Study on Mechanism of Piping and Its Preventive Methods

渗透破坏是高地下水位部位的水工建筑物、围堰中存在的问题。一旦发生渗漏破坏，从经济角度看，会对工程本身和/或邻近地区造成严重破坏。在渗透破坏现象中，还有许多未知的问题需要弄清楚。本文主要研究围堰内土体的渗透破坏问题。首先对Harza和Terzaghi提出的渗透破坏理论进行了详细的分析，提出了一种计算围堰内砂土沸腾稳定性的新方法--棱镜破坏。应用这些理论对典型的三个实例(各向同性和反各向同性单层土和双层土)进行了分析，准确地阐明了这些渗流破坏理论的特点。报道了8个渗流破坏的实例，并对这些问题进行了…分析给出了更多。对这些实例渗漏破坏的可能原因进行了讨论，并将其归类为：(1)设计错误或采取了错误的对策。(2)出现意外情况。(3)蠕变长度过短。(4)地基不均匀(包括多层土)。(5)渗流三维集中进入围堰围堰周围的土体中。(6)土体渗透性具有很强的各向异性。(7)深基坑开挖过程中岩基风化。(8)降水过程中，渗流集中进入围堰内构筑物的末端。(9)随着时间的推移，滤池发生堵塞。最后，进行了47例围堰内土体渗透破坏试验。分析了土体的状态(均质、各向异性)、土体表面形态的变化和等位势的分布、水力水头差与流量的关系、土体开始变形和破坏的水头差以及土壤在水中的休止角。根据棱镜破坏(摩擦)估计土壤开始变形的水力水头差。土的相对密度对破坏方式、变形时和破坏时的水头差有影响。较少

Seepage failure is a problem in hydraulic structures, cofferdams at the location of high ground water level. Once the seepage failure occurs, it causes serious damage to the work itself and/or neibouring places from the economical point of view. There are still many problems unknown, in the seepage failure phenomena, which have to be made clear.In this study, the seepage failure problem of soil within a cofferdam was mainly considered. First the existant theries of seepage failure given by Harza and Terzaghi are investigated precisely, and Prismatic failure -A new method of calculating stability against boiling of sand within a cofferdam is presented. Typical three examples (isotropic and antisotropic one-layred soils, and two-layred soil) were analyzed using these theories, and the characteristics of these theories of seepage failure were then clarified precisely.Second eight examples of case histories on seepage failure were reported and analytical considerations of these problems we … More re given. The possible causes of seepage failures for these examples were discussed and classified as follows : (1) Desige was mistaken or wrong countermeasures were taken. (2) Unexpected situation occurred. (3) Creep length was too short. (4) Inhomogeneous subsoil (including multi-layred soil) was encountered. (5) Seepage flow concentrated three dimensionally into the soil surrounded by a cofferdam. (6) Permeability of soil was highly anisotropic. (7) Rock foundation was weathering in deep excavation. (8) Seepage flow concentrated into the end of a structure constructed within a cofferdam during dewatering. (9) Filter was clogging with the passage of time.Finally, 47 cases of experiments were made on seepage failure of soil within a cofferdam. Analysing the state of soil (homogeneity, anisotropy of soil), changes in formation of soil surface and distribution of equipotentials, the relationship between hydrautic head difference and discharge, hydraulic head differences at which soil starts being deformed and failure occurrs, and angle of repose of soil in water. The hydraulic head difference at which the soil starts being deformed is estimated from Prismatic failure (Friction). Relative density of soil makes effects on modes of failure, hydraulic head differences at being deformed and at failure. Less