【作者】 罗永红；

【导师】 王运生；

【作者基本信息】 成都理工大学 ， 地质工程， 2011， 博士

【摘要】 汶川大地震诱发了数以万计的地质灾害,震后调查揭示,深切河谷谷坡中上部坡折部位、单薄山脊部位、地形突出部位破坏最为严重,具有明显的地形放大效应。国内外专家虽已意识到这一现象,但苦于实测资料少,而且数值模拟与实际监测数据差异大,难以提出符合实际的斜坡不同高程部位地震动放大系数。目前国内重大工程高边坡仍然采用地震部门提供的单一基岩峰值加速度(未考虑地形放大效应)评价地震工况下的稳定性,这在很大程度上埋下了安全隐患。为了深入探讨这一放大规律,作者在中国地调局及国家基金委的资助下,通过收集汶川主震监测数据,开展震后典型滑坡、崩塌现场调查,以及通过掘进斜坡平硐、设立地表观测剖面对汶川余震作用进行监测,采用统计分析、定性描述、半定量分析、数值模拟等方法,对汶川主震动参数特征,龙门山地区复杂斜坡动力响应特征,及汶川余震在斜坡剖面上的动参数变化特征等进行了研究,并对主震地震波作用下不同条件的斜坡响应进行了数值动力分析,首次较为全面地揭示了复杂斜坡地震动响应规律。主要研究成果及结论如下:(1)首次在河谷两岸斜坡(青川东山~狮子梁)不同高程掘进平硐进行地震动观测,而且监测到多次汶川余震完整剖面数据。参照河谷测点(788m),对8次有感~中强地震分析揭示,东山中部(871m)呈现“凸”型响应,放大系数可达2.0,狮子梁中部(893m)呈现“凹”型响应,即河谷两岸中部相近高程呈现相反动力响应。结合各监测点地质条件、边界特征、地脉动测试成果等研究揭示,斜坡地震动力响应程度微地貌是关键,场地条件具有重要影响作用,即地形凸出及多面临空的东山中部斜坡放大效应最为显著,而潜在滑塌体内的地下水增大了狮子梁中部场地的阻尼,对地震动具有消弱作用。(2)国内首次对青川~平武活动断裂带附近局部地形设置了地表观测剖面,并监测了多次有感~中强余震。根据青川桅杆梁斜坡地表观测剖面所监测的18次有感~中强地震,参照河谷785m测点分析揭示,桅杆梁805m测点水平及竖直向地震动峰值加速度放大系数为1~3.0倍,875m测点呈现水平南北向峰值加速度选择性放大,放大系数多在4倍以上,且该分量傅里叶谱主频率集中于2.5~5.5Hz。结合监测点地质条件、边界特征、汶川地震的破坏特征及监测结果等研究表明,桅杆梁凸出地形、近EW走向的条形山体对地震动具有明显控制效应,较大振幅及中低频率地震波与地形产生明显的共振效应,使得水平南北方向峰值加速度放大非常明显并导致沿单薄山脊的震裂破坏。(3)国内首次对前山断裂通过的斜坡地形设置了地表观测剖面,并监测到多次有感~中强余震。根据绵竹九龙镇山前斜坡地表地震动观测剖面所监测的11次有感~中强地震,参照山前平原776m测点分析揭示,斜坡873m测点水平及竖直向动峰值加速度呈现减小,908m测点水平及竖直向地震动峰值加速度均体现为放大,放大系数1~4.8倍。结合斜坡各测点地质条件、边界特征及地脉动测试成果等研究表明,908m测点近距临空面平台地形有利于地震动响应,而873m测点的缓坡地形及断层破碎带不利于地震动响应。(4)对龙门山及邻近地区振幅大于30gal的228条汶川地震波监测数据进行了峰值加速度及傅里叶频谱的统计分析。揭示了汶川地震动峰值加速度竖向与水平向比值1/2以上占61.9%~64.48%,其中大于2/3占22%~30%,且在距离断层上盘70km以内该比值大于2/3,而断层下盘20~70km,该比值趋于1/2~2/3。傅里叶频谱分析揭示,竖直向主频率值小于3Hz占57%,水平向主频率值小于3Hz约占40%,而上盘竖直向以高频为主且总体大于水平向,下盘则相反。分析表明发震断层的逆冲作用对上盘的竖向能量贡献明显,对震害发育的“上盘效应”具有贡献作用。(5)对青川县境内的刘家湾、赵家山等滑坡进行了深入调查与分析,揭示了典型地震滑坡的基本特征及成因机理。在此基础上,对斜坡地震地质灾害的发育与断裂活动、地形地貌、岩性、岩体结构、地震波等影响因素进行了耦合分析,提出了强震条件下高位滑坡的形成条件及其运动程式:即滑体沿结构面、风化卸荷带或岩层分界面等快速拉剪破坏→突然的触发效应形成水平抛出或斜向上抛出→空中滑翔→俯冲着陆或铲刮崩撞→碎屑流化过程。(6)基于震区大量斜坡剖面地脉动卓越频率测试成果(3~6Hz),结合汶川主震地震波傅里叶频谱分析,认为汶川地震丰富的高振幅、中低频率地震波与山区斜坡的共振效应是加重山地次生灾害异常发育的主要因素之一。(7)对不同地貌、不同性状岩性等斜坡的数值动力分析揭示,地震动峰值加速度放大系数一般可达1.0~2.0倍,局部地形及性状差异岩性的影响作用可使放大系数达到3.0~6.0倍。数值分析显示“内硬外软”性状差异岩性组合斜坡在分界面产生拉应力集中,坡体地震动峰值加速度响应最为强烈,结合调查研究认为在高陡地形条件下“外软”岩体易形成抛射效应。(8)综合平硐观测剖面、地表观测点监测数据,以及数值动力计算分析等研究表明,随斜坡高程增加地震动峰值加速度呈非线性放大,放大系数一般为1~3.0倍,局部地形的共振效应,岩体物理性状差异变化等作用可使得动峰值加速度的放大系数达到4.0倍及以上。更多还原

【Abstract】 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.更多还原