ويژگيهاي زمين شناسي مهندسي آبرفتهاي درشت دانه و سيماني شده تهران

Engineering geology aspects of cemented coarse grained alluvium of Tehran

The coarse-grained alluvium of Tehran area has undergone cementation processes through the time that notably affects its strength. The rate of cementation is influenced by many factors including original texture, type and shape of grains, geological history, ground water conditions, tectonically pressures, depth and weathering processes. The main geotechnical features of the alluvium such as strength, stiffness, bearing capacity, and stability of slopes and underground excavation is directly related to the degree of cementation of this deposit. This paper reviews the process of cementation of Tehranʹs alluvial deposit and considers the effect of cementation on the mechanical behaviour of the deposit. Cementing material of Tehran alluvium predominantly was deposited by groundwater. The composition of the cement material is carbonated and is mainly consisted of calcite. The original texture and gradation of the soil has a considerable effect on cementation process and strength. When the content of fine materials is less than about 15%, the cement material bonds directly the coarse grain particles. On the other hand, when the content of fine grains exceeds 15%, the contacts between coarse particles is dominantly by that material, so the strength of deposit is mainly affected by fine materials. The results of in-situ and laboratory tests show that the cementation increases the strength and stiffness of soil, and changes the ductile behaviour of soil to brittle during shear. Moreover, the cementation increases the failure envelope curvature.