Model Test And Numerical Analysis On Construction Of Hefei Shield Tunneling

Shield tunneling inevitably induces strata settlement and changes of earth pressure of which the laws are different in soil of different kinds.Therefore,a thorough study on the relations between physical and mechanical property of soil and the laws of surface settlement and change of earth pressure has enormous significance in ensuring shield tunneling safely and smoothly.With shield construction of Hefei subway transit as background,model tests using typical sand and clay of Hefei region as test material are firstly conducted to investigate variation laws of settlement and earth pressure change which are caused by shield excavation.The results show that the sand ground is more sensitive than the clay ground to the disturbance of excavation.Change of settlement and earth pressure in sand is much faster than that in the clay after excavation,as well as the rate of stability recovery.Moreover,there is larger variation between settlement values collected by monitoring points which are adjacent to each other in sand and the curve of final settlement trough of sand model has more fluctuation.Whereas the settlement and earth pressure of clay model change more smoothly and the curve of final settlement trough of clay model is relatively flat.For both kinds of soil model,results also show that surface settlement of excavation stage counts for 45%~50% of total settlement with a difference less than 5%.The law of earth pressure decrease rate is basically identical with that of settlement rate during four experimental stages and can be used to reflect the surface settlement rate in practice.The maximum value of settlement is observed at the formerly excavated side while the maximum value of settlement rate at the latterly excavated side.The final double-peak settlement trough can be seen as the superposition of the two troughs generated by excavating both sides separately,yet the final settlement trough is not perfectly symmetric but the formerly excavated side is slightly deeper than the latterly excavated side.Secondly,Finite Difference Method and Particle Flow Code,as the difference between the physical and mechanical property of sand and clay is taken into consideration,are applied to verify the correctness of the model tests and probe further into mechanism of ground surface settlement and earth pressure change.The settlement nephogram,distribution of earth pressure and maximum value of surface settlement obtained through Finite Difference Method are all in excellent accordance with those of model tests.Meanwhile,whole distribution and change process of earth pressure around tunnels are also gained with Finite Difference Method.Complete processes of settlement propagation and earth pressure adjustment after excavation in sand,which reveal the mechanism of ground surface settlement and earth pressure change,have been simulated through Particle Flow Code.Lastly,the correctness of models tests and numerical simulation is proved once again through contrast between actual monitoring results of typical clay and sand sections and results of model tests and numerical simulation.Results have been achieved in this paper can provide reference for following shield tunneling construction in Hefei region.