Last modified: 2019-08-14
Abstract
Karst Gunung Sewu is a tropical karst area which have a sinkhole or dolina. Sinkhole or dolina is usually formed due to subsidence. Ponjong, Rongkop and Semanu Subdistricts in Gunung Kidul Regency are areas that include the Karst Gunung Sewu area which has a high frequency of land subsidence. This study aims to identify the physical characteristics of soil around subsidence in Ponjong, Rongkop and Semanu Subdistricts by identifying the physical condition of the soil in subsidence that occurs in Ponjong and Semanu Subdistricts, Gunung Kidul District on a geomorphological unit scale. The method of this research is by mapping the subsidence point, geotechnical investigations, and laboratory tests to determine the physical properties of the soil such as index properties—which include soil textures, attachment limits, and soil hydro physical conditions—which include permeability, bulk density and porosity. The unit of analysis used is geomorphological unit analysis. There are 41 points of subsidence in in the study area, which consists of old subsidence and new subsidence. There are six geomorphological units in the study area, which are the Alluvial Plain of Karst, Dry Valley, Middle Slopes of Light Fire, Plato Kars, Polje, and Conical Kars Zone. The physical characteristics of the soil around subsidence in the study area show that the soil has a medium texture with the characteristics of fine clay. Atterbag limit values on subsidence soils that are more than 50% cause the soil to have a high degree of plasticity. The hydrophysical condition of the soil in subsidence also shows a low permeability value and poor porosity. Besides that, it has deep to shallow soil thickness with good drainage until it is obstructed. These characteristic of the soil causes the potential for ground movement such as subsidence, especially when there is a water thrusting during a flood.