| During rock cutting by TBM(Tunnel Boring Machine),the disc cutters usually suffer normal wears as well as numerous abnormal destructions which are induced by the operation of TBM and the variability of stratum conditions.It not only greatly increases the tunnel construction cost,but also reduces the TBM excavation efficiency.The reasonable explanation for rock fragmentation mechanism by TBM disc cutter is the theoretical basis for the design of disc cutter and cutterhead and the optimization of TBM operation strategy.It is also of great significance for improving the efficiency and economy of TBM operation.Meanwhile,the accurate acquirement of real rock damaging and cracking evolution information under disc cutter is an important part for the reasonable rock cutting model by TBM cutter.Based on the previous researches,this study conducted the rock indentation test by wedge indenter under the plane stress state and the disc cutter indentation test under biaxial confining stress.During the test,some non-destructive testing methods like IRT(infrared thermography)and AE(acoustic emission)were applied to obtain the rock damaging and cracking information,based on which the quantitative analysis for the development law of the induced damage and fracture by cutting interaction was conducted.On the other hand,to explain and analyze the rock fragmentation mechanism under TBM cutter,a computation method of the heterogeneous model was introduced to the combined finite-discrete element method(FDEM),and then the rock cutting by TBM cutters under different loading conditions was simulated,and the rock damaging and cracking behavior and acoustic evolution were analyzed.The main conclusions are as follows:(1)The IRT and AE location monitoring methods were applied in the rock indentation test by wedge indenter,and then the development law of induced rock damaging and cracking was analyzed.During the test,the uneven temperature rise appeared on the surface of the rock specimen under the penetration of wedge indenter,which can be explained by the cavity expansion model(CEM).The temperature rise mechanism of the core zone,the plastic zone and the elastic zone in the model respectively correspond to the thermofracture effect,thermoplastic effect and thermoelastic effect of rock.Different thermo-mechanical coupling effects have different energy dissipation rates and thus lead to different temperature rises,which can be used for the quantitative division of the corresponding damage zone range.It shows that the plastic zone and core zone of rock initiate and develop sequentially during rock indentation,which is inconsistent with the classical CEM.The latter assumes that they initiate and develop simultaneously.Comparing the rock cutting by wedge indenters with different wedge angles,it shows that the wedge indenter with 120° wedge angle is more efficient when cutting rock,especially for rock with medium strength like sandstone,due to the wider rock damage zone.When penetrated by wedge indenters with a specific wear flat,the form of rock damage zone is the semiellipse shape and has a larger damage zone in the penetration axial direction.It is also found that an obvious brittle-ductile transition occurs in the rock failure pattern under indentation when the lateral confining stress exceeds the critical value.Meanwhile,the rock damage zone range obtained by IRT observation and that obtained by theoretical value of the classical CEM were compared,and some shortcomings of the latter were discussed.(2)In the quantitative analysis of the AE monitoring results of rock indentation test under biaxial confining stress,an improved AE localization algorithm was introduced and developed,and accurate AE location could be obtained.Meanwhile,the hemispheric statistical model was applied to analyze the range of rock damage zone and its development law.The results show that the expansion speed of rock damage zone under the disc cutter is fast firstly and then slow.For the same rock,the unit damage radius induced by CCS(constant cross section)cutter is larger than that induced by V-type cutter.For the same disc cutter,it induces lager unit damage radius in hard rock than that in the soft rock.The differences of force response and rock fragmentation pattern induced by CCS cutter and V-type cutter were also compared.SEM observations of rock fracture faces revealed that the fracture mechanisms of crushed zones and major cracks of different rocks under the penetration of disc cutters are shear type and tensile type respectively.On the other hand,when the confining stress is insufficient to cause the rock cracks which are parallel to the free surface,the rock fragmentation induced by disc cutter is inhibited more strongly with the increasing confining stress,and the unit damage radius induced by disc cutter also decreases.However,when the confining stress reaches the critical value,some rock cracks occur in the rock which are parallel to the free surface and then connect with those induced by cutter penetration.The newly generated crack network can promote rock fragmentation under the cutter penetration.Finally,in regard to the existence of a single main crack surface in the rock sample,a method to estimate the evolution of macroscopic fracture surface of rock based on clustering-filtering calculation of the discrete AE events was proposed,but the method needs to be further improved.(3)In order to simulate the progressive damaging and cracking processes and acoustic emission activity of rock under loading,a heterogeneous model establishment method based on random distribution was introduced to the FDEM.Moreover,the corresponding determination method of model parameters was proposed.Then,the rock cutting processes under single,double and multiple cutters were simulated respectively,and the influences of confining stress and cutter spacing were also studied.It shows that there exists a crushed zone filled with massive shear microcracks under the cutter,and the range and damaging degree increase with the penetration of the cutter.When the normal force of cutter reaches the peak value,the lateral crack begins to aggregate and then rapidly expand and some peelable rock piece were formed.The serve rock cracking and force drop can be observed in the process.The statistical results of acoustic emission events and their energy release show that the shear-type microcracks occupy the dominant position in both quantity and energy release during the rock cutting process.On the other hand,it shows that the increase of confining stress has the significant inhibitory effect on the rock fragmentation induced by disc cutter.For rock cutting by double cutters,it is difficult for the lateral cracks induced by penetration of cutters to coalesce when the cutter spacing exceeds a certain value,thus failing to form an effective rock slices.The simulation results of the rock cutting by adjacent multiple cutters prove that the most violent energy release and dissipation occur below the cutter accompanied with the development of the crushed zone,but the energy consumption of the formation of effective rock slices only occupies a small part.The progressive rock damaging and cracking processes induced by cutter penetration under different conditions can be simulated well using FDEM method with the heterogeneous model. |
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