反滤层粒度组成及厚度对大坝渗流特性的影响

Influence of grain composition and thickness of filter layer on seepage characteristics of dam

反滤层的粒度组成和反滤层厚度及其结构对大坝渗流特性具有显著的影响。针对某黏土心墙砂砾石坝,设计了黏土心墙两侧反滤层材料的颗粒级配范围线(上、下包线),配制了材料颗粒级配,并对其进行了4组渗透变形试验,获得了试验材料的临界坡降、渗透系数和渗透破坏形式。根据渗透试验结果和双层反滤结构,采用有限元数值方法对两层反滤料厚度的11种组合(厚度变化范围为0.5~2.5 m)的大坝渗流特性进行了模拟分析。结果表明:(1)反滤层的第一层粒度较细,包线内土料粒度由细变粗时,对渗透系数的影响较小,对临界坡降和破坏坡降的影响较大,且破坏类型由流土变为管涌形式;第二层反滤砂砾石颗粒较粗,且粒径5 mm以下细颗粒含量很少,渗透特性取决于粗粒材料的含量,为过渡型破坏类型。(2)当双层反滤层总厚度不变(厚度3 m)时,改变反滤层的厚度组合和粒度组合,对大坝单宽渗流量和心墙出逸点高程的影响较小。第一层反滤料厚度从0.5 m增加到2.5 m,心墙和反滤料的出逸比降均呈非线性增长,第一层反滤料出逸比降增幅为92.7%,第二层反滤料的出逸比降增幅为70.0%。第一层反滤料粒度变化比第二层反滤料粒度变化对心墙和反滤料出逸比降的影响小。(3)反滤层厚度保持3 m时,建议第一层反滤料厚度取1.0~1.5 m,相应地第二层反滤料厚度取2.0~1.5 m;粒度组成选取两层反滤料都靠近下包线位置,即粒度较粗为最优。

The grain size composition and thickness of filter layer and its structure have significant influence on the seepage characteristics of dam.For a clay core dam project,the particle gradation of filter material on both sides of the clay core(upper and lower cladding lines)was designed,and the particle gradation of the material was prepared.Four groups of seepage deformation tests were carried out,and the critical hydraulic gradient,permeability coefficient and seepage failure types of the test materials were obtained.The double-layer filter structure was designed according to the test results.Eleven combinations of the thickness of the two-layer filter material(the thickness vary from 0.5 m to 2.5 m),the seepage characteristics of the dam were analyzed by finite element method.The results show as follows:(1)The grain size of the first filter is fine,there is little influence on the permeability coefficient of the particle size in the inclusion line of soil material,the influence on critical hydraulic gradient and failure gradient is large,and the form of seepage failure is changed from soil flow to piping;the second filter sand gravel particles are coarse,and the content of fine particles below 5 mm is very small,and the permeability characteristic is determined by the content of coarse grain material,which is transitional failure.(2)When the total thickness of the filters is invariant(3 m),the change of the thickness combination and particle size of the two filters has little effect on the seepage per unit width and the height of the overflow point of the core.The first filter thickness increases from 0.5 m to 2.5 m,the core and the filter material exit gradient shows a nonlinear growth.The exit gradient of the first filter is increased by 92.7%,and second filter by 70%respectively.The particle size change of the first filter is less than that of the second filter.(3)The thickness of the first filter is suggested to be 1.0~1.5 m,the corresponding thickness of the second filter is 1.5~2.0 m;and the particle size selection of both layers of filter material is close to the lower envelope line,and the coarse particle size is the best.