Study On The Fatigue Degradation Of Jointed Rock Slope And Its Anchoring Effect Under Cyclic Loading

Due to the prosperous development of China’s economic construction, the high-speed railways, highways and other infrastructure constructions continue to extend to the mountainous areas, which are inevitably faced with a lot of slope projects, especially steep and complicated rock slope projects. These rock slopes along with highways and railways are not only located in the complicated geological environment with a large number of structural planes, but also affected by the long-term impact of cyclic loading of highways and railways. Under such long-term cyclic loading, jointed rock slope will have fatigue degradation and becomes great hidden troubles to treat the long stability of rock slope. Quite a few domestic and overseas scholars started to make some groundbreaking studies on the characteristics of rock fatigue damage, failure laws and other aspects. However, the theoretical analysis of fatigue degradation specially for jointed rock under cyclic loading, its anchoring effect in the process of the fatigue damage, and the analytical investigation of anchoring mechanism need to be further studied; moreover, real-time tracking test studies on the fatigue damage process of jointed rock by fatigue damage online monitoring auxiliary testing system and ultrasonic testing methods are rarely mentioned, and the researches on dynamic response and long-term stability of jointed rock slope under cyclic loading also need to further developed.On the basis of previous studies, this paper put forward the research topic on fatigue degradation of jointed rock slope and its anchoring effect under cyclic loading, the actual rock project was used as background, sandstones were used as prototype, and similar materials were made into rocklike model specimens (intact specimen, fractured specimen, anchored mtact specimen, anchored fractured specimen, etc.) to develop similar researches on the jointed rock under cyclic loading. The study focused on the deformation law, failure characteristics and mechanism of fatigue degradation of joint specimens under cyclic loading, anchoring effect and anchoring mechanism of anchored specimens in the process of fatigue degradation, tracking tests of ultrasonic testing methods on the fatigue degradation process of jointed specimens and its anchored specimens, and the dynamic response of jointed rock slope under cyclic loading.The major study is concluded as follows:(1) The sandstones were used as the prototype, and the similar materials were designed and made into rocklike model specimens and anchored model specimens. The specimens were including:intact specimens, specimens with different fractured angles (a= 0°、30°、45°、 60°、90°, a referred to the included angle between pre-fractured surface and horizontal direction), intact anchored model specimens, anchored model specimens with different fractured angles (a= 0°、30°、45°、60°、90°, a referred to the included angle between pre-fractured surface and horizontal direction), of which fractured specimens included single and double fractured specimens. The set positions for the anchor bolt of fractured specimens were two kinds including vertical pre-fractured direction and vertical loading force direction.(2) The electro-hydraulic servo loading system was applied to make uniaxial compression fatigue test for the model specimens under cyclic loading. The fatigue damage online monitoring auxiliary test system was used to measure and obtain more sensitive damage parameter value A in the process of damage, while the curve of A value described the change laws of three stages for the specimens during the process of fatigue damage; the dynamic signal test analysis system was used to research the fatigue damage laws of irreversible deformation in axial direction for the specimens with different fractured angles, the fatigue damage process of model specimens were accordingly divided into:initial deformation stage, constant deformation stage and accelerative deformation stage. These three stages were basically corresponding to three stages of fatigue damage described by damage parameter curve value A; and then, the fatigue damage forms and major failure modes of the specimens with different fractured angles were analyzed, and its fatigue damage mechanism was also discussed.(3) The real-time online monitoring system was combined to make comparative study on deformation laws, compressive strength, fatigue life and damage forms for the anchored specimens, non-anchored specimens and specimens with different anchored angles, and the failure modes of anchor bolts in test were analyzed; through the anti-pulling experimental study on the anchored intact model specimens, the attenuation law of anti-pulling force under cyclic loading was obtained, attenuation coefficient of anti-pulling force was defined, and the security of anchor under cyclic loading was visually analyzed.(4) The real-time ultrasonic testing research was made for the model specimens under cyclic loading. The comparative analysis of time domain parameters (ultrasonic wave velocity, wave form, wave amplitude of first wave) for fractured specimens (single fractured, double fractured) and anchored specimens (vertical pre-fractured direction and vertical loading force direction of anchor bolt) were studied. In general, with the development of fatigue damage, the changes of ultrasonic wave velocity were more sensitive, and presented "inverted S" attenuation law of three stages; the wave form was gradually distorted into horn form from semicircle or approximate semicircle, while the waveform correlation coefficient and the first wave amplitude were generally on declining trend.(5) The frequency spectrum analysis on acoustic signal was realized by Fast Fourier Transform (FFT), and the comparative study on frequency domain parameters (basic frequency, maximum amplitude of frequency domain) was made for model specimens. It was found, during the process of fatigue damage, the changes of basic frequency and maximum amplitude of frequency domain were relatively insensitive. The basic frequency and maximum amplitude of frequency domain usually had great declines after the specimens had macroscopic cracking but their change laws were not stable. Meanwhile, wavelet transform theory was applied to decompose the acoustic wave signal with weighted processing; it was found, during the process of fatigue damage, the sensitive degree of weighted frequency domain parameters was larger than that of the frequency domain parameters from Fast Fourier Transform (FFT). while the basic frequency was especially obvious.(6) On the basis of the fatigue deterioration test of model specimens under cyclic loading, basic theory of damage mechanics was combined and ultrasonic wave velocity was selected to define the damage variable D to study on the initial damage and fatigue damage evolution law for the model specimens with different fractured angles. Based on the evolution laws of fatigue damage accumulation of specimens in three stages, the models for nonlinear three-stage accumulated damage were put forward. From the tests of this paper, the significance and value range for parameters of models were analyzed, the fitting analysis was made for the test data of this paper, and the fitting degree was relatively good. The prediction formulas of fatigue life under constant amplitude loading and variable amplitude loading were put forward. Comparing with the test data of this paper, the results from prediction formula had higher accuracy.(7) The anchoring jointed rock slope under the traffic cyclic loading was used as research background, and through the numerical simulation study, it was found axial force had a decreased and increased "small platform" of anchor cable in the range of free section. This "small platform" was located in the small range of anchor cable through the joint, which helped the inference on the locations of loose, broken structural surface or cavity in the rock mass. Under the traffic cyclic loading of slope, the characteristics of dynamic response were analyzed. It was found that the dynamic response of each monitoring point was not only related to the load size, frequency and duration of vibration, but also related to other aspects, such as:distance between each monitoring point and traffic loading, the physical and mechanics parameters of the current layer, whether there were joints, cracks and other structural surfaces, whether the supporting measures were adopted, etc. When the frequency of traffic loading was close to the natural vibration frequency of the slope, the resonance phenomena was easily occurred to increase the damage and destroy of rock and soil mass.