PROPERTY INDEX AND HYDROPHYSICAL CONDITIONS OF SOILS AT SUBSIDENCESS IN PONJONG, RONGKOP AND SEMANU SUBDISTRICTS, GUNUNGKIDUL DISTRICT

Abstract

Karst Gunung Sewu is a tropical Karst area with unique features like a sinkhole or doline.
Sinkhole or doline usually formed due to subsidence. Ponjong, Rongkop, and Semanu subdistricts in
Gunungkidul District are areas that included in the Karst Gunung Sewu area, which has a high frequency of
land subsidence. The purpose of this research is to identify the soil physical characteristics in the subsidence
area in the karst units. This research method includes mapping, geotechnical investigation, and laboratory tests
to determine soil physical properties. In the research area, 41 subsidence points occurred in six karst units. The
physical characteristics of the soil around subsidence in that subdistrict show that the soil has a medium texture
with the attributes of fine clay. Atterbag boundary values in subsidence soil are more than 50%. It was causing
the soil to have a high level of plasticity. The soils conditions in the subsidence area have low permeabilities
and reduced porosity. It has deep to shallow soil thickness with good drainage until obstructed. These
characteristics of the soil cause the potential for ground movement, such as subsidence, especially when there
is a water thrusting during a flood.