Analysis On Key Factors Of Small Diameter Shield In Gravel Stratum

In this paper,the shield construction of gravel stratum in Hexi Branch of Beijing south to North Water Diversion Project is studied as a case.The main equipment of shield construction in anhydrous gravel stratum in the study area is earth pressure balance shield machine.At present,the diameter of shield machine in domestic metro and other projects is generally about 6m.According to the water delivery scale of the project,the outer diameter of the shield required for Hexi branch pipeline is about 4m,and the diameter of the selected shield machine is smaller than that of the conventional shield machine.At the same time,the tunnel crossing section of the project is basically full section anhydrous gravel stratum,which is a typical unstable stratum.The gravel stratum has large particle size,high content of large particle size,poor soil flow plasticity,large friction resistance,and the control of shield construction is difficult.Firstly,in view of the geological problems faced in the construction of small-diameter shield machine in gravel stratum of Hexi Branch project,from the perspective of engineering geological survey,design and construction,this paper makes statistics,analysis and summary on the distribution characteristics,physical and mechanical properties and shape characteristics of large-size particles in gravel stratum,and puts forward the fractal principle theory It is used to describe the content of gravel particles.It is concluded that the particle size,shape and distribution characteristics of the Quaternary Holocene alluvial proluvial pebble layer in the study area have less influence on shield construction than that of the Quaternary upper Pleistocene proluvial pebble layer.Therefore,during the construction period,through design changes,the shield longitudinal crossing stratum is adjusted from the Quaternary upper Pleistocene proluvial pebble layer to the Quaternary Holocene proluvial pebble layer.Secondly,through the derivation of the components and mathematical formulas of the thrust and torque of the EPB shield machine,the starting thrust,cutter head design torque and equipment torque of the EPB shield machine with different diameters are calculated under the ideal state of static balance,and the conclusion is that the starting thrust,cutter head design torque and equipment torque of the small-diameter shield machine are less than those of the conventional diameter shield machine.Finally,through the comparative study of the sensitivity factors of two layers of gravel layers in different sedimentary ages to shield construction,it is concluded that the content of boulder with large particle size,roundness,aspect ratio and fractal structure theory are higher in the Quaternary upper Pleistocene proluvial gravel layer than in the Quaternary Holocene proluvial gravel layer.Comparing the theoretical calculation results of two kinds of shield machines with different diameters,it is concluded that under the same conditions,the small-diameter shield machine has higher wear on the cutting tool and segment compared with the conventional diameter shield machine and worse crushing ability on the gravel particles.