Standardization of the strength of large-lump soils

Abstract

Today’s continuous increase in prices paid for mineral fuel, and stringency of requirements to protect the environment from pollution by hot-house gases throughout the entire world is promoting growing interest in the development of water power. In Russia, for example, construction is therefore being planned for a number of new HPP, and prospects for resumption of construction on the Rogunskaya HPP in Tadzhikistan and the Kambartin HPP in Kirgizia are being actively revisited. High dams, including those built of earthen materials are a component part in all large-scale HPP. The most important problem encountered for these dams is the assurance of their reliability and safety; this is achieved primarily by the strength of the soils employed. Large-lump soils, the volumes of which are frequently very large, are some of the basic materials utilized for the construction of high earthen dams. Thus, the volume of soils in the Nurek and Rogunskaya (designed by the Sredazgidroproekt) Dams is 54.2 and 72.1 million m3, respectively. Although man has used conventional soils as a construction material for several thousands of years, large-lump soils, the size of individual lumps of which reaches 500 mm and more, has come into use only recently since the middle of the last century after the appearance of heavy-duty construction equipment capable of excavating and transporting them, and placing and compacting them in a structure [1, 2]. The two basic types of large-lump spoils are gravellyconglomerate mixtures, and a quarried mass of broken stone. Here, the strength of large-lump soils has been standardized only for sandy and gravelly-sandy soils for hydraulicked dams, whereupon standardization has been to a large degree conditional for densities that vary over a rather broad range. It is also possible to note that the design strength for these soils has also been established with insufficient accuracy; this has necessitated its substantial reduction in Construction Rule and Regulation 2.06.05–84 as compared with Construction Rule and Regulation II-53–73 (Table 1). As for large-lump soils in rolled filled dams, the Constructions Rules and Regulations recommend the density be determined from results of direct tests of the soils, or their standard mixtures. Strength investigations for large-lump soils are very labor-intensive, and require considerable time; tests with these soils are therefore conducted only within the framework of plans for specific large-scale structures, and in very limited volumes. Due to the high labor intensiveness and duration of direct strength tests for large-lump soils, these investigations for each specific case are, as a rule, restricted to the testing of only a small number of specimens (3 – 5) for different normal-stress values o. Experimental data are reduced in o – o coordinates. The resultant o = f(o) line or curve also serves as the basis for determination of the desired strength parameters tan o and c. Here, the standard strength is assumed equal to the average value. Moreover, the design strength is generally conditional in nature, since it is impossible here to satisfy requirements of the Construction Rules and Regulations regarding its level of significance, for which the 90-percentile strength, or strength corresponding to a confidence level of 0.95 is adopted. A large number of points would be required for this purpose. And mainly, all test points on the experimental o = f(o) curve are, as a rule, obtained during the testPower Technology and Engineering Vol. 43, No. 2, 2009