Combining geology, geomorphology and geotechnical data for a safer urban extension: Application to the Antananarivo capital city (Madagascar)

Metropolitan and suburban areas in Africa are rapidly expanding with, in general, a poor safe urban management. Urban development in zones not suitable for construction leads to significant economic loss. The aggregate building damage combined with other environmental treats like drought, flooding or landsliding are enhancing African city vulnerability. Simple cartographic products made available to the general public could facilitate the first stage of planning and management considering parameters that affect building foundation. We combine here in ARCGIS geological, geomorphological and geotechnical data to provide such a framework focusing on the capital city of Antananarivo in Madagascar. The city was initially constructed on basement hills covered with a thick loose weathered layer and is expanding into the alluvial lowland watered by the Ikopa River. About 221 boreholes with pressumeters and laboratory data provide a picture of the underground characteristics, and allow the computation of the soil bearing capacity, the key parameter we used to assess building suitability. Lithology, slope, land-use and ground water level are combine to build a first geotechnical map that is completed using a mapping of the soil bearing capacity combined with land-use and geohydrological constraints. The geotechnical maps divide the city into zones with different constructive conditions that can be use as a first tool for urban development planning.