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Metro sinkhole in Guangzhou 07 Feb 2013
TunnelTalk reporting
Recovered site of the collapse

Recovered site of the collapse

A collapse of metro tunnelling works on the Guanzhou Metro in China on 28 January, 2013, created a sinkhole of 10m deep and 150m2 at the intersection of Kangwang Road and Shanmulan Road. The sinkhole occurred following construction of the turn line for the Culture Park interchange station for the city's Metro Line 6 and Line 8. There were no fatalities.
A correspondent in China for TunnelTalk, reports Metro officials as saying that the main structure of the station and the tunnel, which had already been finished, were not affected.
Reports state that at about 16:00 hours on 16 January, 12 days before the incident, the contractor of the works found some geological anomalies, including gushing water and inflow of basal sand. With an immediate response, nobody was hurt.
Location of the metro works and the collapse site

Location of the metro works and the collapse site

Residents nearby said they often heard the noise of explosions. The contractor claimed that the accident was due to the fact that the real rock layer thickness was much less than site investigation information predicted.
The Metro Line 6

The Metro Line 6

Engineering geology
Geological conditions on the Metro Line 6 alignment are described as complicated and include karst features, soft soil, sand soil, and fracture rock. The thickness of the Quaternary layer is reported as between 13m and 16m, and includes mainly artificial backfill, muck soil, silty-fine sand, and medium-coarse sand. An undulating top of bedrock interface and change in the thickness of each geological layer is severe. Moderately weathered soil consisting mainly of silty mudstone and argillaceous siltstone, both of which are granular in nature and easily weakened, with some hard intercalation, are the main host layers at the tunnel elevation. More accurate details of the geology were not available.
Culture Park Station, in the east to west direction, was constructed as an open cut excavation to about 23.12m deep.
The tunnel was excavated at between 15.3m and 22.6m deep using a composite tunnelling shield of 6.2m diameter which has a cutterhead equipped with both disc cutters (for rock) and scraper tools (for soil). It is alleged that the contractor was using drill+blast to excavate rock in the cross passages between the parallel shield-driven running tunnels without sufficient pre-investigation.
References
Symptoms of the collapse syndrome - TunnelTalk, July 2010

           

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