Numerical analysis of dislocations of the face slabs of the Zipingpu Concrete Faced Rockfill Dam during the Wenchuan earthquake

The Zipingpu Concrete Faced Rockfill Dam （CFRD） was obviously damaged during the Wenchuan earthquake in 2008. A wide range of dislocations occurred along the horizontal construction joints at EL. 845m, between the face slabs constructed in the second and third stages. The maximum displacement of the dislocations reached 17cm. In this study, the slab dislocations were investigated using finite element （FE） analysis. The method based on strain potential was applied to compute the permanent deformation of the Zipingpu Dam during the Wenchuan earthquake. The calculated magnitude of the slab dislocation showed good agreements with the field measurements. The dislocation mechanism was discussed. The results show that the dislocation of the concrete slab is a subsequent damage after the permanent deformation of the rockfill materials. The effects of the shear strength and the direction of the construction joints, the reservoir water level and the seismic waves were studied. The shear strength and the direction of the construction joints, reservoir water level and have a significant effect on the dislocation displacement. The dislocation can be effectively reduced by measures such as changing the direction of the construction joints or improving the shear strength at the horizontal joints.

Analysis on residual strain of Zipingpu Concrete Faced Rockfill Dam after Wenchuan earthquake

The Zipingpu Concrete Faced Rockfill Dam （CFRD） was subjected to significant local damage in the ＂5.12＂ Wenchuan earthquake. It is the first rockfill dam of more than one hundred meters high to encounter a strong earthquake anywhere in the world. Based on the finite element smoothing method, the residual strains at a typical cross-section and a downstream slope of the dam were obtained by processing the dam monitored displacement data. The position of and reason for the dam settlement and deformation ofrockfill dilatancy in the earthquake were analyzed according to the section residual strain. The results show that the maximum settlement ratio on the dam body approximately occurs at 2/3 of the dam height; dilatancy occurs from the dam crest to 25-30 m in the upstream and downstream slope; the immediate cause of the face slabs horizontal construction joint dislocation is excessive residual shear strain. Meanwhile, the position of and reason for the dam fissure in the earthquake were analyzed according to the dam slope residual strain.

The Focal Mechanism and Characteristic of Stress Field in the Zipingpu Area before the M_S8.0 Wenchuan Earthquake

Based on seismic wave records of the Chengdu digital seismic network and Zipingpu reservoir digital seismic network from August 16,2004 to May 12,2008 when the Wenchuan MS8.0 earthquake occurred,the parameters of focal mechanisms of 486 earthquakes with magnitude larger than M1.6 in the Zipingpu reservoir area were calculated using amplitude ratio method,meanwhile the temporal-spatial variation characteristics of mechanisms and stress field were analyzed based on these parameters.Results show a low ratio of thrust earthquakes and an increased number of strike-slip earthquakes in the reservoir water area in the period from 2006 to 2008.While in the areas far from the reservoir waterfront,the thrust earthquakes took up a high proportion and the strike-slip ones did not increase.The direction of mean principal compressive stress field was deflected and disturbed differently in each area before the Wenchuan MS8.0 earthquake.

Research on Seismicity and Source Parameters of Small Earthquakes in the Zipingpu Reservoir before the Wenchuan M_S 8. 0 Earthquake

Based on the digital seismic waveform data observed from regional and reservoir seismic networks,the seismicity and source parameters in the Zipingpu reservoir area from January 1,2000 to May 11,2008 before the Wenchuan MS8.0 earthquake are studied. The analysis shows a continuous activity of small earthquakes before the Wenchuan MS8. 0 earthquake. The level of seismic activity was from ML2.0 to ML3.0 in recent years. The impoundment of Zipingpu reservoir started in September 2005 and the obvious increment of seismic activities occurred in February 2008,but according to the analysis of the longer time series,the frequency and intensity of seismic activities do not exceed its range,which means it is still a fluctuation of regional seismic activities or tectonic activity. At the same time,we also studied the spatio-temporal distribution and the change of apparent stress difference in the source region before the great earthquake,and the results show that the apparent stress values of small earthquakes are generally higher than fitted values of apparent stress in the reservoir area in the nearly three months before the Wenchuan MS8.0 earthquake. It can be seen from the spatial distribution of apparent stress difference,that the epicenter of the earthquake is located at a low stress distribution area,and the relatively high apparent stress difference is at the east of the epicenter. Apparent stress increment occurred in local areas.

Distribution Characteristics of the Seismicity of Zipingpu Reservoir Region after the Wenchuan Earthquake

815 earthquakes recorded by 12 seismic stations of the Zipingpu reservoir seismic network in 2009 were relocated using the double difference algorithm to analyze the seismic activity of the Zipingpu reservoir.Relocation results show that the earthquakes are concentrated relatively in three zones.The distribution characteristics of focal depth are obviously different among different concentration zones.This means earthquakes in different concentration zones may have different causes.Compared to relocation of earthquakes taking place before the Wenchuan earthquake done by other researchers,the seismic concentration zones in the reservoir area shifted obviously after the Wenchuan earthquake.These variations are related to local stress adjustment in the reservoir area and may also be related to the diffusion depth and range of increased pore pressure caused by rock failure in the course of Wenchuan earthquake.

Using Ambient Noise to Study the Seismic Velocity Changes Caused by the Rise and Fall of the Water Level in the Zipingpu Reservoir Region

We study the feature of media changes beneath the Zipingpu reservoir and discuss the process of permeation with the water level rise and fall of the reservoir from January 2005 to January 2008 from ambient noise cross correlation by using continuous seismic data recorded by the stations of Zipingpu seismic network and YZP station. A moving-window cross-spectrum technique has been used to calculate the relative seismic velocity changes between station pairs. Results revealed an obvious relationship between relative seismic velocity, and the water level changes with a time delay that may be caused by permeation during three main impoundments and two large scale disemboguements. Impoundment generates a fast and large impact on the superficial layer, and the changes of seismic velocity is the result of increased pressure and permeation during the impoundment. At the first impoundment, the main effect factor is pressure. During the next two process of impoundment, permeation becomes the main effect factor, affecting the fault at a depth of about 8kin.

Investigation of the 5.12 Wenchuan Earthquake damages to the Zipingpu Water Control Project and an assessment of its safety state

On May 12, 2008, a major earthquake measured 8 on the Richter scale jolted Wenchuan County in Si- chuan Province, China with the epicentral intensity of XI degrees. Zipingpu Dam is located 17.17 km from the epicenter and the influence intensity at the damsite reached IX―X degrees. It is the first time in the world for such a high concrete-faced rockfill dam (CFRD) in China to experience such a strong earthquake occurring in such a short distance. Due to its location of special importance to the down- stream areas, the safety state and damaging condition of the Zipingpu Dam has attracted great atten- tions from various circles. Based on site investigation of the damages to the Zipingpu Dam and com- parison analysis of the observation data obtained before and after the earthquake, the paper makes a comprehensive assessment of the key technical issues relevant to this high CFRD, including its safety state and the high slope stability after the earthquake.

Numerical simulation of the separation between concrete face slabs and cushion layer of Zipingpu dam during the Wenchuan earthquake

The Zipingpu concrete-faced rockfill dam(CFRD)experienced strong ground motion from the 2008 Wenchuan earthquake.Separation between concrete face slabs and the cushion layer was observed after the earthquake.The separation voids under the stage III slabs make up 55%of the total area of the stage III slabs.The observed maximum height of the separation voids was nearly 23 cm at the top of the stage III slabs.Separation voids were also observed locally below the top of stage II slabs near the left abutment,with a maximum height of 7 cm.In this study,a static and dynamic elasto-plastic finite element analysis on Zipingpu CFRD was conducted to capture the separation during the Wenchuan earthquake.The rockfill materials were described using a state-dependent elasto-plastic model that considered particle breakage.The model parameters of rockfill materials were obtained from feedback analysis.The numerical results were largely consistent with the field measurements during construction and after the Wenchuan earthquake.A three-dimensional state-dependent elasto-plastic model that can trace the separation and re-contact of a soil-structure interface was employed to investigate the interaction between concrete face slabs and a cushion layer.The analysis showed the distribution of separation voids observed in the Zipingpu CFRD has a close relationship to the water level and slab dislocations at the time of the earthquake.The phenomenon of the separation from the Wenchuan earthquake was successfully captured by the proposed numerical procedure.

Application of flow driven pore-network crack model to Zipingpu reservoir and Longmenshan slip

The study has analyzed the relationship between the water-drainage sluice process of reservoir, stress triggers and shadows of earthquake and porosity variability of fault slip zone. First, the pore pressure, pressure gradient, viscous stress and Reynolds stress to reservoir-earthquake fault slip problem are analyzed, and these are un-negligible factors of the extended coulomb failure stress under ultra-high temperature and pressure condition. Second, the porosity tensor and permeability tensor are studied, the relationship between Zipingpu reservoir and Longmenshan slip has been analyzed, and the extended viscous stress and Reynolds stress as function of time and infiltration process are obtained. Last, some primary conclusions about the flow-solid coupled facture mechanism to the Zipingpu reservoir and Longmenshan slip problem are presented, which can help understand the flow-solid coupled facture mechanism of reservoir-coseismic fault slip problem.

利用三维孔隙弹性模型探讨紫坪铺水库对汶川地震的影响

本文基于三维孔隙弹性理论,建立了紫坪铺水库及周边地区的有限元模型.根据紫坪铺水库开始蓄水到汶川地震发震时刻的水位变化情况,计算了整个区域的孔隙压力和库仑应力.详细讨论了断层及周围地层的弹性模量和扩散系数对计算结果的影响.计算结果表明:从弹性角度看,断层的弹性模量对汶川地震震源处的库仑应力影响很小;震源处的库仑应力随着断层和周围地层的扩散系数增大而增大.当给定弹性模量和扩散系数代表性值的时候,计算结果表明在汶川地震发震时刻,震源处的库仑应力变化量为+1kPa左右,这表明紫坪铺水库使得汶川地震发震断层更加危险.是否这个量级的库仑应力就能够触发汶川地震还需要进一步探讨.通过分析库区周边小震的分布,发现小震分布区域均是库仑应力增加的地区,因此紫坪铺水库周边的小震应该与紫坪铺水库蓄水有直接关系.

紫坪铺水库与汶川地震关系的讨论

利用紫坪铺水库专用地震台站和加密地震台站资料,使用双差法完成了震前930次地震、汶川8级主震和5789次余震的重新定位,探讨了紫坪铺水库与汶川地震之间的关系,获得了有利于证明它们之间密切关联的事实:1)紫坪铺水库2005年蓄水之后出现了水磨、深溪沟和都江堰震群,地震应变能释放增加了200%,与库水位变化密切相关,在汶川地震前呈现出加速释放的现象;2)野外调查表明存在1条贯穿紫坪铺水库库区、走向NE的同震地表破裂带,与中滩铺断层位置一致;3)余震分布以中滩铺断层为界,西北上盘多,东南下盘少;4)利用P波初始到时重新确定出的汶川主震发震时刻为27分59.5秒,初始破裂深度在6～9km左右,直接位于水磨震群之下;5)汶川主震与发生在2008年4月5日水磨震群中的1次小震的各个台站到时差相同。

紫坪铺水库库区地震精定位研究及分析

利用双差定位法,对紫坪铺水库台网8个地震台站记录到的2004年8月16日至2008年5月10日的1569个小震进行重新定位。结果显示,重新定位后的小震主要集中分布在龙门山推覆构造带的中央北川—映秀断裂、前山灌县-江油断裂和后山汶川—茂汶断裂上,其中大部分地震位于北川—映秀断裂并收敛于紫坪铺水库的两端,走向NE向;震源深度集中在5～15km范围内,平均深度为7.8 km,15～30 km也有少量地震发生。水库区域小震活动频度与水库水位存在一定的相关性。精定位后,到时残差的均方根平均值为0.048 s,震源位置的估算误差在水平方向上平均为0.63 km,在垂直方向上平均为0.51 km。文中还讨论了紫坪铺水库区震源分布和地质构造的关系。

紫坪铺水库区小地震震源机制研究

用紫坪铺水库专用台网的地震记录测定紫坪铺水库区262次小震震源机制参数,结果表明:节面I的优势方向与断裂走向展布一致性很好;库区地震断层有逆断层、正断层、左旋和右旋走滑断层,走滑断层占45%;库区断层66%陡峻,35%几近直立;受水库蓄水的影响,2006年7月1日后以走滑断层活动方式明显增多,水库地震群时段走滑断层活动方式表现突出。

紫坪铺水库区域地壳Q_s成像及其与渗透关系研究

利用衰减结构层析成像方法尝试对紫坪铺水库区域地壳Qs进行成像,获得该区浅层地壳静态及蓄水前、后的Qs变化特征,结合实验研究成果、岩体岩性、断裂构造、水文地质条件探讨库水的渗透作用及其对地壳介质的影响,对研究区内地震活动类型及发震成因的介质物性变化进行了讨论.研究结果表明:紫坪铺水库区域地壳Qs横向不均匀变化显著,库区周边近似存在低Qs值环形区域,主要包括水库东北、东南、西南、西北及库区中段五个区域.其中,水库东北、库区中段、西南低Qs区域与库水渗透关系密切,初步认为库水可能沿着通济场断裂中段和两端的岩石破碎带及节理、裂隙发育地区向地下渗透,使岩石孔隙中充满流体,内摩擦增大,地震波大大衰减,从而导致Qs值大幅下降.同时,在西南区诱发了水库小震群,在东北区和东南区诱发了部分水库小震群.另外,紫坪铺水库区域东北、东南、西南三个小震群基本位于高Qs值(低衰减)和低Qs值(高衰减)过渡区域,可能由于低Qs区地震波衰减大,不易积累能量,而高、低Qs值的过渡区域,介质介于"软"、"硬"之间,有可能积累应变能,孕育地震.当高、低Qs过渡区域岩石裂纹饱含水或部分含水后,水对裂纹边界起潮湿和润滑作用,降低了发震断层的抗剪强度,使滑动容易产生,从而诱发地震.

“5.12”汶川地震对紫坪铺混凝土面板坝的影响及原因分析

"5.12"汶川大地震导致紫坪铺混凝土面板堆石坝产生了最大为100 cm的沉降和60 cm的水平位移,如此大的地震永久变形,使得上游混凝土面板大面积脱空,库水位以上混凝土面板分期施工缝出现明显错台,面板垂直向接缝发生挤压破坏。在现场调查的基础上,对该坝地震损伤原因进行了分析,并对今后混凝土面板堆石坝的地震反应计算分析研究、抗震设计工作以及紫坪铺混凝土面板堆石坝震后修复原则提出了相应的建议。

汶川地震中紫坪铺混凝土面板堆石坝震害分析

汶川地震中,紫坪铺混凝土面板堆石坝坝坡、面板和结构缝均出现一定程度的破坏,大坝发生了整体变形.根据这些震害现象,结合振动台模型试验和数值分析的成果,从坝坡破坏性态、坝顶加速度反应、面板应力与变形、大坝的地震变形和面板缝的破坏形式几方面讨论了面板堆石坝破坏机理和抗震性能.在此基础上,对面板堆石坝抗震设计的着力点和抗震措施提出了建议:在面板堆石坝抗震设计中,高面板堆石坝上部面板在地震中可能出现的高应力区是着重点之一;应该考虑坝体地震永久变形对面板附加应力的影响;应特别注意坝顶区堆石体的稳定,建议选择钉结护面板加固方案,从而提高地震时坝顶区堆石体的整体稳定性.

紫坪铺大坝汶川地震震害分析及高土石坝抗震减灾研究设想

紫坪铺面板堆石坝作为"5.12"汶川大地震强震区高土石坝的典型,重点总结了其主要震害,包括坝体地震永久变形,面板的脱空与垂直缝挤压破坏和施工缝错台,下游坝坡局部震损,防浪墙等坝顶结构震损,渗漏量变化等;并针对本次汶川地震土石坝震害的特点,提出进一步开展有关高土石坝抗震减灾设计研究的一些设想,包括土石坝震害及地震资料的调研与获取,高土石坝动力破损特征与地震灾害机理研究,高土石坝地震安全评价与震害评估方法研究,以及高土石坝抗震加固及应急处置措施研究等。

汶川地震紫坪铺面板堆石坝地震波输入研究

对国家强震动台网中心紫坪铺面板堆石坝区域台站实测主震记录以及大坝台网实测余震记录进行分析,研究主震与强余震地震动的基本特征。分别选取茂县地办、郫县走石山、成都中和这3组基岩台站实测主震地震动,紫坪铺台站2008年11月6日实测余震地震动以及按水工抗震规范人工生成地震动作为数值计算的地震动输入,对紫坪铺大坝进行三维动力有限元分析,并与实测结果进行对比。研究表明,郫县走石山与成都中和2个远场台站位于断层下盘,其实测地震动的加速度反应谱长周期(0.65 s以后)分量过于丰富,不宜作为断裂带附近紫坪铺大坝的地震动输入;紫坪铺大坝台站实测的余震地震动1 Hz附近(大坝基频)的频率成分相对较少,且持续时间较短,以至于难以激发大坝响应;对比坝顶实测地震动加速度反应谱和数值计算反应谱,建议汶川地震中紫坪铺大坝动力计算时可采用茂县地办台站实测地震动或按抗震设计规范反应谱人工生成地震动。

四川紫坪铺水库库区地震剪切波分裂研究

本研究利用四川紫坪铺水库数字地震台网2004年8月17日～2008年5月11日的地震观测波形资料,使用剪切波分裂SAM系统分析方法,获得了四川紫坪铺水库库区8个数字地震台站的快剪切波偏振结果.结果表明,紫坪铺库区台站的快剪切波偏振优势方向主要为NE或NW方向;台站的快剪切波偏振优势方向与区域主压应力方向或活动断裂走向一致;快剪切波偏振方向变化可能与汶川大地震前区域应力场的增加和龙门山断裂带微破裂增加有关,慢剪切波时间延迟的变化与四川紫坪铺水库水位升降变化相关.

汶川8.0级地震前紫坪铺水库小震活动及震源参数研究

利用区域和水库地震台网记录的数字地震波资料,研究了汶川8.0级地震前的2000年1月1日-2008年5月11日紫坪铺水库区的地震活动及震源参数。结果表明：汶川8.0级地震前该区域小震活动持续,地震活动水平为ML2-3,自2005年9月紫坪铺水库蓄水以来,特别是2008年2月地震活动明显增加,但从长期的序列来看,地震活动频次和强度仍在区域地震活动的正常起伏范围。汶川8.0级地震发生前近3个月内,中小地震的视应力差值Δσapp普遍大于0,计算得到的视应力总大于其拟合值。汶川8.0级地震震中位于低应力区,而相对较高的视应力差值分布在震中以东地区,局部出现视应力增加的现象。