Method of calculating seepage through a high earth dam with cracks in the core

Abstract

In mountainous regions where it is usually difficult to haul in construction materials, earth dams made of local materials are the most expedient type of retaining structures. The impermeability of high dams is most often secured by creating a cutoff of low-permeability soil. In regions with high seismic activity additional requirements are imposed to assure reliability of the structures. In this case the stability of darns should be secured by mandatory consideration of factors determining both the long-term and short-term adverse effect on their behavior.The practice of constructing and operating dams shows that among the most unfavorable short-time factors adversely affecting the reliability of high dams is disruption of the continuity of the cutoff element by the formation in it of longitudinal and transverse cracks caused both by nonuniform deforrnadons of the dam during the postconstruction period and by earthquakes during operation. An underestimation of such damages when designing dams can be dangerous.The considerable adverse effect of cracks on the behavior of structures is confirmed by the published data of an analysis of the causes of earth dam damage in the USA [1]. These data show that about 40% of all such damage is due to seepage through dams, about 30% is due to construction defects, and 30% is due to unsatisfactory operation of the discharge devices. All those cases which were accompanied by a noticeable increase of seepage and the occurrence of piping are considered to be seepage-caused. In our opinion, the effect of seepage, and consequently the formation of cracks causing it, on the failure of dams was responsible in a much greater number of cases than has been recorded by investigations. Actually, it has been possible to notice the formation of cracks only in those dams where by virtue of certain circumstances these cracks were exposed and caused serious operational impairment tothe structure, and then its damage.Longitudinal cracks form usually due to differences in compressibility of the soils in the core and transitional zones, and also nonuniform temporal self-consolidation of clay, especially when the core is wetted on the upstream side during filling of the reservoir. The occurrence of transverse cracks is due mainly to nonuniform deformation of the body and foundation of the core over its length, especially under the effect of seismic forces [2]. The character and size of these cracks are determined by a number of factors, including the canyon outline at the dam site, height of the structure, and deformative properties of the soils in the body and foundation of the core. Longitudinal cracks can have only an insignificant adverse effect on the operation of the core as a cutoff element, and therefore they can be neglected when calculating seepage through it.The most dangerous disruption of the continuity of the core occurs with the appearance of transverse cracks, which propagate in a direction parallel to the seepage movement through the core and threaten to produce the direct communic...