基于汶川地震震害经验的土石坝抗震设计规范修编

基于汶川等地震震害、地震响应、震害机理、安全评价等方面的研究成果,主要从4个方面论述了《水工建筑物抗震设计规范》修编中有关地基和土石坝抗震设计的进展。1抗震设防标准及基本原则:调整了抗震设计标准和设计地震的确定方法;对特别重要工程按最大可信地震校核不发生库水失控下泄灾变安全裕度;一定情况下设计地震反应谱的确定需要考虑地震近场效应、上盘效应及频谱不平稳性等;对抗震甲类工程抗震设计需制定防震减灾应急预案;规定了土石坝地震反应谱,取加速度最大反应谱值max?=1.60,对应的阻尼比为20%。2场地与地基:调整了工程建设场地分类方法,场地土类型由4类调整为5类,场地类别划分更为详细;对标贯击数判别液化方法中的液化临界标贯击数计算公式进行了调整,并修订了考虑上覆有效应力影响的标贯击数校正公式。3土石坝抗震计算与安全评价:扩充了抗震计算的内容要求,规定抗震计算包括抗震稳定计算、永久变形计算、防渗体安全评价和液化可能性判别等内容;扩大了要求采用动力法进行地震作用效应分析和安全性评价的范围,规定对设计烈度Ⅶ度且坝高150 m以上,设计烈度Ⅷ、Ⅸ度且坝高70 m以上,地基中存在可液化土层等3种情况,应同时进行基于有限元法的动力分析,对覆盖层厚度超过40 m的土石坝宜进行动力分析;补充了对土石坝地震作用效应进行动力分析的原则要求,包括本构模型选取、计算参数确定、残余变形计算、稳定分析要求等;补充规定了根据动力法成果对土石坝的抗震安全性进行综合评价的原则要求。4面板堆石坝抗震工程措施:主要根据紫坪铺大坝设计、建设及震害经验,增加了针对面板堆石坝的抗震工程措施,包括坝体地震变形控制、坝顶及其附近坝坡防护、面板及垂直缝抗挤压、水平施工缝抗错台及接缝细部构造设计等。

Advances in seismic design of earth-rockfill dams and foundations in China and revision of code based on lessons from Wenchuan Earthquake

The advances in seismic design of earth-rockfill dams and foundations and the revision of code for seismic design of hydraulic structures in China (SL203-97/DL5073-2000) are presented based on research results and lessons from Wenchuan Earthquake disasters. The main revisions and advances focus on four aspects: (1) Seismic fortification criterion and its basic principles, in which the seismic design criterion, design parameters of ground motions and method for determining seismic waves are revised. For especially important projects, the seismic safety margins of the structures under MCE shall be assessed in order to prevent any collapse resulting in reservoir water discharging catastrophically. The near-field effect, hanging wall effect and unsteadiness of the frequency spectrum shall be considered when determining the design earthquake response spectrum. Earthquake prevention and disaster mitigation emergency contingency plans are required for the structures with engineering seismic fortification category of Class A. The seismic response spectrum for earth-rockfill dams is stipulated, and the value of the maximum acceleration response spectrum ( ) is taken as 1.60, whose corresponding damping ratio is 20%. (2) Site and foundation, in which the criterion for classification of site soils and site itself are revised. Site soils are classified into 5 types instead of 4 types in the old code according to the shear wave velocity of soil layer, and are classified in more detail than before according to the type of site soil and overburden thickness. For the liquefaction potential evaluation method, the formula for determining the critical standard penetration test N-values (SPT N-values) and the formula for correcting the influence of the effective overburden stress on SPT N-values are revised. (3) Seismic calculation and safety evaluation of earth-rockfill dams, in which contents for seismic calculation are supplemented. Seismic calculation should include seismic stability calculation, permanent deformation calculation, safety evaluation of the anti-seepage body and liquefaction potential evaluation. The scope for adopting the dynamic analysis method for the analysis of seismic effect and safety evaluation is supplemented. For the earth-rockfill dams with the following conditions: a) higher than 150 m and with a design intensity of VII; b) higher than 70 m and with a design intensity of Ⅷ or Ⅸ; and c) presence of liquefiable soils in the foundation based on finite element method is necessary for dynamic analysis to comprehensively evaluate its seismic stability. For the earth-rockfill dams with deposits thicker than 40 m, finite element dynamic analysis is recommended, and the principles for dynamic analysis of earth-rockfill dams are provided, including selection of constitutive model and determination of model parameters, calculation of the permanent deformation and requirement for stability analysis. The basic principles for evaluating seismic safety of earth-rockfill dams according to the dynamic results are provided. (4) Earthquake resistant measures for earth-rockfill dams and foundations, in which earthquake resistant measures for CFRD are proposed according to the experience of the design, construction and lessons from earthquake damages of Zipingpu Dam in Wenchuan Earthquake, including seismic deformation control, protection of dam crest and nearby slope, anti-squeezing of vertical joints, anti-dislocation of horizontal construction joints and detailed structure design of joints.