13.4.2 Lesson learned

The following situation should be strictly avoid：

①Over⁃excavation：The original design depth was 17 meters，but the actual pit had been excavated to 20.3 meters，causing the retaining wall to be hanging wall；

②Overtime：The service period of the foundation pit support structure was one year，and the actual excavation had been nearly three years；

③Overloading：overloading at the top of pit such as dump truck，crane and backhoe excavator；

④Ignoring geological response：The rock layer was shallow and inclined into the pit.The designers used the software to design and checked the foundation pit，and ignored the fact that the rock layer was inclined 25°.It was found that the displacement of the wall on the south side was too large，so this part of the wall was reinforced with anchor cables but the range was too small which only covered 20⁃30 m along the west side of the south.Constructors believed that the insufficient reinforcement wes caused by unreasonable design.