Density and strength of dams of earth materials

Abstract

Compaction of the soil during construction of dams of earth materials (earth and earth--rock dams)is one of the most important technological operations. The density of the soil in the structure is a generalized characteristic of other important physical and mechanical properties of the material and structure as a whole. 1.Effect of Density on the Properties of Earth Materials Reduction in Compressibility. Many soils are able to change their volume to a considerable extent when a load is applied to them, for example, as the height of an earth-fill structure increases. Large and differential settlements of homogeneous dams on a compressible foundation with the formation of cracks at the crest are the result of insufficient compaction of the dam soil during construction.It is especially important to attain the required density in the cores of earth and earth--rock dams. Due to the fact that the periods of consolidation of the soils in the shoulders and clay core are different, a decrease in vertical stresses occurs in the core owing to differential settlements of the core and shoulders and tile danger of socalled hydraulic splitting of the core arises with the formation in it of a horizontal through crack (Boulderhead dam in England and certain others) [1,2].It is known that noncohesive soils when first saturated with water slump with a rapid decrease in the volume of voids. A marked intensification of settlements upon filling a reservoir is observed on almost all rolled-fill earth dams with vertical cores whose upstream shoulders are made of rocks or sand--gravel alluvium or other such materials. On the Infiernillo dam (Mexico) settlements occta~ed during 10 days of filling the reservoir to half of the dam height which were equal in magnitude to the settlements measured dm-ing the 6-month period of observations after completing its construction [3]. During settlements of the upstream shoulders their upper part shifts toward the reservoir and substantial vertical deformations of the shoulder and core are observed. Such deformations occur nonuniformly over the length of the dam, especially in the case of a variable height of dam, which occurs in mountain regions.Regardless of the design of earth dams. the elevation of the crest of which for economic considerations slightly exceeds the maximum water level in the reservoir, the material in the dam should be compacted to such a degree that settlement to permit overflow of water is not a11owed. The method of calculating settlements and slumping of especially high dams in seismic regions is so far from perfect that one cannot rely on the accuracy of the calculated settlements.Increase in Strength. The most important result of compacting soils is to increase their shear strength, which determines the slopes of the dam and ultimately its volume and cost. Figure 2 shows the results of investigating the shear strength of gravel--pebble material placed in the dam of the Nurek hydroelectric station as a function of its density,* The material is a mixture of gravel and pebbles w...