Seismic Microzonation of Central Khartoum, Sudan

A preliminary seismic microzonation of Central Khartoum, Sudan is proposed. Khartoum, the capital of Sudan, is located at the confluence of White and Blue Niles. The city is heavily populated. The Central Khartoum with its high rise buildings is the center of governmental and business activities and is located on strip adjacent to the Blue Nile. Geological and geotechnical data indicated that the subsoil conditions at Central Khartoum are characterized by alluvial deposits underlain by Nubian Sandstone below a depth of 20 m. The alluvial deposits locally known as Gezira formations, consist of clays grading into silt and sand with depth. Macro seismic zonation of Sudan and its vicinities, developed by the authors, gave the ground acceleration at the bed rock surface. The effect of alluvial deposits at Central Khartoum on propagation of seismic motion parameters to the ground surface is investigated in this study. Correlations are proposed for pertinent cyclic soil properties such as shear modulus, damping, and shear wave velocity. The classical shear beam model developed by Idriss and Seed is used to study the effect of local soil conditions on ground motion parameters. In absence of strong motion records, artificial time histories of ground motion parameters are used. Plots showing the time histories of ground motion parameters at the ground surface are obtained. The results indicated amplification of ground acceleration of up to 1.15. Because of the presence of saturated loose to medium dense sand at some locations within Central Khartoum, the risk of earthquake-induced liquefaction is evaluated. The susceplity of subsoils in Central Khartoum to liguefaction is evaluated probabilistically by modifying the classical method developed by Seed and Idriss. The risk of earthquake-induced liquefaction is computed by combining the seismic hazard and the conditional probability of liquefaction. The study showed that the risk of liquefaction is low