The Yangzhou Slender West Lake tunnel passes through the hard plastic clay stratum by slurry shield machine in full section.During the construction process

surface subsidence accidents occurred many times after halting the slurry shield machine.The disintegration and softening of hard plastic clay in soaking condition are investigated by experimental study.The analysis of reason of surface collapsing by combining with field data

and the discussion about the procedure of surface collapsing propose corresponding preventive measures on collapsing in slurry shield construction in the hard plastic clay stratum.This study concludes:disintegration and softening of hard plastic clay occur when the slurry shield shutdown in slurry soaking condition

which generates the local cavity above the excavation face.With the time prolonging

the local cavity gradually develops to the surface

and ultimately forms the ground collapse.It is suggested to avoid halting machine when slurry shield go through full section hard plastic clay stratum.When halting machine is inevitable

reducing the halting time

avoiding the building region

meanwhile strengthening the monitoring and grouting reinforcement in the stratum above the excavation face are suggested to prevent collapsing.