Study On Long-term Mechanical Performance Of High-Filled Cut-and-Cover Tunnel Based On Loess Creep

With the influence of "the Belt and Road Initiative",the construction development in the area that is full of loess in mid-west of China is greatly accelerated.However,this area is mountainous that the usable land here is limited,and a great many traffic roads lay in the bottom of the deep valleys.To reclaim usable land with traffic roads unaffected,high-filled cut-and-cover tunnel(HFCCT)is put forward.For HFCCT in this loess-covered area,when loess is used for backfilling,the loess can be remolded before backfilling,and the creep behavior of the remolded loess is significant,which possibly affects the long-term safety of HFCCT.And,though the remolded loess is compacted during backfilling,its creep behavior is still obvious.Therefore,considering the creep behavior of the backfilled loess,numerical simulation was conducted to investigate the long-term mechanical perfonnance of HFCCT,so that the long-term safety of HFCCT can be further understand.Several works were conducted in this paper,as follows.(1)The previous studies with regard to the creep behavior of remolded loess were summarized.According to these previous studies,a proper constitutive model was selected to simulate the creep behavior of remolded loess when numerical simulations were conducted.As a result,the Burgers model was selected.(2)Considering the creep behavior of the backfilled loess,the long-term evolution of internal force and deformation in HFCCT lining structure were investigated based on numerical simulation.The numerical simulation,which consists of numerical model establishing and numerical computing,was conducted by using the finite difference program,FLAC3D.The simulating results show that the HFCCT lining structure suffers more vertical compress than lateral compress all time,but the difference between vertical and lateral compress changes over time.In post-construction,both internal force and deformation in HFCCT lining structure initially fluctuate over time,which indicates the difference between vertical and lateral compress sometimes increases and sometimes decreases.Later,the fluctuations in both internal force and deformation weaken over time,and vanish at the same time.After the fluctuations have vanished,the absolute value of bending moment generally decreases;the axial force in different lining structure parts approach to each other;the absolute value of shear force generally decreases in the upper parts of lining structure,and generally increases in the bottom parts of lining structure.Meanwhile,the deformation is reduced over time,which indicates the difference between vertical and lateral compress gradually decreases.Generally,the creep behavior of the backfilled loess benefits the long-term safety of HFCCT in some extent.(3)Considering that the creep behavior of load reduction materials for HFCCT can be obvious,which is the same as the backfilled loess,more numerical simulations were conducted to investigate the long-term evolution of internal force and deformation in HFCCT lining structure with the influence of creep behaviors of both backfilled loess and load reduction materials,so that the long-term effect of HFCCT load reductions can be further understood.The results show that after load reduction materials with significant creep behavior is applied,both the internal force and deformation still experience fluctuating,fluctuation vanishing,and approaching to constant over time,when the creep behavior of backfilled loess is considered.In fluctuating,the peaks of internal force and deformation significantly decrease due to load reductions,the load reduction effect is obvious in this period.After fluctuation vanishing,the changes in internal force and deformation that are resulted from load reductions decrease over time,which indicates the weakening of load reduction effect.