Research On Excavation Deformation Characteristics And Its Control Measures Of Shallow Buried Tunnel In Water-rich Loess

In recent years,with the increase of national infrastructure construction and the continuous investment in the development of the western region,more and more high-grade highways are planned and built across the loess area in order to meet the growing traffic demand.The loess can maintain good wall standing under the condition of low water content,but its strength decreases sharply after being soaked in water.The surrounding rock is difficult to stabilize for a long time.This paper aims at the characteristics of rich water and shallow buried section of Pangwan tunnel of Xihui Expressway,such as fast disintegration after excavation,weak self-supporting capacity,especially insufficient bearing capacity of arch foot and large deformation rate of initial support.(1)Based on the similarity theory,a model of shallow buried tunnel with soft plastic soil is made by using similar soil that has similar engineering properties in Pangwan tunnel.The evolution law of pressure and deformation of surrounding rock of hard plastic soil and soft plastic soil during excavation is explored,and the characteristics of tunnel excavation clearance displacement under two conditions are compared.The evolution law of pressure and deformation of surrounding rock of hard plastic soil and soft plastic soil during excavation is explored,and the characteristics of tunnel excavation clearance displacement under two conditions are compared.(2)Based on the field test of Pangwan tunnel,the typical section is selected for analysis.During the excavation of the middle bench,the settlement of the arch crown reaches 34 mm /day,and the bearing capacity of the arch foot of the upper bench is insufficient.The design of the conventional arch foot support cannot control the stability of the arch foot.After taking a series of measures,the tunnel clearance displacement is controlled,and the construction is carried out smoothly.(3)FLAC3D software is used to establish a three-dimensional model to further analyze the displacement field and stress field of soft plastic loess tunnel,and carry out three-dimensional fluid solid coupling numerical calculation to analyze the deformation characteristics of shallow loess stratum under the action of groundwater,and compare the results only considering the mechanical calculation.(4)Based on the numerical analysis of different supporting conditions,the paper compares the settlement and deformation control effects of three steps of Φ 89 anchor bolt with different down slope angle,different arch foot size expansion and middle pipe shed + small pipe support for shallow buried water rich tunnel.The results show that: 1)the down slope angle of different steps of anchor pipe is not the same for the control of tunnel net space displacement,and the upper step anchor pipe is in the control of tunnel net space The displacement effect is more significant than that of the middle and lower steps,and the length of the anchor pipe can be increased properly in the upper step;2)the large arch foot support has an obvious effect on the control of the arch settlement of the shallow buried water rich loess tunnel,the settlement is reduced by about 10.9%,while the control effect on the horizontal convergence of the side wall is limited,in which the setting of a large arch foot of 60 cm ～ 80 cm in the upper step is more favorable for the settlement control of the tunnel;3)The middle pipe shed+the ground settlement and vault settlement of small conduit support are reduced by 10% and 22.8% respectively.