Research On EPB Shield Cutterhead And Tools Wear Characteristics In Sandy Pebble Stratum

Rail transit is favored for its safety,rapidity,large capacity,low energy consumption,less pollution and other advantages,which is a green transportation in modern cities.Shield method is widely used in the construction of urban subway because of its advantages such as high speed,safe construction,high degree of mechanization and good adaptability to strata.In the process of subway tunnel construction in China,some cities have encountered a large area of sandy pebble stratum,such as Beijing,Chengdu,Lanzhou,Shenyang,etc.When shield tunneling in sandy pebble stratum,due to the strong abrasion of pebble,the tool wear is serious,so it is necessary to open the cabin frequently to repair the cutter.Therefore,the efficiency of shield tunneling is low,which seriously affects the construction period and leads to the increase of project cost.In this research,theoretical analysis,laboratory test and numerical simulation are combined to study the tool wear characteristics of shield cutterhead in sandy pebble stratum.The main research contents include:tool wear mechanism analysis,tool stress and wear state analysis,formation abrasiveness analysis and tool wear calculation formula optimization.The main research results are as follows:（1）Based on the tribological theory,the wear mechanism of the cutting tools in the sand and gravel formation is analyzed.The results show that the wear types of the cutting tools are mainly abrasive wear and adhesive wear.The main factors include relative hardness index and interaction force,and the application of relative hardness index in cutting sandy cobble stratum is analyzed.It is determined that quartz and mineral particles with higher hardness are the main abrasive materials in the formation when cutting sand and gravel stratum by cutting tools.（2）Based on the PFC^3D numerical calculation software,the force and wear analysis method of cutter head cutting sand gravel stratum is proposed.The method of gravel and pebble particle generation based on clump template and tool model generation method of complex cutter head based on model import method are summarized.The boundary conditions of the simulation model are determined by analyzing the environment state of the cutting system in the process of cutter head cutting.Based on consolidation test and triaxial compression test,the calibration method of sandy pebble stratum parameters is proposed.The force state of cutting tool is recorded by fish language in PFC^3D.Based on the analysis of tool wear mechanism,the method of Fl wear index to represent the actual wear state is proposed.Finally,the visualization software is used to realize the visual display of various states.（3）Based on the proposed numerical simulation method,the force and wear of cutterhead in sandy pebble stratum are analyzed.The results show that the force on the cutting surface is the largest in the cutting process,and the tool wear state is similar to the force state.The influence of different overburden pressure and cutting speed on tool force and wear is analyzed.The increase of two factors will cause the increase of tool surface force and wear,and the relationship between them can be expressed by linear function.For the scraper,the influence of cutting speed on the force is greater than that of the overburden pressure,and the influence of the overburden pressure on the wear is slightly greater than the cutting speed.For the ripper,the influence of the overburden pressure on the force is slightly greater than the cutting speed,and the influence of the overburden pressure on the wear is obviously greater than the cutting speed.In the process of cutterhead cutting sandy pebble stratum,the maximum force on the front of cutterhead is about 10 times of that on the back,about 2～3 times of that on the side.There are a few areas with large force on the front of cutterhead.These areas are mainly located in the narrow gap of cutterhead.The wear of the cutterhead is small on the inside and large on the outside.The wear on the front of the outer ring of the cutterhead is about 4 times as much as that at the center lock,and the wear on the outside of the outer ring is about half of that on the front.The wearloss of some areas in the front of cutterhead is relatively large,which is basically the same as that of the area with large force.（4）Image analysis,X-ray diffraction analysis and LCPC test were used to analyze the shape characteristics,EQC and abrasiveness of sandy pebble stratum soil particles in Beijing Daxing International Airport Express.The test results show no significant differences in the shape characteristics and EQC of particles with different diameters obtained from the same sandy pebble stratum.For the same EQC,the main factors affecting the LAC are particle size distribution and moisture content.An optimized effective size(ES_op)index was established to represent different particle size distributions.The test results showed that the ES_op index is more accurate than other indices in determining the LAC of soil samples,and the LAC had a quadratic relationship with the ES_op index.Soil samples with moisture content less than the saturated moisture content have an LAC larger than that of a dry soil sample,and soil samples with moisture content greater than the saturated moisture content have an LAC similar to that of a dry soil sample.The influence of test time on the LAC of the soil samples could be divided into two stages,where the first and second stages are represented by a quadratic function and a linear function,respectively.（5）Based on the example presented in this research,the wear coefficient of the high ripper was 102.31～113.73μm/km,that of the low ripper was 35.94～42.55μm/km,and that of the scraper was 38.60～40.34μm/km.When using the JTS model to calculate cutter wear,the wear of high and low cutters had to be separately calculated.The outermost cutter wear was significantly higher than that of the inboard cutter,making the correction coefficient necessary when calculating the wear of the outermost cutter.In this research the wear coefficient of the low ripper and scraper was approximately 40%of that of high ripper,and the correction coefficient of the outermost scraper and ripper was 1.2～1.4 and2～3,respectively.Theoretical deduction and measured data analysis proved that the distribution of strata in the shield excavation area,the value of the wear coefficient of single strata,and the cutter installation radii all had a significant influence on the composite stratum wear coefficient.The wear coefficients of cutters with different installation radii in composite strata were quite different.In the calculation process,the wear coefficients of each installation radius had to be separately calculated to ensure the accuracy of the calculation results.Base on the trajectory weighting method,the JTS model was optimized.The tool wear coefficient of different installation radius in composite formation is calculated separately,and the optimization model is verified according to the measured data.At the same time,several tool wear calculation model are compared and analyzed.The results show that the trajectory weighting method proposed in this research is more suitable for tools wearloss calculation in composite stratum.