Hangzhou metro pit collapse

Poor construction and safety management have been blamed by Chinese national authorities for last month’s fatal collapse at the open cut tunnel excavation for a new metro station in Hangzhou, Zheijiang province.

Official figures put the death toll at 21 in the collapse caused by the failure of the reinforced concrete walls and steel pipe bracing. The collapse happened mid afternoon on 15 november, at the Xianghu station area on the southern end of the Line 1 project. Those killed were construction workers buried under mud that half filled much of the 16m deep excavation.

Traffic was running on the road skirting the west side of the pit when the ground dropped behind the failed wall, which split, kicked-in and tilted. The pit bracing failed, generally falling out of place, intact, and both walls moved and cracked, and saturated soil flowed into the excavation. However, the edge of one section of failed wall looked to have a long vertical, clean, brittle fracture through its depth and there was little sign of steel reinforcement.

Water from the nearby river also began to seep into the sodden ground and an elongated, flooded crater on the west side, resulting in flooding of some metres deep which worsened the slurry conditions and emergency pumping requirements. A string of at least 11 vehicles were stuck in the crater, but higher up a car fell into a large crack that opened up in the road. Following the collapse, nearby buildings had to be demolished on unstable or suspect ground. There were no casualties among the public and residents.

There were up to four hundred rescuers on site. Emergency services and workers had found the first bodies – four by the end of the next day, in mud – and they still tried, digging with shovels, to find if there were survivors among the 17 then missing, but later recovered as dead. Fifteen other workers who escaped, a number by hoist, were treated in hospital.

In total, the section of works affected by the collapse was 100m long by 50m wide, reported the State Administration of Work Safety (SAWS). The depth of the crater was given as 6m, though at the deepest point the road could be seen to have dropped more than the height of two lorries.

Following a preliminary investigation, SAWS outlined five main problems that it concluded had led to the fatal collapse. For the project parties, the failures range from poor construction management to insufficient technical and safety training, and non-standard employment of workers. The state body also criticised the local government for inadequate supervision of the construction works.

Following the accident, the Chinese authorities ordered immediate safety checks on all ongoing construction projects in the province and nationally, especially metros.

In Hangzhou, the Line 1 metro project is being developed by the city’s mass transit rail authority. SAWS said the section involving works at Xianghu had the following parties involved: design was done by Beijing Urban Construction Design Research Institute; supervision was provided by the project consulting unit of Tongji University, in Shanghai; and, construction was being performed by China Railway Group’s fourth engineering bureau, known as China Tiesiju Civil Engineering Group. China Railway Group was formerly China Railway Engineering Corp (CREC).

Local, unconfirmed reports say workers had allegedly noted cracking and subsidence movement in the adjacent road weeks before the collapse, and that there had been patch up and strengthening work. Reports also said a number of workers on the job were from rural areas and their training had been limited.

The works at Xianghu are part of Phase Two of the Line 1 metro development, most of which commenced in mid-2007, said the provincial government. Xianghu station is located on the south bank of the Qiantang river in Xiaoshan district, in the south east of Hangzhou. Out of the eight stations on Phase Two, the construction work on Xianghu started last due to design changes, noted the provincial government.

The other stations in the second phase are Binhelu, Xixing, Binkanglu, Zhanongkou, Genshanmen, Xiashaxi and Qichecheng.

The works for Phase One began in March 2007 and has involved a staggered construction schedule for six stations – Jiubao East, Binjiang, Wenze Road and three stations to be located underground in the Qianjiang New City area. Completely unrelated to the incident last month, a section of tunnels near Binjiang station is being bored by Shanghai Tunnel Engineering (T&TI, September 2007, p6).

With the first sections of Line 1 due to open before 2012, the entire length of the metro route will be 48km with almost 42km of the route to be built underground. The completed line is to have 30 stations. Early concept and feasibility studies for Line 1 were undertaken by MTR Corp, of Hong Kong, and Parsons Brinckerhoff. The preliminary design was approved about two years ago.