Chapter3 Site

,*,单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,SCHOOL OF URBAN CONSTRUCTION, YANGTZE UNIVERSITY,1,Main contents of this course,Chapter 1 Introduction of Earthquake,Chapter 2 Seismic Conceptual Design,Chapter 3 Site, Subsoil And Foundation,Chapter 4 Earthquake Action,Chapter 5 Seismic Design of RC Structure, Chapter 6 Seismic Design of Multi-story,Masonry Structure, Chapter 7 Isolation And Energy Dissipation,1,Chapter 3 Site, Subsoil and Foundation,3.1 Site 场地,3.2 Construction site categories,3.3 地震时地面运动特性,3.4 Seismic checking for subsoil and foundation,3.5 Soil Liquefaction,3.6,桩基的抗震验算,2,Some words,Subsoil,subgrade地基,foundation基础,Fault rupture 发震断裂,Natural period 自振周期,overlaying thickness覆盖层厚度,3,概述,从破坏性质和工程对策角度，地震对结构的破坏作用可分为两种类型：,场地、地基的破坏作用和场地的震动作用,。,(1)场地和地基的破坏作用一般是指造成建筑破坏的直接原因是由于场地和地基稳定性引起的。,场地和地基的破坏作用大致有地面破裂、滑坡、坍塌等。,这种破坏作用一般是通过,场地选择,和,地基处理,来减轻地震灾害的。,(2)场地的地震动作用是指由于强烈地面运动引起地面设施振动而产生的破坏作用。,减轻它所产生的地震灾害的主要途径是,合理的进行抗震和减震设计和采取减震措施,。,为此要,确定工程场地的设计地震动参数。,4,3.1 Site,场地,场地 site,Site means the places have similar response spectra. Its range is equal to a factory area, a living area, natural country area, or area not less than 1.0km,2,.,工程群体所在地，具有相近的反应谱特征,其范围相当于厂区、居民点或自然村或不小于1.0km,2,平面面积。,Favorable plats should be selected while unfavorable plats shall be avoided except appropriate seismic measures have been takenThe buildings assigned fortification category A, B and C shall not be constructed on the hazardous plats.,5,3.1 Site,地段类别,地质、地形、地貌,favourable plat,有利地段,稳定基岩，坚硬土，开阔、平坦、密实、均匀的中硬土等,Common plat,一般地段,不属于有利、不利和危险的地段,unfavourable plat,不利地段,软弱土，液化土，条状突出的山嘴，高耸孤立的山丘，非岩质的陡坡，河岸和边坡的边缘，平面分布上成因、岩性、状态明显不均匀的土层（如故河道、疏松的断破裂带、暗埋的塘浜沟谷和半填半挖地基），高含水量的可塑黄土，地表存在结构性裂缝等,hazardous,plat,危险地段,地震时可能发生滑坡、崩塌、地陷、地裂、泥石流等及发震断裂带上可能发生地表错位的部位,6,水边地的地下水位较高，土质也较松软，容易在地震时产生土壤滑动或地层液化。,山坡地在地震时会产生土壤滑动,用另外的土石来填补地基，常有土壤密实度不足情形，导致建筑物在地震时产生倾斜、沉陷。,冲积地的土质松软，地震时容易塌陷，如果此处有地下水层，还容易发生液化。,7,临近悬崖，,容易滑落,谷地或低地，这里的建筑物容易在地震发生时，受土石崩塌破坏。,萨尔瓦多地震引发了一巨大的泥石流，数百户人家被埋在泥石里，估计有1200多人遇难,地裂,8,3.1.1,发震断裂带,的地震影响,活动断裂 active fracture,晚更新世以来有过活动且将来有可能再度活动的岩层断裂。它是地震可能发生地点的重要标志，分为发震断裂与非发震断裂两种。,发震断裂：具有潜在地震活动的断裂。,断裂带是地质上的薄弱环节，浅源地震多与断裂活动有关。,发震断裂带附近地表，在地震时可能产生新的错动，使建筑物遭受较大的破坏，属于地震危险地段。,9,10,3.1.1,发震断裂带,的地震影响,1952年7月21日加州南部克恩县地震，主震7.7级。在白狼断裂带，由于断层破裂和位错而引起的破坏主要局限于4条铁路隧道与此断层交汇处，但附近未与此断层交汇的其他11条隧道破坏轻微，附近的桥梁无破坏。,11,1999年集集地震断裂引起的建筑破坏,12,图10 图9中所表示的建筑物遭受断裂,破坏作用以后的平、剖面示意图,13,断层出露地表一定距离范围内建筑震害,14,3.1.1,发震断裂带,的地震影响,发震断裂带上可能发生地表错位的地段主要在高烈度区，全新世以来经常活动的断裂上面。,一般来说, 地震震级愈高, 出露于地表的断裂错动与断裂长度就愈大 ; 覆盖层厚度愈大, 出露于地表的断裂错动与断裂长度就愈小; 断裂的活动性还和地质年代有关, 对一般建筑工程只考虑全新世,(1万年来）,活动过的断裂, 在此地质期以前已活动过的断裂可不予考虑。,15,3.1.1,发震断裂带,的地震影响,对符合下列规定之一的情况，可忽略发震断裂错动对地面建筑的影响：,（1）抗震设防烈度小于8度；,（2）非全新世活动断裂；,（3）抗震设防烈度为8度和9度时，隐伏断裂的土层覆盖层厚度分别大于60m和90m。,中文教材P25,16,3.1.1,发震断裂带,的地震影响,中文教材P25,17,3.1.2 局部地形的影响,1994年云南昭通地震，芦家湾某村坐落于山梁上，山梁长150m，高60m.距震中18km。,“重灾区里有轻灾，轻灾区里有重灾”,产生的原因是局部地区的工程地质条件不同,烈度为9度,烈度为8度,烈度为7度,1920年海原地震中，位于渭河谷地的姚庄烈度为7度，而2km以外的牛家庄因处于高于百米的黄土梁上，烈度达到9度。,18,3.1.2 局部地形的影响,高突地形距离基准面的高度愈大，高处的反应愈强烈;,离陡坎和边坡顶部边缘的距离愈大，反应相对减小;,从岩土构成方面看，在同样地形条件下，土质结构的反应比岩质结构大；,高突地形顶面愈开阔，远离边缘的中心部位的反应是明显减小的；,边坡愈陡，其顶部的放大效应相应加大。,19,20,3.1.3场地土和水文条件的影响,场地土和水文条件的影响主要体现在场地土坚硬程度、覆盖层厚度(土层的性质、厚度)、场地自振周期和粉、砂土的液化等方面。,21,3.1.3场地土和水文条件的影响,1964年3月27日，美国阿拉斯加州威廉王子海峡发生一次8.4级强烈地震。,强地震动周期与持续时间长。估计安科雷奇市地震动周期大于0.5秒，持续时间达3分钟左右。许多自振周期长的十至十四层房屋震害严重，而强度低的低层砖石房屋却几无损坏。,液化引起的震害严重。安科雷市最大灾害来自四个大滑坡，滑坡原因在于地下18米处有一薄砂层，在持久的振动下液化。,22,3.1.3场地土和水文条件的影响,1964年6月16日，日本新泻发生一次7.1级地震。,这次地震的主要特点是砂土液化现象普遍，并使震害加重，河流、港湾、铁路、桥梁、给排水、电力设施等工程设施多见由地基失效引起的破坏现象，如下沉、倾斜、滑坡、变形等；而邻近地区地基土密实，结构破坏甚微，木造房屋屋顶挂瓦也未震落。,23,3.1.3场地土和水文条件的影响,1957年7月28日,墨西哥格雷罗海湾附近发生7.5级强震。,根据地基土壤情况，墨西哥市可以分为A、B、C三个区。A区位于古湖床上，地表土层为10230米厚的火山灰软粘土，可压缩性很高，平均含水量达300%，近地表土的抗压强度为60kPa，地表千米下为基岩。C区为火山坡前的低山地带，地基土为凝灰岩、胶结或密实砂土和类似的低压缩性土壤，无侧限抗压强度通常超过500kPa。B区为前两区的过渡区，在密实沉积物之上常有厚度小于10米的低至中等压缩性的砂质粘土或粉质砂土、或高塑性软粘土。,墨西哥市震害较重。在已知的1500栋房屋中，受害较重者近1000栋，其中在低山区（C区）者不到0.3%，在过渡区（B区）者约4%，其余95%均在湖区（A区）；倒塌的房屋都在A区内。墨西哥市的这种震害特点屡见于该地以后每次大地震。,24,3.2 Construction site categories,土的刚度可按土的剪切波速划分.,横波在土层中传递的速度称为土层剪切波速,shear-wave velocity 。剪切波速的大小与土层的密度和剪切模量有关.,25,某浅层粘质砂土，泊松比0.496；压缩模量E=0.045*10,6,kN/m,2,;自重取17.2 kN/m,3, 则剪切波在在该土层中的传播速度,以C20混凝土为介质材料，压缩模量E=25.5*10,6,kN/m,2,;泊松比0.25;自重取24.5 kN/m,3,则剪切波在该混凝土中的传播速度,26,3.2 Construction site categories,表明，介质（土层）的坚硬程度与横波在该介质中的传播速度有着密切关系，波速越快土质越坚硬，反之土层越软弱。,27,Classification of site soil,Type of the soil,Shear-wave velocity of soil layer (m/s),Rock,V,s800,Stiff soil or Soft rock,800,V,s500,Medium- Stiff site soil,500,V,s250,Medium-soft site soil,250,V,s150,Soft site soil,V,s150,The site soil should be classified according to the shear-wave velocity (,剪切波,)of the soil layer.,28,对于场地土为分层土时，需计算其等效剪切波速,average velocity of shear wave in soil layer,Where:,d,0,=,calculated depth, in m; it shall be taken as the minor of both the overlaying thickness and 20m.,d,i,=,the thickness of the,i,-th soil layer within the range of calculated depth, in m.,V,si,=,the shear-wave velocity of the i-th soil layer within the calculated depth, in m/s.,n,= number of soil layers within the range of calculated depth.,中文教材P23,29,3.2 Construction site categories,场地条件对建筑震害的影响因素是：,(1),场地土的刚性（即坚硬或密实程度）大小；,(2),场地覆盖层厚度。,Construction sites shall be classified into four categories according to type of site soil and the overlaying thickness at the site.,30,2.,The thickness of overlaying layer,场地覆盖层厚度,1)一般情况下，应按地面至剪切波速大于500m/s且其下卧各层岩土的剪切波速均不小于500m/s的土层顶面的厚度。,2)当地面5m以下存在剪切波速大于相邻上层土剪切波速的2.5倍的土层，且该层及其下各层的剪切波速均不小于400m/s时，可取地面至该土层顶面的距离确定。,3)剪切波速大于500m/s的孤石、透镜体，应视同周围土层。,4) 土层中的火山岩硬夹层，应视为刚体，其厚度应从覆盖土层中扣除。,31,实例2,32,中文教材P22，英文讲义P22,33,3.3,地震时地面运动特性,场地土的卓越周期,H为场地覆盖层厚度，v为土的剪切波速。,中文教材P30式(2-19),单一土层时,多层土时,34,3.3,地震时地面运动特性,从震源传来的地震波是由许多频率不同的分量组成, 其中在振幅谱中幅值最大的频率分量所对应的周期 , 称为地震动的卓越周期。,场地覆盖土层相当于一个滤波器和放大器，与场地卓越周期一致的频率波将被放大。地震波通过土层后，地表地震动的卓越周期很大程度上取决于场地固有周期。,凡建筑物的自振周期与场地的卓越周期相等或接近时， 则共振效应使得地震效应明显增强。因此，在建筑抗震设计中，应使建筑物的自振周期避开场地的卓越周期，以避免发生类共振现象。,35,Some words,seismic check抗震验算,tensioned stress 拉应力,Saturated饱和的Condensation固结,pore water pressure 孔隙水压力,Discrimination of the liquefaction potential液化的判别,standard penetration tests 标准贯入试验,silt,粉土loess黄土,Epipleistocene of Quaternary (Q3)第四纪晚更新世,groundwater table,地下水位,buried depth of foundation基础埋置深度,characteristic depth 特征深度,36,3.4 Seismic check for subsoil and foundation,3.4.1,need not be checked,由震害调查得到下面结论：只有少数房屋是由地基的原因而导致上部结构的破坏。导致上部结构破坏的地基大多是液化地基、易产生震陷的软土地基和严重不均匀地基。大量的一般性地基具有良好的抗震性能，极少发现因地基承载力不够而产生震害。,我国抗震设计规范对量大面广的一般地基和基础不作抗震验算，对容易产生地基基础震害的液化地基，软土地基和严重不均匀地基规定了相应的抗震措施，以避免或减轻震害。,37,For the following types of buildings, the bearing capacity of natural subsoil and foundation,need not be checked,for earthquake resistance:,(1) Buildings that seismic check for the upper-structure is not necessary in accordant with this code.,抗震规范规定可不进行上部结构抗震验算的建筑,38,need not be checked,(2) Buildings that without soft cohesive soil in the main load-bearing layer of the subsoil;,1)The ordinary single-story factory buildings and single-story spacious houses;,2)Masonry buildings;,3)The ordinary civil framed buildings and frame-wall buildings no more than 8 stories and 24m in height;,4)Multi-story framed factory buildings and multi-story wall buildings with base load equivalent to point 3).,基础荷载与上述民用框架房屋相当的多层框架厂房和多层混凝土抗震墙房屋。,39,3.4.2,The seismic bearing capacity check,Except the building listed above, the bearing capacity of natural subsoil and foundation,need be checked,for earthquake resistance.,应采用地震作用效应标准组合，且,地基抗震承载力在静力设计承载力基础上调整。,40,天然地基抗震承载力验算：,seismic soil-bearing capacity,调整后的地基土抗震承载力；,f,a,=soil-bearing capacity characteristic values after depth and width adjustment,41,the seismic adjusting factor of soil-bearing capacity,42,On the one hand, because of chanciness and instantaneity of earthquake and economy of engineering, the safety factor of seismic design can be less than that of static loading.,On the other hand, because of quick and repeat change of earthquake, it is too late for soil produces enough deformation, so the soils strength in earthquake is greater than strength under static loading.,调整的出发点：,1）地震是偶发事件，地基抗震承载力安全系数可比静载时降低。,2）多数土在有限次的动载下，强度较静载下较高,43,the mean pressure on the base of the foundation and the maximum pressure at the edge of the foundation shall comply with the following equations:,44,3.5,Soil Liquefaction,3.5.1Cause and damage of soil liquefaction,Under earthquake, the grain of,saturated,sand and powder soil under groundwater has the trend to,condensation,. Since the water cannot be expelled on time, the pore water pressure ascends. If the pore water pressure is equal to the pressure, soil grain is in weight-lost status having no pressure to transfer. The relation between grains is destroyed, so soil becomes flowing liquid. The phenomenon is called as soil liquefaction.,45,saturated soil,Soil that has all its micropore and macropore spaces filled with water.,The soil cannot retain additional water until water is deleted from some of these pores.,返回,46,压缩：土在压力作用下，体积将缩小。这种现象称为压缩。,固结condensation ： 土的压缩随时间增长的过程称为固结,返回,47,式中 剪切面上有效法向压应力（粒间压应力）；,剪切面上总的法向压应力；,剪切面上孔隙水压力；,土的摩擦角。,48,唐山地震时，严,重液化地区喷水高度,可达8米，厂房沉降,可达1米。,天津地震时，海,河故道及新近沉积土,地区有近3000个喷水,冒砂口成群出现，一,般冒砂量0.1-1立方,米，最多可达5立方,米。有时地面运动停,止后，喷水现象可持,续30分钟。,液化的震害：喷水冒砂淹没农田，淤塞渠道，淘空路基；沿河岸出现裂缝、滑移，造成桥梁破坏，等等。,49,3.5.1Cause and damage of soil liquefaction,液化使建筑物产生下列震害：,1.地面开裂下沉使建筑物产生过度下沉或整体倾斜；,2.,不均匀沉降引起建筑物上部结构破坏，使梁板等水平构件及其节点破坏，使墙体开裂和建筑物体形变化处开裂；,3.,室内地坪上鼓、开裂，设备基础上浮或下沉。,50,影响,液化的因素,地质年代：,地质年代越古老的饱和砂土越不容易液化,土中粘粒含量：,粉土中粘性颗粒多的要比粘性颗粒少的不容易液化。这是因为随着土的粘聚力增加，土颗粒不容易流失。,土层埋深：,土层液化深度很少超过15m。,土的密实程度：,颗粒均匀单一的土比颗粒级配良好的土容易液化；松砂比密砂容易液化；细砂比粗砂容易液化,上覆非液化土层厚度和地下水位深度,：,随着液化砂土层埋深增大，砂土层上的有效覆盖应力增大，就不容易产生液化。随着地下水位的上升，液化的可能性就越高。,地震烈度和持续时间,：地震烈度越高，越容易发生液化；地震动持续时间越长，越容易发生液化,51,3.5.2 Discrimination of the liquefaction potential,A two-step discrimination method of the liquefaction potential is adopted in the code for seismic design of building .,That is, preliminarily discrimination (初步判别)and the standard penetration tests method(标准贯入试验判别).,52,3.5.2 Discrimination of the liquefaction potential,初步判别的作用：,排除一大批不会液化的工程，少做标准贯入试验。凡经初步判别为不液化或不考虑液化影响的就可不进行第二阶段判别，以节省勘察工作量。,当经初步判别不能排除地基土液化的可能性时，采用标准贯入试验判别。作用是判别液化程度和液化后果，为工程处理措施提供依据。,53,1.,preliminarily discrimination,If one or following condition is satisfied, the,saturated sand and saturated silt,(loess not include) may be primarily discriminated as non-liquefaction or consequence of liquefaction need not be considered:,饱和的砂土或粉土（不含黄土),，当符合下列条件之一时，可初步判别为不液化或可不考虑液化影响：,（1）,The geochron of soil is Epipleistocene of Quaternary (Q3) or earlier it may be discriminated as non-liquefied soils for intensity 7 and 8.,地质所代为第四纪晚更新世Q,3,及其以前时，7、8度时可判别为不液化。,54,1.,preliminarily discrimination,(2) The percentage of contain of clay particles (diameter less than 0.005mm) in silt is not less than 10%, 13% and 16% for Intensity 7, 8 and 9 respectively, it may be discriminated as non-liquefied soil.,（2）粉土的粘粒（粒径小于0.005mm的颗粒）含量百分率，7度、8度和9度分别不小于10、13和16时，可判别为不液化土。,55,1.,preliminarily discrimination,(3)For buildings adopted natural subsoil the consequence of liquefaction need not be considered when,the thickness of the overlaying nonliquefiable soils and the elevation of groundwater table,comply with one of following conditions,（3）天然地基上的建筑，当上覆非液化土层厚度和地下水位深度符合下列条件之一时，不考虑液化：,56,d,w,elevation of underground water level地下水位深度（m），宜按建筑使用期内年平均最高水位采用，也可按近期内年最高水位采用；,d,u,thickness of the non-liquefiable overlaying layer上覆非液化土层厚度（m），计算宜将淤泥和淤泥质土层扣除；,d,b,buried depth of foundation基础埋置深度(m),不超过2m时应采用2m；,d,0,characteristic depth of liquefaction potential soil液化土特征深度,57,上面判别式（d,b,=2)亦可用下图表示：,1 2 3 4 5 6 7 8 9 10,1,2,3,4,5,6,7,8,9,10,d,w,(m),不考虑液化影响区,须进一步判别区,砂土,1 2 3 4 5 6 7 8 9 10,1,2,3,4,5,6,7,8,9,10,d,w,(m),不考虑液化影响区,须进一步判别区,粉土,7度,9度,8度,7度,8度,9度,db2时，在,du,、,dw,中减去（db-2),后再查图确定。,1.,preliminarily discrimination,58,查液化土特征深度表,解：按判别式确定,9m,8m,7m,砂土,8m,7m,6m,粉土,9,8,7,烈度,饱和土,类别,不满足判别式，需要进一步判别是否考虑液化影响。,例 图示为某场地地基剖面图,上覆非液化土层厚度d,u,=5.5m,其下为砂土，地下水位深度,为d,w,=6m.基础埋深d,b,=2.5m,该场地为8度区。确定是否考,虑液化影响。,d,w,=6m,d,u,=5.5m,d,b,=2.5m,59,例2 图示为某场地地基剖面图,上覆非液化土层厚度d,u,=5.5m,其下为沙土，地下水位深度,为d,w,=6m.基础埋深d,b,=2.5m,该场地为8度区。确定是否考,虑液化影响。,解：按土层液化判别图确定,需要进一步判别是否考虑液化影响。,d,w,=6m,d,u,=5.5m,d,b,=2.5m,1 2 3 4 5 6 7 8 9 10,1,2,3,4,5,6,7,8,9,10,d,w,(m),不考虑液化影响区,须进一步判别区,砂土,7度,8度,9度,60,Given.,A drilling geological data are listed in following table. The geochron of soil is after Epipleistocene of Quaternary (Q3). The elevation of groundwater table is near the ground, so dw=0. The buried depth of foundation d,b,=3m. Intensity 7.,Required.,Primary discrimination of liquefaction potential.,61,Solution.,Since the geochron of soil is after Epipleistocene of Quaternary (Q3), so the first item of primary discrimination isnt satisfied.,Intensity 7 and the max percentage of contain of clay particles is 8%, the second item of primary discrimination isnt satisfied either.,As to the third item, dw=0, db=3m;,du= 9.8-2.2 (mucky soil) =7.6m,d0= 7m,The first formula: 7.67+3-2=8,The second formula: 07+3-3=7,The third formula: 7.6500,74,Exercise,3-3 Given.,A drilling geological data are listed in the following table. The geochron of soil is after Q3. The elevation of groundwater table dw=2.0m. The buried depth of foundation d,b,=3m. Intensity 7.,Required.,Primary discrimination of liquefaction potential.,75,76,