ANALISIS KESTABILAN LERENG DI DESA TERBAH DAN SEKITARNYA, KECAMATAN PATUK, KABUPATEN GUNUNG KIDUL BERDASARKAN SLOPE STABILITY PROBABILITY CLASSIFICATION

Desa Terbah merupakan salah satu desa di Indonesia yang rawan longsor. Daerah ini berada pada sedimen Tersier yang telah mengalami deformasi dan pelapukan kuatyang berpotensi longsor,sehingga diperlukan analisis kestabilan lereng untuk upaya mitigasi. Tulisan ini bertujuan mengidentifikasi karakteristik satuan geologi teknik lereng dan nilai probabilitas tingkat kestabilan lereng. Metode penelitian yang digunakan yaitu pengamatan tingkat pelapukan, pengukuran bidang diskontinuitas (spasi, kekasaran, lebar bukaan) pada lereng berdasarkan metode Rock Mass Rating (RMR), pengujian laboratorium menggunakan point load test , dan analisis kestabilan lereng dengan metode kinematika dan Slope Stability Probability Classification (SSPC). Hasil penelitian menunjukkan bahwa terdapat lima satuan geologi teknik batuan, yaitu: Satuan  Breksi Andesit 1, Satuan Breksi Andesit 2, Satuan Tufa, Satuan Batupasir Tufan, dan Satuan Batupasir. Hasil pengukuran pada 35 lereng menunjukkan 14 lereng berada dalam kondisi tidak stabil berdasarkan kestabilan lereng orientasi independen (tidak terpengaruh diskontinuitas), dan 18 lereng berpotensi terjadi longsor gelinciran ( sliding ), dan 14 lereng berpotensi terjadi longsor robohan ( toppling ) berdasarkan orientasi dependen (terpengaruh diskontinuitas). Terbah village is one of the rural areas in Indonesia that has high vulnerability to landslide. This area occupies the Tertiary sediment which had been deformed and highly weathered, therefore prone to landslide. It is important to analyze the slope stability of the study area as a part of the mitigation measures. This paper aims to identify the engineering geological units and the probability values of the slope stability. Methods included the observation of weathering degree, measurement of slope discontinuities (space, roughness, width of opening) according to Rock Mass Rating (RMR), point load test in the laboratory, and slope stability analysis using kinematic method and Slope Stability Probability Classification (SSPC). Results show that there are five engineering geological units: Andesitic Breccia Unit 1, Andesitic BrecciaUnit 2, Tuff unit, Tuffaceous Sandstone Unit, and Sandstone Unit. Measurements of 35 slopes show that based on orientation independent analysis 14 slopes are unstable, and based on orientation dependent analysis 18 slopes are prone to sliding and 14 slopes are susceptible to toppling.