The last 10–15 years in our country and abroad are marked by a greater interest to weak carbonate rocks (chalks, chalky marls, certain types of limestones and dolomites) in connection with their use as the medium or foundation for large structures and as natural building materials. The example is the successfully operating Euphrates Project founded on chalks, the largest hydroelectric project in the Middle East, built by the Soviet design and technical assistance.

Great volumes of chalks and chalky rocks were placed in the bodies of the Gavins Point, Fort Randall, Harlan Country dams in the USA and the dam of the Euphrates Project in Syria and a great many complex estimates have been made and complex technical problems have been solved. There can be no doubt that in the near future chalk or chalky rocks will be used in other structures under the similar conditions.

Owing to some specific peculiarities the chalks and chalky rocks feature rather a wide range of variation of physical and mechanical properties and interaction with water. To some extent the variation is conditioned by tectonics and epigenetic processes; however, the main salient features of chalks and chalky rocks develop during the sedimentation of carbonate silt and its early diageneaia. In this connection to understand the features of the rocks and use them properly in the engineering practice, it is necessary to know their composition, microstructure and formation conditions. Only based on these data it is poassible to study comprehensively the weak carbonate rocks and analyze and generalize the available data. According to the data of scanning electron microscopy, the chalks and chalky marls consist mainly of calcite shells of single-celled cocco- lithophorid algae or their fragments - crystallites ("powder-like calcite"), and their content and proportions in the rock can differ. In addition to the above components there are usually other admixtures, both organic and inorganic ones. The first ones contain mainly foraminifer shells and their fragments (from fractions of per cent to 10- -15 per cent and 40 per cent as an exception), or inoceram shells (approximately the same percentage). The remains of other organisms (echinoids, crinoida, sponge speculea etc.) make up usually fractions of per cent and have no appreciable influence on the properties of rocks, so they can be practically ignored. The inorganic admixtures are represented on the main by clay material, in which the minerals of the montmorillonite group, mixed-layered formations and hydromica are usually the prevailing ones. The other clay minerals(kaolinite minerals, chlorite and others)are encountered comparatively rarely and in small amounts. The presence of other authigenous and terrigenous admixtures, such as zeolite, glauconite, metal oxides, dolomite, barite, celestine quartz etc., is rather insignificant and they are mainly of genetic origin. However, in some typical varieties of chalks and chalky marls their percentage can be rather high. For instance, the dolomitic chalks of the Oligocene deposits of the Arabian platform contain a considerably greater amount of dolomite(5); while the sandy chalks contain a great amount of terrigenous admixtures (quartz, feldspar and glauconite(10).

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