Journal of Applied Geology, vol. 7(2), 2022, pp. 83–100 DOI: http://dx.doi.org/10.22146/jag.76532 Engineering Geology and Slope Stability of West Pit Coal Mine of PT. Tawabu Mineral Resource, East Kalimantan, Indonesia Rama Tri Saksono, I Gde Budi Indrawan * , and Wahyu Wilopo Department of Geological Engineering, Faculty of Engineering, Universitas Gadjah Mada, Yogyakarta, Indonesia ABSTRACT. The research area was located in the west pit of the open pit coal mine of PT. Tawabu Mineral Resource (TMR) in Bengalon District, East Kutai Regency, East Kaliman- tan Province, Indonesia. Several landslides drove the research in the area; however, the re- maining slopes’ engineering geological conditions and stability have not been evaluated. This study’s objectives were to understand better the engineering geological conditions and stability of the research area. The geological engineering conditions (i.e., geomorphol- ogy, rock and soil, geological structure, and groundwater conditions) were evaluated by photogrammetric analyses, field observations, and analyses of borehole logs and labora- tory test results. The slope stability analyses were first carried out by conducting back sta- bility analyses of failed slopes on the northern lowwall slope segment. The shear strength parameters obtained from the back analyses were then used for forward stability analyses of the remaining 10 lowwall and highwall slopes. The slope stability analyses involved deterministic and probabilistic analyses, under static and dynamic, using the limit equi- librium method (LEM). The results showed that the research area and the surroundings consisted of two geomorphological units: the alluvial plain and structural hills. Rocks in the study area consisted of claystone, sandstone, and coal with a general layer strike direction of N59°E – N63°E with a dip of 19°–26°. These rocks were grouped into two lithological units: alternating sandstone and claystone unit, and alternating claystone and coal unit. The geological structures were identified on the highwall, from west to east, namely a major sinistral shear fault with a relative direction of NNE–SSW, two minor sinistral shear faults with a relative direction of NE–SW, and a major dextral shear fault with a relative direction of NW–SE. These geological structures were interpreted as being formed by the folding process. The groundwater level was estimated at -45 m to 20 m. The slope stability analyses showed that only the East HW-4 slope, located on the east highwall, was unstable. It is recommended to optimize the slope by either lowering the groundwater elevation by 4 m from the actual level or by reducing the overall slope angle to 31°. Keywords: Back stability analysis · Coal mine · Engineering geology · Forward stability analysis · Slope optimization. 1 I NTRODUCTION Characterization of geological engineering con- ditions needed to be carried out in evaluating slope stability at a research location. Ulusay * Corresponding author: I G.B. I NDRAWAN, Depart- ment of Geological Engineering, Universitas Gadjah Mada. Jl. Grafika 2 Yogyakarta, Indonesia. E-mail: igbindrawan@ugm.ac.id (2013, 2019) emphasizes the importance of har- monizing engineering geology with rock engi- neering on the stability of natural and engi- neered rock slopes. Gonzalez de Vallejo and Ferrer (2011) explained that engineering geo- logical investigations need to be carried out to provide a geological and geotechnical descrip- tion of the excavation area, to obtain the param- 2502-2822/© 2022 The Authors. Open Access and published under the CC-BY license.