Ground fissures within the Main Ethiopian Rift: Tectonic,lithological and piping controls |
| |
Authors: | Jan Valenta Kryštof Verner Karel Martínek Tomáš Hroch David Buriánek Leta Alemayehu Megerssa Jan Boháč Muluken Kassa Ferdawok Legesse Meheret Yakob Bisrat Kebede Jiří Málek |
| |
Affiliation: | 1. Institute of Hydrogeology, Engineering Geology and Applied Geophysics, Charles University, Prague, Czech Republic;2. Institute of Petrology and Structural Geology, Charles University, Prague, Czech Republic;3. Institute of Geology and Palaeontology, Charles University, Prague, Czech Republic;4. Czech Geological Survey, Prague, Czech Republic;5. Institute of Petrology and Structural Geology, Charles University, Prague, Czech Republic Czech Geological Survey, Prague, Czech Republic;6. School of Earth Sciences, Addis Ababa University, Addis Ababa, Ethiopia;7. Geological Survey of Ethiopia, Addis Ababa, Ethiopia;8. Institute of Rock Structure and Mechanics, Czech Academy of Sciences, Prague, Czech Republic |
| |
Abstract: | Ground fissures, especially if they open due to a sudden collapse of the surface, is a serious risk for populated areas. Their common occurrence in unconsolidated sediments of the Main Ethiopian Rift was found to be mostly a result of piping. The fissures start by piping in linear sub-horizontal underground voids, which often propagate upwards resulting in ceiling collapse and formation of deep and long ground fissures with vertical walls. In the southern and central Main Ethiopian Rift the fissures pose a serious risk to infrastructure and settlements. The ground fissures are often linear (up to several kilometres long and often tens of metres deep) and accompanied by sinkholes (along the length). A detailed field mapping of the geological (rock composition, orientation and character of lithological boundaries, primary fabrics and brittle structures) and geomorphological features (especially a length, width and depth of fissures, sinkholes and gullies) followed by in situ seismic anisotropy measurements and a laboratory determination of the geomechanical properties of volcanoclastic deposits was carried out to investigate the ground fissures' origin. The conditions and factors leading to the formation of the ground fissures have been linked to: (a) the presence of regional normal faults and the associated extensional joints and (b) the alternation of lithological units with contrasting hydraulic permeability. The latter corresponds to a sequence of less permeable hard rocks (e.g., rhyolitic ignimbrites) overlain by heterogeneous, soft and permeable, unconsolidated volcaniclastic deposits with a low amount of clay (less than 10%). The ground fissures' occurrence has shown affiliation to areas which have a significantly high seismic anisotropy (more than 20% at the study sites), which can be used as a proxy to map out high risk areas prone to piping and ground fissure formation. |
| |
Keywords: | geomechanical properties pipe collapses pyroclastics seismic anisotropy soil subsurface erosion surface waves unconsolidated sediments |
|
|