Factors controlling building susceptibility to earthquakes: 14-year recordings of Islamic archaeological sites in Old Cairo, Egypt: a case study

Abstract

Cairo City has a large number and different forms of Islamic archaeological sites, in particular, at El-Gammalia and El-Moez streets, as well as Coptic archaeological sites, e.g. at Mari Gergis. Human interference and activities at these historical areas resulted in flooding such sites’ foundations with domestic water, deteriorating its basal courses by salt weathering. The 1992 earthquake is another natural environmental hazard severely affecting many of these sites. The aim of the current study is to examine some factors (of bedrock and buildings) that are expected to control building susceptibility to damage by earthquakes by taking 38 Islamic archaeological sites in the El-Gammalia area as a representative case study. Detailed field recordings of site damage category before and after the quake and continued recording of damage features generated by the 1992 quake over the last 14 years, measuring depth to sub-surface water, measuring buildings’ height before the quake and bedrock sampling at these sites for geotechnical investigations were all considered for achieving this aim. The data has been processed mathematically and graphically (using the Excel package) to examine the main factors responsible for building susceptibility to damage by earthquakes. The selected archaeological sites give an excellent representation of the factors controlling building susceptibility to damage by quakes; it is found that the sites with heights (before the quake) ranging from 12 to 14 m are the most affected ones; the sites with the highest damage category before the quake were more susceptible to more damage by the quake; the sites that had been built on alluvium soil were more affected than those built on the Eocene limestone. The age of these sites has, to a small extent, indirect control on sites’ susceptibility to damage by the quake, particularly in parts flooded with domestic water (i.e. affected by salt weathering). The depth to sub-surface water is an effective parameter on sites’ basal courses (through salt weathering), which, indirectly, control a building’s susceptibility to quakes, particularly where the depth of water ranges from 0.6 to 1.6 m in alluvium bedrock. The alluvium soil at the study area has a liquid limit ranging from 62% to 82%, plastic limit from 37% to 86% and plasticity index from 26% to 46% and free swelling from 27% to 81%. These geotechnical limits for such alluvium bedrock indicate that its clay minerals are mostly montmorrillonite.