Study On Methods Of Prediction For Disc Cutter Load And Wear Of Shield Tunneling In Composite Strata

The variability and complexity of stratum properties when shield tunneling in composite strata lead to great challenges to the selection and design of the shield machine.As the key role of composite excavation,it is essential but difficult to calculate the disc cutter load accurately and configurate the cutting tools reasonably in shield machine selection.At the same time,the rock of high strength high abrasiveness and high variability are widely distributed in the excavation face when tunneling in composite stratum,which causes a complex and harsh working environment as well as some adverse consequences for disc cutters,such as uneven wear or fracture of the cutter rings,damage of cutter shaft,sharp rotation load increasing,laborious tunneling and even jamming of the shield machine.To figure out the interaction between the excavation face and disc cutter under different geological conditions,analyze the influence of stratum properties and tunneling parameters on disc cutter load and realize the mutual adaptation of shield cutter head,stratum property and tunneling parameters,as well as to reduce the risk and cost caused by chamber opening when check and change cuttes,and improve the construction efficiency of shield tunneling,the load calculation method and wear prediction model of shield disc cutter are studied.The main contents and contributions of this paper are as follows:(1)Taking several shield tunnelling projects in multiple composite strata as examples,the statistics of shear strength index of strata are carried out.By introducing the probability density function of multimodal distribution for fitting,the calculation method of strength limit value and stratum difference coefficient between different types of strata in composite strata are obtained,which provides a basis for subsequent calculation of disc cutter loads in composite strata.According to the statistical results,the stratum strength distribution in each tunneling projects is obtained,and the value bounds of spacing and advancement of the disc cutter with different difference coefficients are given based on the theory of rock breaking angle.The main reason for the eccentric wear of disc cutter is that the tangential load is not enough to provide its starting torque in soil,soft rock and other strata.Therefore,the layout of disc cutter on the cutterhead should meet the common requirements for bearing capacity and starting torque of disc cutter.On this basis,the cutterhead layout design procedure of the shield machine used in the composite stratum is established.(2)Due to the different cutting mechanisms,the existing CSM model is not suitable for the load calculation of disc cutter in the soil stratum.Therefore,a calculation model of disc cutter loads in the soil stratum is established,in which the disc cutter loads include soil compression force,lateral dynamic shear force,soil acceleration inertia force and dynamic friction resistance.Further,a calculation example of disc cutter loads is given to validate the proposed.Considering difficulties in selecting a suitable disc cutter load calculation model in soil rock transition layer,the load calculation method of disc cutter tunnelling in soil rock transition layer is proposed based on the strength limits of the soil layer,rock layer and transition layer,and the piecewise solution of disc cutter loads in composite strata is realized.A CSM model considering the strain rate effect of rock strength is established,which provides a theoretical basis for the study of the influence of cutterhead rotation speed on disc cutter loads and cutter head torque.(3)In the annular variable material tunneling test,the loads increase step by step and always fluctuates when the disc cutter cuts silty clay,M2.5 cement mortar,C15 concrete and C30 concrete in turn.The loads of the disc cutter increase with the increase of the cutter spacing to a certain extent,but the increases are small.The loads of the disc cutter increase with the penetration of the cutter ring when cutting different materials,and the phoneme is more obvious when cutting brittle materials,which is consistent with the conclusion of the CSM model.With the increase of cutting depth,the fluctuation range of disc cutter loads also increases to a certain extent.The disc cutter loads increase with the cutterhead rotation speed,which is due to that the increase in cutting speed increases the dynamic strength of concrete and other materials.Based on the test data,the strain rate effect parameters in the CSM model considering the dynamic strength of rock are calibrated.(4)According to the results of the circumferentially and radially variable material tunneling test,when an equivalent free surface occurs caused by that the surrounding materials can not provide enough constraints for the cutting area,the disc cutter loads will decrease sharply.At this time,the variation of confining pressure will cause the uncertainty of material strength,so the CSM model can not be used.Therefore,the piecewise solution of disc cutter loads in composite strata proposed in this paper is recommended for calculation.The results show that the variation range of disc cutter loads at the interface of different materials is much larger than that of the average load in each area and increases with the cutterhead rotation speed.As a result,the disc cutter loads will fluctuate sharply and the fatigue of the cutter ring and shaft may occur.At the same time,higher cutterhead rotation speed may shorten the service life of the disc cutter.(5)According to the results of the shield tunneling test,the rate effect parameters and damage parameters in HJC constitutive model and FHWA constitutive model are compared and modified,and then the numerical calculation of rock breaking and soil cutting by disc cutter are carried out,respectively.The simulation results show that:when the cutting speed increases,the average value of disc cutter load for rock breaking increases in the form of the logarithmic function,but its fluctuation amplitude decreases.The results show that not only the average value of disc cutter load for rock breaking increases with the cutting depth,but also its fluctuation amplitude increases sharply.The average value of of disc cutter load for soil cutting increases in the form of the second power function with the cutting depth,which is different from the linear increasing trend of rock breaking.The load level of soil cutting increases linearly with the overburden earth pressure,and the law can be used to calculate the size of cutter load under different buried depths,which makes up for the defect that indoor disc cutter tunneling test can not consider the confining pressure state of excavation face.(6)Based on the accurate disc cutter load model proposed,the relative hardness index CAI is introduced to the Archard Wear Law,and the prediction model of the uniform wear of the cutter is established based on the plastic removal mechanism and the analysis of the relative motion between the cutter ring and the excavation face,which can be used to predict the relationship between the uniform wear of the disc cutter and the driving distance in practical engineering.The prediction results show that the predicted value of cumulative wear of disc cutter reflects the overall level and change trend of disc cutter wear in practical engineering.Considering the strong variability of stratum properties and shield tunneling parameters during shield tunneling,the failure probability prediction model of cutter wear in composite strata is established to avoid the misjudgment of wear caused by the variations of geological parameters and construction parameters.