Dynamic Response Of The North Slope Of Fushun West Open Pit

Fushun open pit is located in the southeast of Fushun. It has mined for over a hundred years and till now has formed a pit of 16.6 km long east west, 2.2 km wide north south and about 400 ~ 500 m deep, known as "Asia’s largest open pit”. It has a total product of 1.7billion cubic meters. The north slope of Fushun west open pit is close to the urban area of the Fushun city. In the event of a landslide disaster, it will cause a large number of casualties,property damage, and thus endanger the safety of the entire city.The north slope of Fushun west open pit is located in the eastern flank of Tan-Lu seismic belt. Although there has not been a destructive earthquake in Fushun area so far, minor earthquake activities are very frequent. Between 1975 and 1980, the late period of Haicheng earthquake, 36 small earthquakes occurred in Fushun, and the seismic frequency was significantly higher than other periods in the history. It is easy to infer that the Haicheng earthquake relevant to the new period tectonic activity of Tan-Lu fault zone. Along the Hunhe fault spread to Fushun, the influence of earthquake intensity reached degree VI. That is, after the Haicheng earthquake, the residual strain energy spread to Fushun area through Tan-Lu fault zone conduction. According to earthquake statistics, tectonic belts experienced major earthquakes before are prone to repetitive major earthquakes. So it cannot be ruled out that large tectonic earthquake happen in the river fault zone on the Fushun city in the future. This study carried out a series of analyses on the north slope of Fushun West open pit related to earthquakes. Based on the results of static calculations, the seismic stability and dynamic response of the slope is further discussed.In this study, published work on slope stability analysis has been reviewed and summarized. Engineering geological conditions of main achievements of the north slope Fushun west open pit have been surveyed in detail. Based on this information, the finite element method is applied to calculate and analyze resonance characteristics, static and dynamic stability and dynamic response of the north slope of Fushun west open pit. Mainachievements of this thesis are as follows:1. Comprehensive field geological investigation of the north slope of Fushun west open pit is taken. Geological, hydrological data are collected. Topographic, lithological, geologic structural and hydrogeological conditions are studied. A typical geological section is selected,and the geological model, the calculation model of the slope is established and the calculation scheme is planned. The static safety factor of the slope is 1.5. From the results of the static analysis with only gravity, the north slope of Fushun west open pit is stable. Besides, the static results provide initial static stress field for the seismic stability analysis.2. The resonance characteristics of the north slope of Fushun west open pit are analyzed.Through the modal analysis and harmonic response analysis by using the finite element method, the slope natural frequency and mode of vibration, and the relationship between displacement and frequency(resonance curve) are obtained. The resonance effect of the slope is found to be excited by the first three natural frequencies. The peak of the vertical resonance peak appears at about 1.1Hz, and the frequency of the peak appears at the frequency of1.38 Hz. The comparison of the frequency response curve shows resonance peak slope with vertical amplification and free surface amplification.3. The seismic stability of the north slope of Fushun west open pit is calculated and analyzed. By using the finite element dynamic time history method, the factor of safety time history curve of the seismic stability of the north slope of Fushun west open pit under earthquake is obtained and the minimum dynamic factor of safety value is 1.14. On this basis,it is judged that the slope is stable as a whole. In addition, if the slope is judged stable as a whole, the key parts like the slope shoulder and the slope toe are set to analyze the stress and vibration velocity effect for the local stability analysis. The results show that some cracks appear near the slope shoulder, but it does not affect the overall stability of the slope, which is basically consistent with the actual survey data.4. The dense cretaceous rock mass of the F1 upper wall locates above the green mudstone group, which is characterized by a relatively smaller density, smaller stiffness and is easy to deform. The soft rock of the green mudstone group is characterized by large creepdeformation, and its slip deformation will become bigger combined with inclined gravity of cretaceous rocks above. The slip deformation and toppling deformation, which are significant,dominate in the upper slope and surface deformation. Under the action of gravity of the cretaceous rock mass of the F1 upper wall, normal stress and shear stress develop, causing an increase in the relative decline of rock mass. However, the green mudstone of footwall rock mass locate in the crumple zone, with the weakest rock mass strength. The anti-dumping dip strata pass down force to squeeze the lower soft rock mass. This will form the first sliding surface along with the oil shale and therefore constitutes the deformation failure model in which the upper slope surface will subside and the lower slope will bedding-slip.5. Slope dynamic responses of Fushun West open pit mine in North Slope are analyzed and discussed. 14 monitoring points were set up in different locations of the slope(slope surface, the sliding surface and the slope body) for calculation and analysis purposes. The calculation results show that, in the vertical direction, due to the existence of F1 fault, seismic wave generates refracting and reflecting effects on the fault plane. The peak value of the dynamic response of any point in the slope and the ratio of the acceleration peak value of input seismic wave(PGA amplification coefficient) did not show a monotonical increase with the height of the slope. The slope dynamic response of acceleration focused on the fault, and the acceleration response value mutation and gradually decay to both sides. The earthquake acceleration maximum value is enlarged 3.6 times in faults. Displacement increases as the elevation increases in the vertical direction. Due to the presence of faults, the slope is in unfavorable deformation state. In addition, the residual deformation of fault zone is large so that the top of the slope has a larger deformation. The peak value of ground motion acceleration and displacement of upper wall are significantly larger than the lower wall.Under the action of earthquake, the acceleration and displacement of the surface of the middle part of the slope surface become the greatest. It is obvious that the earthquake is more likely to cause damage in the middle of the slope, which is mainly related to the structure of the slope and the lithology of each layer. To compare the horizontal displacement values in vertical monitoring points of the slope surface and the sliding surface, the free surface andvertical amplification effect is obvious and the free surface amplification effect is larger.Under the action of earthquake, rock and soil mass at the slope surface are very easy to be destroyed.6. The impacts of three factors on the dynamic response of the slope are analyzed. It is found that the amplitude of the power input only lead to the absolute amount of displacement and acceleration of slope dynamic response values into magnification, but their distribution pattern has not changed. In general, the absolute amount of acceleration and displacement response increased substantially in linear relationships with the input of seismic wave amplitude. However, the degree of amplification of each part of the slope remained constant,and the distribution pattern in each slope section has not changed. The displacement and acceleration amplification coefficient remain unchanged and they are not affected by the amplitude, depending only on the rock and soil material and spectrum characteristics. With increasing input frequency of seismic waves, the slope of each point on the PGA amplification coefficient decreased. When input the original wave frequency, PGA amplification coefficient is the highest, as the predominant frequency of the original wave minimum slope on the most obvious amplification effect and transmission. The higher the frequency of the seismic wave,the more the high frequency energy is. The filtering effects of the rock and soil mass can be enhanced and the influence of the duration on the peak response of the slope is not obvious.In conclusion, based on the investigation of the engineering geological conditions of the north slope of Fushun west open pit, the geological model and the calculation model have been established in this study. The dynamic stability analysis, the resonance characteristics analysis and the dynamic response analysis of fushun west open pit are conducted by using the finite element method. Results of this study indicate that northern slope of Fushun West open pit didn’t slide under rare earthquakes while there were some cracks near the slope shoulder. In addition, it is suggested that the variation trend of slope response, the position of the response maximum and the frequency segment of resonance response can be analyzed and used to avoid the problem of failure under earthquakes. Corresponding seismic measures are given in the meanwhile.