Abstract:
The coal mine slopes in the Warukin formation consist mostly of sandstone and claystone. Moreover, those
stones are part of sedimentary rocks. One of the factors that influence the instability of sedimentary rock slopes was
the presence of the groundwater level because it can reduce the internal strength of the slope and increase the slope
load. The essential job of a geotechnical engineer is to know the presence, type, and level of groundwater. The purpose
of this study was to determine the effect of the groundwater level variation on slope instability. Characterization of
groundwater parameters in this study used the slug test and seepage mapping. The slug test was used to determine the
value of soil hydraulic conductivity, and the seepage mapping was used to determine the distribution pattern of the
seepage point in the study area. From the slug test, the hydraulic conductivity values ranged from 10-5
to 10-6 m/s. It
means that the aquifer layer in the study area was identified as fine sandstone aquifer. The seepage mapping activities
showed that the most seepage was found in the sandstone layers with a percentage of 60%. The results of the slope
stability analysis obtained the value of the safety factor (SF) for cross-section A-Aˈ 1.56 at the low wall and 1.42 at
the high wall. In cross-section C-Cˈ, the SF value was 1.77 at the low wall and 1.26 at the high wall. Linear correlation
between the safety factor and the groundwater level were obtained in this study after analyzing slope stability with
groundwater level variations. The slope safety factor decreases when the groundwater level increases; however, the
safety factor increases when the groundwater level decreases.
