Research On The Dynamiac Responses And Permanent Displacement Of Slope Under Earthquak

As one of important subjects in geotechnical engineering and earth environmental engineering, the problem of slope stability under earthquake actually involves two aspects, namely, the seismic stability analysis of specific slopes and seismic landslide hazard evaluation on regional slopes, and there are many issues need to be researched for each aspect. The problem of seismic slope stability in our country is particularly serious because of the special geological conditions and seismotectonic conditions. Currently, our country's infrastructure construction such as highway, railway and water conservancy, etc. is in a period of rapid development, which puts forward higher request to anti-seismic design of specific slopes and landslide risk assessment of regional slopes, yet the existing theories and practice can not meet this requirement. Wenchuan earthquake not only caused numerous landslides, but also greatly changed the geological disasters environment, which made the fragile geological environment of Longmen Mountains even worse. Therefore, evaluating the degree of geological disasters environment evolution has become a key scientific problem need to be solved urgently in reconstruction after earthquake.To meet the national requirement of capital construction and post-disaster reconstruction in Wenchuan earthquake-stricken areas, several scientific problems, including dynamic response laws of slope, permanent displacement calculation, displacement-based anti-seismic design method, permanent displacement prediction for regional slopes and geological disasters environment evolution laws in Wenchun seismic area have been researched systematically in this dissertation, concerning issues related to slope stability analysis on micro and macro level. The research approaches include earthquake disaster investigations, large-scale shaking table model test, numerical simulation and theoretical analysis, etc. Major work and findings are as follow:Understanding of the dynamic response laws of slope under earthquake and their influencing factor is helpful to explore the seismic failure mechanism of slope, and thus to guide aseismatic design. Based on two groups of shaking table contrast tests on single-side slope and free field, single-side slope, concave slope and convex slope, combined with numerical simulation, the dynamic response laws of acceleration and earth pressure, as well as the influence of ground motion parameters and slope shape on the responses under earthquake were analyzed. Results show that the slope has vertical and surface amplification effect to input seism waves, the amplified coefficients of PGA along slope surface decrease with the increasing earthquake amplitudes, i.e. shows a characteristic of "magnitude saturation". Contrast test on slope shape shows that single-side slope is the best shape on acceleration dynamic response, and concave slope's acceleration extremely increases above shape-changed point. The permanent displacement of slope evidently increases with the increasing earthquake amplitudes and duration while decrease with the increasing frequency. Input seismic waves'spectrum properties will be changed obviously after propagation through the model soil, the soil in slope has amplification effect to input seism waves at high frequency and filtering effect at low frequency, the changing amplitude has a significantly increase around the natural frequencies of the model slope compared to other frequency bands.As the inherent characteristics of slope, deeply understanding the dynamic characteristics' variation laws and influencing factors under earthquake helps to understand the dynamic damage process and failure mechanism from the view of internal factors of slope. Based on the traditional modal analysis and transfer function theory, identification technique on slope dynamic characteristics was discussed, the imaginary part of absolute transfer function was suggested to utilize on identifying the dynamic properties of slope. Dynamic characteristics variation laws of slope and its influencing factors were researched base on two groups of shaking table contrast test between single-sided slope and reinforced slope. Results show that the natural frequencies of model slope decrease with the increasing vibration number, and the decrease range expands with the increasing earthquake amplitudes, while the damping ratios increase with their increasing, the reduce rate of natural frequencies increase when the predominant excitation frequencies of input seismic waves approach to the natural frequencies of the model slope. Dynamic characteristics variation laws of reinforced slope and their influencing factors are similar to pure soil slope, yet reinforced materials can remarkably enhance the natural frequency of the model slope, make it away from the predominant frequency of seismic wave, and consequently reduce the effects of earthquake. Meanwhile, the reduce rate of natural frequencies of reinforced slope is far less then that of soil slope after the same load law.Wenchuan earthquake damage investigation on slope reinforcement structure shows prestressed anchor cable reinforced slopes have excellent seismic performance. In order to reveal their aseismic mechanism thus to guide design, a shaking table model test of prestressed anchor cable reinforced slope was carried out. Results show that the acceleration amplification factor of reinforced slope model is obviously less than the model without anchor, especial on the moderate earthquake case and the strong earthquake case. The restraining effect on acceleration amplification by anchor cable reinforcement increases with the increasing height of slope under strong earthquake condition. Anchor cable increases the natural frequencies of slope, as a result to avoid meeting the resonance frequency, tends to reduce the seismic effect, and the reduce rate of natural frequencies of anchor cable reinforced slope is far less then that of soil slope after the same load law. The axial force of anchor cable increase with the increasing earthquake amplitudes, the frequency of seismic waves has less effect on the cable axial force, while the type of input seismic wave has remarkable effect on it.With the farther research on slope dynamic stability, the dynamic stability evaluation method using single seismic safety coefficient has been found insufficient. The permanent displacement can characterize the damage degree of slopes quantificationally, thus provides an effective criterion on slope stability evaluation. According to the principle of energy conservation, the energy response process during earthquake was analyzed; a permanent displacement calculation formula of the slope with energy method was presented. Result shows that the reverse displacement can not be neglected when the slope has a small angle of sliding surface and weak stability, while the earthquake is strong, otherwise the result may be rather conservative. The critical acceleration is a constant, acd, regardless of vertical acceleration, yet it will become an acceleration time history, acd(t), vary with time when take the vertical acceleration into account, and there may be a considerable difference on permanent displacement calculation with acd instead of acd(t). Earthquake plays an important role on triggering landslides, and the permanent displacement is mainly determined by gravitational potential energy. Reinforcement can significantly increases positive critical acceleration, i.e. improve the aseismatic performance of slope. An anti-seismic design method based on displacement controlling may improve the efficiency of aseismatic reinforcement, and it is more scientific compared to the pseudo-static method. With the illustration of anchor cable reinforced slope, the anti-seismic design method based on displacement controlling had been discussed.A more effective way to deal with large scale and wide distribution seismic landslides in state's basic construction is to consider this problem during the construction sites selecting stage, displacement predictive models provide a convenient tool for regional seismic landsides risk and landsides disaster evaluation after earthquake. Based on a large number of strong-motion records from Wenchun earthquake, a regression model predict permanent displacement suiting to Sichun and adjacent provinces was put forward. Result shows that the models developed based on strong-motion records have zone dependence, and it is necessary to establish displacement predictive models for various regions by accumulating strong-motion record data. Despite the Arias strength can describe earthquake characteristics better than PGA in physical sense, it is complicated to calculate and the model in terms of Arias fails to improve prediction accuracy, the regression model in terms of PAG is recommended to adopt as displacement predictive model.Wenchuan earthquake dramatically changed the mountain disaster environment, made the weak geological environment of Longmen Mountains even worse. Mountain hazards such as fallings, landslides and other mountain hazards may be extremely active for quite a long period of time after earthquake. A displacement calculation model on slope damage was established, displacements of critical residual strength and critical failure were defined to judge the damage degree of slope. A classifying system for environment evolution of mountain disaster based on seismic permanent displacement was established, its main contents are as follow:Regions of class I where seismic intensity isâ…§and below are the slightly damaged zone of secondary mountain disaster environment, there were fallings and landslides in partial region in earthquake, and shallow or surface secondary mountain disasters may occur mainly after earthquake, geological environment in these areas can be repaired basically in 5 years; Regions of classâ…¡where seismic intensity is IX and weak X are the severely damaged zone of secondary mountain disaster environment, there occurred many fallings and landslides in earthquake, and secondary mountain disasters may be active after earthquake, geological environment in these areas require 5-10 years to recover; Regions of class III where seismic intensity is strong X and above are extremely worse in secondary mountain disaster environment, there triggered quite a lot fallings and landslides in earthquake, large-scale secondary mountain disasters may be easily take place after earthquake, geological environment in these areas need 10 more years to recover. Regions class III are the key zones of mountain disasters preventing, in which rock and soil material has been damaged severely, secondary mountain disasters environment has been changed acutely, and the post-disaster reconstruction should be suspended in these regions.