13.5.1 Cause analysis

The excavation sequence of the shaft and the bypass channel is wrong.The failure of the refrigeration equipment caused the rising of temperature and the underground pressurized water caused sandblasting.These three disadvantages eventually led to the accident.For projects with high construction risks like this，it would be an important cause of the accident if there is lack of emergency response plan，or the on⁃site management and supervision are out of control.

1）Excavation sequence error

There was a large shaft above the tunnel，and other two small shafts 8⁃9 m below the large shaft were excavated to connect the already formed tunnel.According to the construction practice，the bypass channel should be dug first and then the shaft.However，the construction sequence had been changed，which was very likely to cause collapse.When the accident occurred，one small shaft had been dug and the other shaft was excavated about 2 m.

2）The incorrect application of freezing method

Before the accident，the contractor，the project department of Shanghai Branch of China Coal Mine Engineering Co.，Ltd.，arbitrarily adjusted the original construction design while the expert group determined that the adjustment did not strictly follow the relevant provisions of the freezing construction process，resulting in weak layers of the frozen soil in the adjacent channel construction.The adjusted scheme reduced the average temperature requirement for frozen soil，from－10℃in the original scheme to－8℃；the number of vertical freezing tubes at the bypass channel decreased from 24 in the original scheme to 22，while for the original 7 vertical freezing pipes of 25 meters deep，4 of them were shortened to 14.25 meters，and 3 shortened to 16 meters，resulting in weak frozen soil in the intersection below the waistline of the bypass channel and the under tunnel.A row of 6 frozen inclined holes with a hole spacing of 1 meter was adopted.Although the length of the frozen hole was increased，the number was too small and the spacing was too large，so that the freezing effect was insufficient to resist the water and soil pressure of the corresponding part.

3）On⁃site management dereliction

On the morning of June 28th，the small refrigerator of the down line of the tunnel broke down，and the cooling was stopped for 7.5 hours.At about 2 p.m.，the construction crew installed a hydrological observation hole in the down⁃line tunnel and found that there was always pressure water leaking.Although some measures such as blocking the tunneling surface with wood were adopted，the effect was not good.At 3 o'clock on the morning of the 29th，the water pressure measured at the water valve was close to the confined water pressure of the seventh external layer.The danger was first revealed，but no one at the scene reported the situation to the contractor or supervisor，resulting in a gradual increase in the risk.

It was in such a dangerous situation that at 0：00 on July 1st，although the deputy manager was aware of the serious potential dangers in the frozen soil layer of the connection passage，and the project had been suspended，he still determined to arrange the construction personnel to partially remove the wood sealing at the excavation face besides the frozen soil，and used the wind chisel to cut the hole with a diameter of 0.2 m to prepare the concrete conveying pipe.It was from this hole that the water emerged，and the water sand continuously rushed in from the lower right corner of the tunneling face and the side wall，so that the sealing became invalid and eventually caused an accident.