Research And Application Of Key Technology Of Shield Tunnel Construction Based On Rich-water Sandy Cobble Boulder Strata In Chengdu

Sandy cobble stratum is one of the difficult strata encountered in the construction of subway tunnels,and it is a typical unstable stratum.In China,sandy cobble strata exist in the central and northern cities such as Nanchang,Changsha and Beijing.For example,the construction of the Beijing subway has relatively accumulated a relatively mature shield construction technology for the anhydrous sandy cobble stratum,and has been widely applied.With the shift of the focus of national economic development,western cities such as Chengdu have also joined the ranks of subway construction.In the areas of Pixian and Wenjiang in the Chengdu Plain,the widely distributed sandy cobble strata have a dual structure.The pebble soil contains large diameter boulders,randomly distributed sand lens bodies in the stratum and migration.Influencing factors such as groundwater make tunnel construction more difficult and more challenging.However,in the geological category of sandy cobble,there is no similar experience for the shield operation under the sandy cobble boulder stratum,and the research work on related technologies is relatively rare.During the tunnel construction process,problems such as shield component wear,cutterhead stop,screw conveyor gushing,shield tail seal failure and slag super-discharge due to geological problems.Based on the characteristics of the Chengdu rich-water sandy cobble boulder stratum,this paper takes the shield tunnel section project in the construction of Chengdu Metro Line 6 as the carrier,and carries out the following research and application of key construction techniques.The main research contents and conclusions include the following:(1)Based on the special conditions of rich pebble,large particle size of boulders,high water pressure and strong permeability in the study area.The distribution law of soil in shield tunnel section,soil particle gradation and shield construction adaptability are studied.With the characteristics,the necessity of solving the construction problems such as shield wear,screw conveyor gushing,shield cutterhead stop,shield tail seal failure and slag super-discharge are analyzed.The selection design of shield machine and the modification method of shield machine for this type of stratum are proposed.The research results show that in the construction,it is necessary to consider the way of digging,passing and discharging.The high reliability waterproof seal should be designed especially for the shield tail and hinge.(2)By analyzing the main theoretical parameters and integrating the key parameters such as earth pressure parameters,thrust parameters,cutterhead torque parameters and grouting parameters collected during the shield construction process,a reasonable control range for the excavation parameters suitable for such strata is proposed.Based on the research results of shield attitude control,boulders treatment,tube assembly,changing cutters,and improvement of slag,the construction technical suggestions for this type of stratum are proposed.The results show that after the injection of 3%foam modifier,the internal friction angle of the soil drops from 42.56° to 37.51°,which improves the total thrust level and reduces the cutter torque.The permeability coefficient increased from 2.315×10-2cm/s before the improvement to 4.672×10-5cm/s,which certain limiting effect on settlement.(3)In the process of shield tunneling,it is inevitable to wear a variety of important urban structures.Taking the old pressurized sewage pipe widely distributed in the study area as an example,a construction method using ground discharge to ensure safe passage of the shield is proposed,and the construction parameters such as allowable soil discharge and grouting ratio during the crossing are given.(4)Combined with the causes and occurrence process of delayed settlement,the numerical simulation of discrete element(FPC2D)based on particle flow theory is carried out to study the influence of factors such as the buried depth of shield tunnel and the formation of damage.Combined with the monitoring data to verify its correlation and rationality,the development law of delayed settlement in this type of stratum shield tunneling construction is further obtained.The research results show that the formation of caves in Chengdu sandy cobble boulder stratum is a gradual process from formation to spread to the ground,and the stratum failure caused by shield construction finally forms a triangular loose area.The buried depth of the tunnel has a direct impact on the generation of collapsed arches in the formation.As the buried depth of the tunnel changes,the formation stress will also change significantly.Lagging settlement mainly occurs in locations where external loads(ground loads,water effects,etc.)are strong,but the locations of occurrence are mainly concentrated in the 2.0D range on both sides of the tunnel.The settlement value at the center of the monitoring point in the study area reached the maximum.The discrete element numerical simulation of the surface settlement and the on-site measured difference was-1.3mm.Part of the surface settlement value showed a discretization to a certain extent.On the whole,the actual measured values on the site are slightly larger than the discrete element simulation values.The comprehensive judgment may be due to the effects of artificial control factors such as slag improvement effect,excavation volume,synchronous grouting and secondary grouting delay during shield tunneling.