Experimental Study On Degradation Characteristics Of Frozen-Thawed Of Wudang Group Schist

Characteristics and mechanism of freeze-thaw damage to rock is one of the important topics of rock mechanics studies. Cold regions is widely distributed in china, with the implementation of strategic plan of The Western Development and Revitalizing the Northeast Old Industrial Base, many major projects both under construction and planning are facing freeze-thaw problems of engineering rock, such as railway, highway, water conservancy, et al. In northwest of Hubei province, Wudang Group schist is widely distributed, and the schistosity is extremely developed in these schist, which leads to the mechanical properties of the Wudang Group schist extremely complex. The northwest of Hubei province is seasonal frozen area, the freeze-thaw damage characteristics of tunnel surrounding rock in this region is more complex. Thus, research schist freeze-thaw damage characteristics and their impact on the long-term stability of tunnel surrounding rock, has important theoretical and practical value.Combining the damage characteristics of Wudang Group schist under freeze-thaw condition, some work was done with the guide of the theory of Engineering Geology, Rock Mechanisms, Petrology and Mineralogy. In the work, we used the field geological survey, laboratory physical and mechanical test, freeze-thaw test, CT scan, ultrasound test, scanning electron microscopy test, polarized light microscopy test, theoretical analysis and numerical simulation. The main work is as followed.1. Experimental study on basic physical and mechanical properties of schist(1) Laboratory test on physical and hydrolytic properties of5schist samples was carried out, the result shows, bulk density and particle density is between2.86g/cm3～2.92g/cm3, porosity is between1.70～1.09. Density of these5blocks is nearly, however, porosity and water absorption is differently. Among them, softening coefficient of No2block is0.887, more than0.75, it indicates that the No2block with weak softening, and with strong frost resistance and weathering resistance.(2) Laboratory uniaxial and triaxial compression test is carried out to the schist, stress-strain curves show, the samples all have experienced four stages, compacting, elastic deformation, yielding and failure during compression, and the failure mode are all brittle. The average uniaxial compressive strength of schist under natural state is46.94MPa, and41.65MPa under saturated state, there are all more than30MPa, thus, the schist belongs to hard rock. The average of deformation modulus, elasticity modulus and Poisson’s ratio under natural state are24.8GPa,26.4GPa,0.449respectively, and the average of deformation modulus, elasticity modulus, Poisson’s ratio, cohesion and friction angle are20.5GPa,24.1GPa,0.298,10.15MPa,30.3°respectively. Samples mainly occurs pure shear failure along the foliation plane direction.2. Research on method of initial crack generate and it quantitative characterization(1) The method that taking30kN axial preload is used to generate initial damage fractured to rock samples, according to the uniaxial compression test results of schist under initial state. Stress-strain curves during preloading indicate that, the sample is still in a state of elastic stage, when the axial pressure load to30kN, the average of axial strain is approximately0.0013, while, the average of axial strain is approximately0.0013, it shows that the pre-press through30kN load, the sample produced a more pronounced axial deformation, which can achieve the effect that generate fissures of initial damage. In order to quantitatively characterize the change of the fracture rate of internal rock by axial preload, wave velocity test is taking to the drying sample both before and after preload. The results shows, longitudinal wave velocity of these three exploring samples after preload test is significantly reduced compared with the initial state, which shows internal fissures rate of the samples increased significantly through the axial preload of30kN.(2) Uniaxial and triaxial compression test is taken to the schist samples after preloading, and obtain that, the average of uniaxial compressive strength, elasticity modulus, Poisson’s ratio, cohesion and friction angle are40.6MPa,23.166GPa,0.418,9.77MPa,30.1°respectively. In general, after initial damage, the uniaxial compressive strength, deformation parameters, mechanical parameters are all reduced to some extent. The samples mainly occurs pure shear failure along the foliation plane since the control of plane foliation.3. Research on damage law of schist under freeze-thaw cycles condition(1) Firstly, all samples were preload by30kN, then, divided them into4groups, and4,8,12,16freeze-thaw cycles test was carried out on these groups of sample respectively. The results show that, deterioration of mechanical parameters of schist is varying degrees, under the action of freeze-thaw cycles. Among them, maximum uniaxial compressive strength, modulus of elasticity, cohesion, internal friction angle was gradually decreased with the increasing number of freeze-thaw cycles, the reduced rate is from fast to slow and gradually stabilized. The relationship between mechanical parameters and the freeze-thaw cycles was fitting by nonlinear method. There has no significant law that Poisson’s ratio with the increases of number of freeze-thaw cycles.(2) CT number and longitudinal wave velocity of the samples in different damage stages were counted, then obtained the variation law of longitudinal wave velocity and CT number. The results show that, CT numbers and longitudinal wave velocity of the samples were gradually reduced under the effect of initial damage and the freeze-thaw cycles, it shows that the damage of preload and freeze-thaw cycles cause the rate of rock fractures increasing. In order to compare the suitability of rock fracture rate Characterization by CT numbers and longitudinal wave velocity, correlation analysis to the mechanical parameters of uniaxial compressive strength, elastic modulus, Poisson’s ratio, cohesion, internal friction angle between longitudinal wave velocity and CT numbers was carried out respectively. The result shows, longitudinal wave velocity has higher correlation with mechanical parameters than CT numbers, and longitudinal wave velocity is more suitable for characterization of fractured schist rate.(3) Characterize fracture rate of schist by using of longitudinal wave velocity, and then, expression of the relationship between longitudinal wave velocity and mechanical parameters, the attenuation of longitudinal wave velocity and the deterioration of mechanical parameters were established respectively. The result shows, there has significant correlation between longitudinal wave velocity and mechanical parameters, the attenuation of longitudinal wave velocity and the deterioration of mechanical parameters, to characterize fracture rate of schist by longitudinal wave velocity is appropriate.4. Research on freezing-thawing damage deterioration mechanism of schist.(1) The deformation failure mode of all samples after the compression tests was statistical analyzed. The results show that, in initial state, minority of samples that shear failure plane is oblique to the schistosity plane after the compression tests, and minority of samples that shear failure plane is parallel to the schistosity plane. The number of samples that shear failure plane oblique to the schistosity plane is increasing gradually with the initial damage and freeze-thaw cycle go on. Initial damage and freeze-thaw cycle causing the internal crack extension, the damage deterioration degree of schistosity plane is higher than that of rocks between the schistosity planes. Therefore, pure shear failure along the schistosity plane is easier occur in compression test.(2) The surface morphology of failure plane of schist after the compression tests was compared. The results show that, in initial state, shear failure plane is very rough and irregular, there has few rock debris with high content and powder are distributed on the surface. While, Shear failure plane becomes smooth and regular with the increase of freeze-thaw cycles after preload, on which gray rock debris with low content and powder with high content are distributed. Based on above, the fractal dimension according to the typical fluctuation curve of failure plane was calculated, the result shows, fractal dimension and the waviness of shear failure plane decrease with the increase of numbers of freeze-thaw cycle after the compression test. The main reason is that, the freeze-thawing cycle causes rapid deterioration of mechanical properties of schistosity plane, which led to the the control action that schistosity plane to deformation and failure of sample was enhanced, thus, shear failure plane is smooth while samples with higher degree of damage deterioration.(3) According to the comparison of rock slice obtained from before and after the freeze-thaw cycle as well as the compression test through polarization microscope test, the freezing-thawing damage properties of schist was effected by the size of the mineral particle filled in the schistosity and the directional arrangement of mica Foliation is filled with quartz, feldspar and calcite mineral particles, when these mineral particle size in the foliation of the larger, more quantity, the binding effect of mica creep deformation along the foliation direction of the more obvious when the compression test. If the mica is arranged densely and continuously with no mineral filled, the internal extending crack easily to be versed in will reduce the mechanical properties of schistosity greatly in freeze-thaw cycle, thus leading to the creep deformation along the direction of schistosity of mica when pressed. If the schistosity of much space is filled with minerals of relatively high hardness such as quartz and feldspar, poor continuity of schistosity will cause the microcracks hard run along the direction of schistosity in freeze-thawing cycle, thus leading to the overall creep deformation along the direction of schistosity except partial distortion or dislocation because of the resistance of other minerals.(4) Based on the comparison of shear failure plane morphology of schist sample through scanning electron microscope, the shear crack direction of schistosity in shear failure plane is oblique to shear direction within45degree for specimen without freeze-thaw cycle after compression, while the degree is70to85for sample with freeze-thaw cycle. It demonstrates that shear failure along schistosity plane occurs easily with the freeze-thawing cycle resulting in the decrease of the shear resistance of schistosity, while compression-shear failure occurs without the freeze-thawing cycle because of the high shear resistance of schistosity. In addition, deformation and failure is not obvious for schistosity rocks without the freeze-thaw cycle after the compression test, while the cranny ratio of mica schistosity increases and the shear strength decreases because of the extrusion stress caused by the dissolution and expansion of feldspar and calcite filled in the schistosity plane.5. Long term stability of tunnel surrounding rock considering freeze-thaw damage degradation of schist(1) Through mechanics test and field geological survey, the quality of the tunnel surrounding rock was classified with the method of RMR. The scores of the six indicators including uniaxial compressive strength, RQD, joint spacing, joint condition, groundwater infiltration, the relation between joint attitude and cavity axis are6,16,10,13,12and-11. The RMR score is46and the surrounding rock from section YD115+038to section YD115+163is in medium level3. Based on the analyses above, the mechanical parameters of rock mass were estimated by using of Hoek-Brown criterion. The m; of intact rock is8, the D representing the stress perturbation of rock mass is0.8, the rock mass density is28.8kN/m3, the buried depth of tunnel is325m and the uniaxial compressive strength of rocks are obtained respectively from the freeze-thaw cycle of0,4,8,12,16and20times. Finally, we get the mechanics parameters of surrounding rocks including elasticity modulus, cohesion and internal friction angle of different number of freeze-thaw cycle.(2) The deformation and stability of the surrounding rock and second lining structure after the tunnel excavation and lining in6conditions including freeze-thaw cycles of0,4,8,12,16 and20times was analyzed by FLAC-3D numerical method, the change rules about the displacement field, stress field and plastic zone of them was obtained. The results show that the surrounding rock and lining deformation, plastic zone size, maximum and minimum principal stress increase gradually with the increase of freeze-thaw cycles. The change is mainly due to the decrease of mechanical properties of surrounding rock, which lead to the following steps:the deformation increases, the loosing zone expands, the steady ability decreases, the load arranged on the supporting structure by surrounding rock increases and finally the stability of the lining is reduced. When selecting the supporting scheme and designing the supporting structure, we should take into account the effect of the damage deterioration of the mechanical properties on the tunnel stability in the long freeze-thawing cycle.This paper has research the rule and mechanism of freezing-thawing damage deterioration through a variety of experiment and methods. The main innovations are as follows:(1) Generation of initial damage crack system in schist and the quantitative characterization of crack rate are proposed. At present, the initial damage of rock is mainly created by blasting and excavating with the artificial regular macro-crack which is not meet with the actual situation. Therefore, certain internal cracks are generated with the method of axial preloading of the RMT system on the premise of the overall specimens for the simulation of initial damage caused by engineering disturbance and the quantitative characterization of crack rate changes reflected by ultrasonic wave velocity.(2) The relationship between the crack rate and the mechanical parameters is analyzed quantitatively under the freeze-thaw cycle. Ultrasonic test and CT scanning are carried out for specimen in different stages. The crack rate changes are characterized indirectly through the longitudinal wave velocity and CT number. The corresponding relationship between longitudinal wave velocity, CT number and mechanics parameters of schist is created. Then the quantitative relationship between the crack rate and mechanics parameters is obtained.(3) Mechanism of freezing-thawing damage deterioration of schist is analyzed from microscopic view. X-ray diffraction, polarized optical microscope and electron microscopy are used to study the mineral composition, arrange of granules and microstructure.