Fatal collapse of metro heading in Foshan 15 Feb 2018

An inrush of water and mud through the tailseal of an EPB TBM working on the Foshan Metro Line 2 is said to have caused the devastating collapse of the heading, claiming the lives of 10 and with another still missing. Unofficial reports to TunnelTalk from contacts in China confirmed that it was inrush through the tailshield, rather than over excavation at the cutterhead, that undermined the segmental lining and led to the massive collapse, burying the TBM and its crew as a consequence.

First surface signs (left) of impending devastating collapse

The failure occurred at 8.40pm on the night of February 7 as the TBM was excavating the 6m diameter metro tunnel beneath the main traffic thoroughfare of Foshan city near Guangzhou in southeast China. The completed parallel running tunnel some 7m distant is unaffected by the 200m² (30m wide x 6m deep) sinkhole over a second TBM drive. Contacts suggest that the same TBM may have excavated both running tunnels.

Working shaft of the ill-fated metro heading

Nine workers in the crew were reported as surviving the incident. It is unknown if the dead and missing were all construction workers. Traffic blocks were implemented immediately to allow for rescue and recovery efforts. A reported 30,000m³ of concrete has backfilled the sinkhole and allowed traffic on the eight-lane highway to resume.

According to local newspaper and TV reports, authorities also shut off water, gas and electricity supplies near the collapse zone and report that Deputy Governor of Guangdong Province, Lin Shaochun, was among the first high officials to be at the scene.

One of the surviving workers told TV reports that he and his workmates had known something was wrong when they saw wet clay running in the tunnel. Seconds later, and after hearing a loud noise, the workers realized there was “big trouble” and started to shout warnings to run. Once on the surface, he said, it was realized that only workers near the working shaft had managed to escape. Among the missing, he said, was an assistant project manager.

Other fatal collapses have been experienced on projects that quantify the rapid expansion of metro systems in China and while China has made considerable progress in improving industrial safety, fatalities in the country's construction and tunnelling industries is still high and higher than global standards. Official figures are reported as recording 38,000 fatalities in workplace accidents in 2017 with an explosion at a high-speed rail tunnel construction site in the southwest province of Guizhou in May 2017 claiming the lives of 12 workers.

CCTV reporting of the accident (CCTV / AP)

How fatalities and accidents are recorded, and how these compare with fatalities and accidents per kilometre of the astonishing lengths of tunnelling excavated in China per year, compared to other countries of the world, and across all tunnel excavation purposes, is a set of research yet to be made available. It is known that some cities build 50km or more of new metro lines within one year. By 2020, there will be some 6,000km of metro in operation in China. In the Pearl Delta alone, and for metro and intercity rail systems alone, TBM tunnelling and excavation for stations, shafts, cross-passages and crossover structures is a continuous rolling programme with many projects and contracts and tens of TBMs operating and progressing simultaneously.

TunnelTalk visited one of the first cut-and-cover metro line construction projects in Guangzhou city in 1990. Since then hundreds of kilometres of metro and rail tunnel systems have been excavated in the region.

The earliest TBMs to operate on these systems were supplied and imported from the international TBM manufacturers of the time, including Robbins and Herreneknecht. In more recent times, Chinese manufacturers have established their manufacturing capabilities and knowhow and are increasingly suppling to national projects and have the intent of supplying international projects.

TBM tunnelling projects in the Pearl River Delta Region, including Hong Kong and Shenzen, and as published by Herrenknecht in 2011

In 2007, Robbins reported that the China Communication Construction Corporation, 2^nd Navigation Engineering Bureau contractor (CCCC), selected two 6.3m diameter Robbins EPBMs for the twin 2.6km long rail tunnels on its Lot 12 contract for the 32km Guang-Fo line running between Guangzhou and Foshan. A further 14 TBMs from different manufacturers worked on the other 11 lots of the project.

The geology on the alignment, which included passage under the Huadi River, was reported by Robbins as a complex layered profile ranging from highly weathered to slightly weathered granite, coarse sand, and silt. Some 70% of the tunnel horizon was through a mixed face, with soft soils above spring line and the bottom half in rock of at least 50 MPa UCS. The remaining 30% consisted of flowing sand with high water content. EPB operating pressures were up to 4 bar.

The two Robbins TBMs launched in December 2008 and January 2009 respectively and brokethrough after seven months of boring. A best month advance of 377m was claimed as higher than any of the 16 TBMs that worked on the project.

In 2011, Herrenknecht reported that it had delivered 48 TBMs to the Guangzhou Metro project (Fig 1) with a further 33 on the Shenzhen Metro. At the time these projects were part of the on-going expansion of metro and rail connections between the major conurbations of the Pearl River Delta which include the islands of Hong Kong and Macau.