Study On Complex Slopes Response Law Under Earthquake Action

Tens of thousands of geohazards were induced by Wenchuan earthquake. Survey revealed that the most serious damage occurred in the transition parts of the middle to upper slope of deep valley, thin ridges and especially parts of prominent terrain. Those positions have obvious topographic amplification effects. Although domestic and foreign experts have been aware of this phenomenon, as a result of less measured data and large differences between the numerical simulations and monitoring data, it is difficult to propose the ground motion amplification factors of a slope in different elevations. At present, the stability evaluation of high slopes in major projects still use single a peak acceleration of bedrock (without considering topographic amplification effect) provided by earthquake departments, which leaves large potential safety hazard. For the sake of deep research on amplification law of slopes, in funding of China geological survey bureau and Nation science foundation, through collecting main earthquake waves, surveying typical landslides and rockfalls after earthquake, excavating slope adits and setting surface observation profiles for aftershock monitoring, methods of statistical analysis, qualitative description, semi-quantitative analysis and numerical simulations are adopted to study the main seismic parameters characteristics, dynamic response characteristics of complex slopes and variation of seismic parameters on the slope profiles aftershocks, moreover, numerical dynamic analysis is performed on the slope response considering the different conditions when excited by main earthquake waves. Based on the above work, for the first time more comprehensive seismic response laws of complex slopes are revealed. The main results and conclusions are reached as follows:(1) It is not only the first time to excavate adits in different elevations of valley slopes on two opposite sides (Dongshan-Shiziliang mountain) for ground motion observations, but also the first time to do the monitoring on several aftershocks. Relative to 788m monitoring point, analysis on eight earthquakes with felt to middle intensity shows the opposite dynamic response in the similar heights on both sides of the valley. The peak acceleration in 871m of Dongshan is amplified at the factor of 2.0 while that in 893m of Shi ziliang is reduced. As reflected by research in combination of geological conditions, boundary characteristics and test results of microtremor, microrelief is one of the key factors and site conditions have an important effect on seismic response extent. The protruding topography and multi-directional free surfaces of Dongshan middle slope have the most obvious amplification, but groundwater in potential landslides of Shi ziliang increases site damping to reduce dynamic reponse.(2) The first time we set surface observation profiles for local topography close to Qingchuan-Pingwu active fault and monitor a large number of felt to middle aftershocks. Relative to 785m monitoring point, analysis on 18 earthquakes shows that horizontal and vertical peak accelerations in 805m of Wei ganliang amplify 1.0 ~3.0 times, while selective amplification is presented at 875m monitoring point in the north-south horizontal component. This component amplifies more than 4 times and its main frequency of Fourier spectrum focuses on 2.5~5.5Hz. Research by combinating geological conditions, boundary characteristics, macro geohazards distribution and monitoring results reveals that the protruding topography and EW-striking mountain of Wei ganliang has control effect on seismic response. High amplitude and low frequency seismic waves produce obvious resonance with topography, making significant peak acceleration amplification and leading to cracking damage along the thin ridge.(3) The first time we set surface observation profile for slope topography passed by the front fracture and monitor a large number of felt to middle aftershocks. Relative to 776m monitoring point of piedmont plain in Mianzhu jiulong town, analysis on 11 earthquakes shows that horizontal and vertical peak accelerations in 908m amplify 1.0 ~4.8 times while they reduce at 873m point. Research by combinating geological conditions, boundary characteristics and test results of microtremor reveals that platform terrain in 908m is beneficial to seismic response, but mild slopes and fault zone in 873m both go against the effect.(4) Statistic analysis is performed on peak accelerations and Fourier spectrums based on 228 recordings of main earthquake with amplitudes large than 30gal in Longmenshan and its neighbouring regions. Results show that the ratio of vertical-to- horizontal peak accelerations greater than 1/2 occupies 61.9%~64.48%, 22%~30% of which is more than 2/3. Within 70km of hanging wall this ratio is greater than 2/3, but between 20 and 70km in the footwall it tends to 1/2~2/3. Fourier spectrum analysis reveals that the predominant frequency less than 3Hz of vertical components occupies 57%, while that of the horizontal components occupies 38%~40%. In the hanging wall, the predominant frequency of vertical components concentrates on the high values and it is greater than that of the horizontal components, which is opposite in the footwall. Analysis shows that thrust motion of the causative fault obviously contributes to vertical energy in the hanging wall, and also contributes to"the hanging-wall effect"of geohazards development.(5) Basic features and genetic mechanism of typical earthquake landslides are reflected through deep investigation and analysis for Liu Jia Wan and Zhao jiashan landslides in the territory of Qingchuan. On this basis, coupling study is conducted on influencing factors such as fracture activities, topography, lithology, rock structure, seismic wave and so on. Then formation conditions and movement mode of high slope landslides caused by a strong earthquake are proposed: quick tension and shear failure of landslide along structural plane, weathering zone or contact surface between rock layers→sudden trigger effect forming horizontal or oblique throwing→gliding in the air→dive landing or scraping and collapse hiting→debris flows process.(6) Based on the predominant frequency test results of microtremor (3~6Hz) and Fourier spectrum analysis, the resonance effect is one of the main factors of secondary geohazards development in Wenchuan earthquake.(7)Numerical dynamic analysis for different topography, lithological characteristics and other factors of slopes reveals that the general amplification factor of peak acceleration is 1.0~2.0, but the influence of local topography and diversity lithology can make amplification factor reach 3 to 6. Numerical simulations show that a slope with internally hard and externally soft lithology combination generates tension stress concentration on the interface of lithology, and it shows the strongest seismic response in peak accelerations. The'externally soft'rock in high and steep topography is prone to generate projection effects.(8) Comprehensive research on adit observation profiles, monitoring data and numerical seismic analysis shows that the peak accelerations display a non-linear amplification with the elevation increase of the slope. The general amplification factor is 1.0~3.0, but resonance effect of local topography and difference in physical properties of rock can make magnification factor up to 4 times or more.