INVESTIGATION OF THE GEOLOGICAL AND GEOTECHNICAL EFFECTS ON THE ORIGIN AND EXTENT OF EARTH FISSURES IN THE EAST SEMNAN

In this study, in order to clarify the main cause of earth fissures development in the East region of Semnan, various factors affecting the earth fissure, including geotechnical properties of sediments and geological conditions of the area along superficial and deep patterns of the fissures, have been studied. Based on the results of these studies, the cracks developing in the area are affected by natural causes that exist in the region. The previously studies introduced subsidence due to groundwater pumping as the main reason of the earth fissures in the East region of Semnan. The cracks pattern that is readily apparent from the satellite (google earth) images of the area indicated a significant difference of these cracks from induced cracks by subsidence. The earth fissures in the study area form polygonal blocks that look identical to the shape of mudcracksare. In fact, these giant cracks are the same desiccation mudcracks or large soil cracks formed in clay layers, yet on an enormous scale. Some giant desiccation cracks are large enough to be mistaken for earth fissures that are caused by subsidence from groundwater pumping. These cracks are formed deep in the earth and, eventually, work up to the surface by collapse of the roof of the cavity due to subsurface erosion. The most common form of a desiccation crack on the surface is one of linear collapse features. The controlling factors in the formation of the giant mudcracks in the area are geological conditions, mineralogy, topography, and hydrology, along with the climate. Based on field studies, the main development of crack in the study area occur at the toes of alluvial fan that are inundated by sheetflow during heavy rain. In addition, based on in-situ and laboratory geotechnical tests, the soil in the areas has low dry density, and the clay minerals are dispersion potential. Dispersion of clay by water that enters the crack system causes enlarged desiccation crack and makes an opening for materials to collapse into.