13.3.2 Lesson learned

After field investigation and preliminary analysis，five reasons of excavation collapse were concluded：（1）using the average shear strength value derived from consolidated quick sheartests and not the standard value from consolidated undrained tests as the strength parameters；（2）canceling the fourth level struts；（3）poor connection of pipe struts with coupling beams and the diaphragm wall；（4）insufficient monitoring instruments and ignoring the unusual performance of the excavation；and（5）using artesian wells for consolidation instead of jet grouting.

①This collapse accident once again proves that excavation of soils should be based on the principles of layered and zoned construction，symmetrical and balanced excavation，early cast of basal slabs，and over⁃excavation is strictly prohibited，especially in soft soil areas.This project was not implemented in accordance with these principles，which is one of the direct causes of the accident.

②When using retaining wall with inner support structure in thick soft soil area，we must consider stability problems in the deep area，including wall settlement，uplift and“toe kick”damage；it should be calculated and verified by a variety of methods.If the requirements cannot be met，the insert depth of walls should be deepened or the passive area should be reinforced.The design of this project was not firm enough，and passive area reinforcement was not carried out.

③If the inner support is made of steel truss or steel pipe strut，a requirement of joint structure and welding should be proposed properly to avoid eccentricity and instability；and the first inner support should be strengthened by reinforced concrete.

④The selection of the shear strength parameters should be matched with the calculation purpose and the safety factor.For the calculation of earth pressure，the related calculation of“toe⁃kick”and“overturning”should choose the results of consolidation quick shear test or triaxial CU test；the calculation of basal heave，sliding and overall stability should choose the results of quick direct shear teat，triaxial unconsolidated undrained test or cross plate shear strength teat when soft clay plas an important role.The design should be based on comprehensive consideration，not just consider the average but also the maximum and minimum data.

⑤The monitoring should be carried out strictly，independently and without interference.When the alarm value is reached，it must be timely reported and must not be concealed.The supervision department should be familiar with geotechnical engineering and should strictly stop major violations.