A 3D Peridynamics Study For The Free-Surface-Assisted Rock Fragmentation Caused By TBM Disc Cutters

In recent years,there has been a great deal of interest in free-surface-assisted rock fragmentation caused by TBM disc cutters,which can effectively overcome the increased difficulty in abrasive or high strength hard rock stratum excavation by TBM,with the promising applications.Due to the short period of the appearance of free-surface-assisted rock fragmentation caused by TBM disc cutters,the related research is still in initial stages.The finite element method and discrete element method used to simulate free-surfaceassisted rock fragmentation caused by TBM disc cutters have limitations in studying rock fractures;and the research on 3D rock breaking by a single disc cutter in vertical free-surface conditions and collaborative rock breaking between adjacent cutters is still inadequate;the study of rock breaking by disc cutter in inclined free-surface conditions is still not systematic.In this paper,we introduce the Peridynamics to the simulation of rock fragmentation by TBM disc cutter for the first time,and carry out a series of studies on three-dimensional free-surface-assisted rock fragmentation caused by TBM disc cutters.This numerical method has some advantages in the fractures simulation and can compensate for the limitations of the finite element method in complex rock fractures simulation and the inconvenience of the discrete element method with its many calibration parameters.The main research and conclusions in this paper are as follows:(1)In this paper,we have simulated wedge-shaped cutter indentation test and punch indentation test using Peridynamics to demonstrate the feasibility of this method to study rock fragmentation by cutters.We propose a non-linear short-range force contact model to calculate the variation of the normal force on the TBM disc cutter with its penetration depth,and demonstrate the reasonableness of the proposed contact model by simulating 7 sets three-dimensional TBM disc cutter indentation tests.The results show that the Peridynamics requires only two calibration parameters to obtain the evolution of rock damage under cutter penetration,and the nonlinear shortrange force contact model proposed in this paper is closer to the experimental trend of normal force on the disc cutter than the traditional short-range force contact model.In addition,once the calibration parameters are determined,the non-linear short-range force contact model can simulate the variation of peak force on disc cutter at different confining stress,and the reasonable rock surface damage can be obtained.(2)By establishing model of vertical free-surface-assisted rock fragmentation caused by TBM disc cutters,the three-dimensional process of rock fragmentation is obtained,the variation of the three-dimensional rock fragment size is analysed,the collaborative rock breaking between adjacent cutters is studied and the fragmentation pattern is compared in double disc cutters with sequential and simultaneous loading.The results show that on the premise of ensuring the rock breaking efficiency,the size of the rock fragment can be desreased by reducing the distance from cutter to free surface or the rock free-surface height to achieve the transport of the rock fragment and prevent the occurrence of cutterhead jamming.In order to avoid the significant reduction in rock breaking efficiency,the spacing design of double disc cutter with sequential loading should take the influence of the distance from cutter to free surface into account,while the spacing design of double disc cutter with simultaneous loading is less influenced by the distance from cutter to free surface,to a certain extent.(3)After the rock fragmentation by disc cutters has been finished near the vertical free surface,the inclined free surface will be formed.In order to further study the influence of the inclined free surface on the subsequent rock fragmentation by TBM cutters,the change of the free-surface angel and the distance from cutter to the free surface are considered in this paper.The results show that in the conditions of this paper,rock breaking appears to internal crack extension mode and internal crushed zone expansion mode.With the decrease of the free-surface angel,rock fragmentation mode gradually changes from the internal crack extension mode to the internal crush zone expansion mode,causing significant reduction in rock breaking efficiency.Moreover,when the free-surface angel is fixed,different distance from cutter to the free surface does not result in exactly the same rock breaking mode.In the engineering,it is necessary to prevent forming the small free-surface angel after the rock fragmentation by disc cutters has been finished near the vertical free surface.At the same time,a reasonable distance from cutter to the free surface should be set so that the rock is broken in internal crack extension mode under the inclined free surface for improving the rock breaking efficiency.