GEOTECHNICAL AND ENVIRONMENTAL HAZARDS IN DESERT NEW CITIES - A CASE-STUDY OF EL MINIA EL GEDIDA SITE, EGYPT

The plateau east of the Nile Valley at El Minia, Egypt, has been chose n for the siting of El Minia El Gedida city. Problems of underground c aves suggested the need for integral geological, geotechnical and envi ronmental hazard studies to examine the suitability of the area as a b uilding site. Geologically, the area is formed of a thick sequence of Middle Eocene fossiliferous limestones and chalk with some layers of s andy, cherty and marry limestones. These rocks have heterogeneous phys ical and mechanical properties and fall within the weak to very weak r ange of limestones. Their mode of failure is almost of the brittle typ e developing extension, wedge and single shear type fractures. The mec hanical properties are controlled by their physical, geological and mi neralogical properties. They increase with density and dolomite conten t and decrease with porosity and calcite content. Empirical equations for some of these relations as well as the relation between field meas ures of rebound index and compressive strength are presented. Both reg ional and detailed structural analyses revealed the presence of major and minor normal faults and fractures trending in NW and NE directions . The NW faults are dominant. They define the topographic escarpments in the area. Most of the caves are aligned along major faults of this trend. Faults, fracture distribution and structural hazard maps of the area are presented. Paleostress tectonic analysis of these fractures suggests that the NW and NE faults are extensional due to tectonic str ess which changes from normal to parallel to the Red Sea at different geologic times starting in post Middle Eocene. The similarity of this paleotectonic setting to present Red Sea tectonics points out the poss ibility of future rejuvenation of these faults. Topographically, the a rea lies in shallow surface drainage basins. The topographical risk is increased by fracturing and it should be considered if the area is su bjected to a long period of heavy rainfall. Radiometrically, El Minia El Gedida city represents one of the lowest natural radioactivity area s within the safe limits recommended by the International Commission o n Radiological Protection (ICRP).