Study On Disturbance Control Of Sandy Cobble Strata Induced By Shield Tunnelling In Lanzhou Subway

As one of the major city in northwest China,Lanzhou is now experiencing an unparalleled development in infrastructure constructions,especially in the subway engineering.Due to the tectonic activity,Lanzhou is located on the alluvial-proluvial fan produced by the Yellow River.The metro line 1 in Lanzhou was recently conducted and went through the heavy sandy-cobble sediments for a long distance(about 11.8km).In addition,metro line 1 passes underneath the Yellow River twice.The coarse sandy cobbles encountered in this project are different from those in Beijing and Chengdu,indicating that there is no engineering precedents/experience that this project could follow.In light of the tough challenges encountered in the excavation of Lanzhou metro line 1,the current author carefully studied the mechanical characteristics during tunneling.Based on the shield engineering practice of Lanzhou Metro Line 1,the engineering characteristics of shield tunneling on coarse-size sandy cobble stratum are studied.Firstly,the physical and mechanical properties of coarse-size sandy cobble stratum are studied.On this basis,the disturbance state and reasonable control of driving parameters of earth pressure balance shield and slurry balance shield tunneling to coarse-grain sandy cobbly stratum and the rational control of driving parameters of slurry balance shield tunneling through the Yellow River are studied in detail.On the one hand,it can guide the construction of the first phase of Line 1.On the other hand,In order to provide a certain reference and practical basis for the subsequent construction of Lanzhou metro or similar stratum shield engineering.The main research contents and conclusions are as follows:(1)In-situ tests and lab-tests were conducted to investigate the physical components and physical and mechanical properties of coarse-size sandy cobble stratum in Lanzhou.The gradation composition,loose coefficient,stress-strain relationship curve,strength parameters and nonlinear elastic model parameters of coarse-size sandy cobble stratum were obtained(2)The disturbance state of coarse-size sandy cobble stratum by shield tunneling is studied by the deep-hole detection method in the field.The results show that the disturbance of slurry balanced shield tunneling to coarse-size sandy cobble stratum is obviously different from that of earth pressure balanced shield tunneling.The disturbance of slurry balanced shield tunneling to coarse-size sandy cobblestone stratum is small,and the stratum deformation shows an overall deformation trend.There is a "strong disturbance area" in the stratum at the top of earth balance shield tunneling,but the surface settlement is very small.(3)From the respective of grain scale,the surface settlement and the kinematic activity of the coarse sandy cobbles were studied through three-dimensional discrete element analysis.Several numerical calculations simulated by EDEM software were conducted,results from which were further compared with the in-situ measurements.The results show that the range of "strong disturbance zone" in front and right above the cutter head caused by the face-plate cutter head shield is about 0.5D(D is shield diameter),and the range of "strong disturbance zone" in front and right above the cutter head caused by spoke cutter head shield is about 0.3d.When the overburden of the tunnel is more than1.5D,the strata above the "strong disturbance area" of shield tunneling in sandy cobble stratum have obvious arching effect.(4)The surface settlement,the ground deformation,and the appropriate parameters control were studied through combinations of the in-situ measurement and the simulated results.Particular attention was then paid to the control of the earth pressure,the development of the ground deformation,the torque and the thrust of the cutter-head.A systematic rule was concluded according to the investigation.(5)Three-dimensional finite element PLAXIS3 D was used to simulate the tunneling process of slurry shield in sandy cobble stratum.The stratum settlement caused by shield tunneling was analyzed,and the results were compared with commonly used analytical formula,empirical formula and field monitoring data.The applicability of various prediction methods for surface subsidence in sandy cobble stratum is analyzed.The results show that good calibration of constitutive parameters and selection of appropriate simulation method can be used to better predict the subsidence of the formation by numerical calculation.In practice,numerical simulation method and empirical formula should be combined and corrected by field measured data(6)Considering the tunneling disturbance,the author analyzed the adaptability of the slurry shield passing underneath the Yellow River.The risk control as well as the tunneling control were also studied.Results provide a series of thumb rules for controlling tunneling disturbance in similar geological conditions.With the development of subway construction in more and more cities in China,sand and pebble stratum will still be a difficult problem and challenge for shield tunneling in the future.The research results obtained in this paper have important practical significance for Lanzhou subway construction and shield tunneling practice of similar coarse sandy cobble stratum.