13.4.1 Case analysis

The main technical reasons for this collapse are

①It can be concluded from the geological exploration data that the site was consist of rock layer with argillaceous siltstone mingled with interlayers of strongly weathered soil within the depth of excavation；during the excavation，it was found that the rock mass on the south side was inclined to the pit，and there was water seepage and mud flowing in the strongly weathered interlayer；according to the deformation monitoring data，it had shown obvious signs of the collapse on the south side of the foundation pit.However，none of these problems had drawn due attention，and no effective measures had been taken，which is the main cause of the accident.

②The initial design depth of the foundation pit was 17.0 m，but the actual excavation depth was 20.3 m.The 3.3 m deep over⁃excavation not only caused the insufficient of the anchoring force，but also made the 20 m deep retaining wall a suspended wall which cannot be adjusted，resulting in a potential danger.

③The time of intermittent construction of the foundation pit was 2 years and 9 months.It was far longer than the period of one year according to the code concerning temporary construction，which led to the softening of the soil layers because of seepage，and the reduction of anchoring force of anchor（cable）because of the corrosion of steel components.

④When the foundation pit collapsed，there was still one backhoe excavator，one crane and one 16 m2 dump truck on the south side of the foundation pit near the Seafaring hotel.When the foundation pit was in danger，these construction loads were also one of the causes of the collapse of the foundation pit.