13.3.1 Case analysis
Through field investigation and preliminary analysis，the reasons for the excavation collapse were found to be the misuse of the soil parameters，over excavation，incorrect installation of steel struts，invalid monitoring data，and inadequate ground improvement.
The soil condition of Hangzhou is unnormal.According to field investigation，the accident was in silty clay.As a main road，Fengqing Road beside the foundation pit has a large traffic and a large load，which has a significant influence on the load⁃bearing wall on the west side of the foundation pit，causing the entire foundation pit to collapse；furthermore，a rare continuous rainfall in Hangzhou in October increased the instability of sand.
The whole design did not consider the influence of the surrounding environment.The demanded demolition and transfer of nearby house was not carried out.The construction method should be closely integrated with the stratum and the surrounding environment；however，no good construction method has been developed in this project.Since a cheaper one has been chosen，which caused a serious damage to the environment，and many problems，it ends up with great losses.
In order to save money and time，the construction was in a rush，the construction period and cost are unreasonable—these are the main reasons that affected the safety and quality.During the project，there is a collapsed sewage pipe leaking for a long time，which causes the formation of the slipping surface；consequently it may cause the collapse with the help of continuous rain.
There are some other causes for the collapse：
1）Direct causes
（1）Over⁃excavation
The design document stipulated that when the foundation pit was excavated to 0.5 m below the support design elevation，the excavation must be stopped，and the support should be set in time，and no over⁃excavation was allowed.
However，during the excavation，when the last strut was supposed to install，some sections have been dug to the bottom of the foundation pit，and the strut installation and the construction of concrete floor are not in time.
（2）Weak links in the support system
The supporting system was diaphragm wall with steel pipe strut；however，the design report did not include a detailed description about the connection of the supporting steel pipe strut and the diaphragm wall，or an example of the joint of strut.There were no corresponding technical requirements put forward regarding to the joint，or welding requirements for steel strut and diaphragm wall embedded parts.No welding was performed.
The excessive lateral displacement of sectional wall together with large axial forces and severely eccentric of the struts resulted in instability of the support system.
（3）Monitoring failed
Before November 15th，the maximum settlement of the ground had reached 316 mm，and the maximum displacement of the inclinator was at the depth of 18 m is 43.7 mm.On November 13th，the maximum lateral displacement of the inclinator had reached 65 mm.All of them had already exceeded the alarm value，but with no alarm.
2）Indirect causes
①The original plan of the designer was reinforcing the foundation pit by strip⁃shaped mixing pile group at the bottom；however，the contractor suggested consolidating the base by dewatering instead of using reinforcement measures.
The design report proposed：
a.The groundwater in the foundation pit be reduced to 1 m below the lowest point of the structural internal members and 3 m below the bottom of the foundation pit；
b.The dewatering test should be carried out before excavation，and on⁃site dewatering should be done 4 weeks in advance；
c.When the ground subsidence exceeds the alarm value，the dewatering should be stopped and reported in time.
But none of these were fully implemented in the actual construction.
②The supervisor did not stop the serious problems that was against the design codes and specifications.
③The results of physical and mechanical properties test were reliable.However，the design of the supporting system had not taken into consideration the comprehensive design and reasonable recommendation according to the characteristics of the local soft soil，which caused the selected parameters too large and reduced the factor of safety of the foundation pit system.