推荐回答(2个)
流砂与管涌的区别与联系
一、流砂
渗流力:地下水在土体中流动时,由于受到土粒的阻力,而引起水头损失,从作用力与反作用力的原理可知,水流经过时必定对土颗粒施加一种渗流作用力。
在向上的渗流力作用下,粒间有效应力为零时,颗粒群发生悬浮、移动的现象称为流砂现象,或流土现象。
这种现象多发生在颗粒级配均匀的饱和细、粉砂和粉土层中。它的发生一般是突发性的,对工程危害极大,流砂现象的产生不仅取决于渗流力的大小,同时与土的颗粒级配、密度及透水性等条件相关。
流砂现象的防治原则是:
①减小或消除水头差,如采取基坑外的井点降水法降低地下水位,或采取水下挖掘;
②增长渗流路径,如打板桩;
③在向上渗流出口处地表用透水材料覆盖压重以平衡渗流力;
④土层加固处理,如冻结法、注浆法等。
二、管涌
在渗透水流作用下,土中的细颗粒在粗颗粒形成的孔隙中移动,以至流失;随着土的孔隙不断扩大,渗透速度不断增加,较粗的颗粒也相继被水流逐渐带走,最终导致土体内形成贯通的渗流管道,造成土体塌陷,这种现象称为管涌。可见,管涌破坏一般有个时间发展过程,是一种渐进性质的破坏。
在自然界中,在一定条件下同样会发生上述渗透破坏作用,为了与人类工程活动所引起的管涌相区别,通常称之为潜蚀。潜蚀作用有机械的和化学的两种。机械潜蚀是指渗流的机械力将细土粒冲走而形成洞穴;化学潜蚀是指水流溶解了土中的易溶盐或胶结物使土变松散,细土粒被水冲走而形成洞穴,这两种作用往往是同时存在的。
土是否发生管涌,首先取决于土的性质,管涌多发生在砂性土中,其特征是颗粒大小差别较大,往往缺少某种粒径,孔隙直径大且相互连通。无粘性土产生管涌必须具备两个条件:①几何条件:土中粗颗粒所构成的孔隙直径必须大于细颗粒的直径,这是必要条件,一般不均匀系数>10的土才会发生管涌;②水力条件:渗流力能够带动细颗粒在孔隙间滚动或移动是发生管涌的水力条件,可用管涌的水力梯度来表示。但管涌临界水力梯度的计算至今尚未成熟。对于重大工程,应尽量由试验确定。
防治管涌现象,一般可从下列两个方面采取措施:①改变几何条件,在渗流逸出部位铺设反滤层是防止管涌破坏的有效措施。②改变水力条件,降低水力梯度,如打板桩。
区别:(1)流砂发生在水力梯度大于临界水力梯度,而管涌发生在水力梯度小于临界水力梯度情况下;(2)流砂发生的部位在渗流逸出处,而管涌发生的部位可在渗流逸出处,也可在土体内部;(3)流砂发生在水流方向向上,而管涌没有限制。
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