by the president of the Taiwan Construction Research Institute, the cause of the Kaoping Bridge (高屏大橋) collapse has been attributed to the loss of riverbed gravel due to quarrying, in addition to design inadequacies.

Since the collapse of the bridge on Aug. 27, engineering experts have suggested that illegal gravel quarrying could be the reason for the accident.

Owners of quarrying operations have disputed the accusations, saying that their activities in the river were halted in 1997.

Chern Jenn-chuan (陳振川), president of the construction research institute, said the group made two inspections of the site before meeting to discuss their findings.

"Our team has concluded that the collapse was primarily attributed to erosion of the riverbed which had been deepened by as much as 8m. We also found that some of the buttresses were poorly protected," Chern said at a press conference yesterday.

"Excessive gravel quarrying in the vicinity is, of course, related to the cause of the deepened riverbed and exposed pillars," Chern said. He refused, however, to blame the problem on quarrying alone.

Chern said that the protective barriers immediately upstream from each pillar of the bridge (蛇籠) were also to blame as it appeared that they were not properly preventing the riverbed from deepening around the pillars. Chern said that this was especially true of the P22 pillar that failed, allowing sections of the bridge to collapse.

One investigator said that the protective barriers were not the same and that this could have been significant.

"The river flow at the time of the collapse was between 5,000 to 6,000 tonnes per minute. This was fairly normal for the Kaoping River. However, the difference in height among the adjacent barriers had produced many hydraulic jumps (水躍) which increased the flow of water and pounded against the pillars," according to Tsai Chang-tai (蔡長泰), a team member and a professor of hydraulic engineering at National Cheng Kung University (成功大學).

Chern said that "stability of the riverbed is key to the safety of all bridges," and that the Kaoping Bridge was no exception. Even though quarrying was to have stopped in the river at least three years ago, he said that there was no doubt that earlier quarrying had deepened the riverbed.

When asked to say who should be responsible for the collapse, Public Construction Commission (公共工程委員會) Chairman Lin Neng-pai (林能白) said that the matter was one for the judiciary and would be considered only after all investigations had been completed.

"We will present our reports to judicial authorities," Lin said.

The team suggested that renovations of the Kaoping Bridge should include cross-riverbed protection works (跨河固床工) to reinforce the foundations of the bridge and that of a new bridge to be built.

The Cabinet last week decided to build a new bridge within three years at an estimated cost of NT$1 billion.