Kajian Teknis Stabilitas Lereng Timbunan pada Waste Dump Batupangah PT. Meares Soputan Mining Kecamatan Likupang Timur Kabupaten Minahasa Utara Provinsi Sulawesi Utara

Abstract

Abstract. PT Meares Soputan Mining uses waste mining material dumping techniques using the outpit dump method which is buried bottom up with a layer thickness that exceeds 1/3 the height of the conveyance and is not compacted using a compactor. Technical differences in hoarding that are not in accordance with the basic provisions applicable to the Minister of Energy and Mineral Resources Republic of Indonesia No. 1827 K / 30 / MEM / 2018, and the actual condition of the steep embankment slope so it is necessary to conduct a geotechnical study to get the optimal slope geometry but remain at safe limits (no landslides). The results of this study note that the characteristics of embankment rock material is a mixture consisting of coarse-textured material in the form of fresh basaltic-andesite, as well as fine-textured material in the form of volcanistic conglomerates and teralterated basaltic-andesite. The weight value of the embankment material is 18,13 - 21,85 kN / m³ (or density 1,85 - 2,23 t / m³). Based on the test results it is known that the pile material's cohesion is 74.0 kN / m2 - 91.24 kN / m² (KPa) and the deep shear angle is 24.02 ° - 35.16 °. Analysis of the stability of a single slope under saturated groundwater conditions on a slope geometry with a height of 15 meters and a slope of 25 ° produces FS values of 2.0 and PF = 0.0%. The single embankment slope has a FS value> 1.1 and PF <5% so that the slope is said to be stable. The results of the analysis of overall slope stability in saturated groundwater conditions resulted in the northwest slope with a height = 45 m and a slope = 20 ° has a FS value of 1.215 and PF = 0.0%. North slope with height = 125 m and slope = 20 ° has FS = 0.874 and PF = 100%. East slope with height = 85 m and slope = 20 ° has FS = 0.957 and PF = 80.6%. And the southeast slope with height = 95 m and slope = 20 ° has FS = 0.914 and PF = 99.5%. North-West Slopes are said to be stable because they have FS values> 1.1 and PF <5%, while North, East, and Southeast slopes are said to be unstable (potentially landslides) because FS values <1.1 and PF> 5%. Stabilization of the embankment slope is by draining ground water to prevent the slope from being in saturated groundwater, carried out by making a Horizontal Drain Hole (HDH). Based on the simulation of the decrease in groundwater level from saturation, the result is that on the northern slope a 25 meter deep groundwater reduction is needed so that stability is achieved at FS = 1.144 and PF = 0.0%. East Slope required a decrease in groundwater as deep as 10 meters to obtain FS = 1.106 and PF = 1.1%. As well as the southeast slope it is necessary to reduce groundwater as deep as 15 meters to obtain FS = 1.149 and PF = 0.0%.

Keywords: Dumps, Slope Stability, Safety Factors, Groundwater.

 

Abstrak. Teknis penimbunan material waste hasil penambangan yang dilakukan oleh PT Meares Soputan Mining menggunakan metode outpit dump yang ditimbun secara bottom up dengan tebal layer melebihi rasio 1/3 tinggi alat angkut dan tidak dilakukan pemadatan menggunakan alat compactor. Perbedaan teknis penimbunan yang tidak sesuai dengan dasar ketentuan yang berlaku pada Kepmen ESDM Republik Indonesia No. 1827 K/ 30/ MEM/ 2018, serta keadaan aktual lereng timbunan yang curam sehingga perlu dilakukan kajian geoteknik untuk mendapatkan geometri lereng timbunan yang optimal namun tetap berada pada batas aman (tidak longsor). Hasil dari kajian ini diketahui karakteristik material batuan penyusun timbunan merupakan campuran yang terdiri dari material bertekstur kasar berupa basaltik-andesit segar, serta material bertekstur halus berupa konglomerat vulkanistik dan basaltik-andesit teralterasi. Nilai bobot isi material timbunan berada pada angka 18,13 – 21,85 kN/m³ (atau densitas 1,85 – 2,23 t/m³). Berdasarkan hasil uji diketahui nilai kohesi material timbunan berada pada angka 74,0 kN/m² – 91,24 kN/m² (KPa) dan sudut geser dalam sebesar 24,02° - 35,16°. Analisis stabilitas lereng tunggal dalam kondisi muka air tanah jenuh pada geometri lereng dengan tinggi 15 meter dan kemiringan 25° menghasilkan nilai FK = 2,0 dan PK = 0,0%. Lereng tunggal timbunan memiliki nilai FK >1,1 dan PK <5% sehingga lereng diikatakan stabil. Hasil analisis stabilitas lereng keseluruhan dalam kondisi muka air tanah yang jenuh menghasilkan pada lereng bagian Barat Laut dengan tinggi = 45 m dan kemiringan = 20° memiliki nilai FK = 1,215 dan PK = 0,0%. Lereng bagian Utara dengan tinggi = 125 m dan kemiringan = 20° memiliki FK = 0,874 dan PK = 100%. Lereng Timur dengan tinggi = 85 m dan kemiringan = 20° memiliki FK = 0,957 dan PK = 80,6%. Serta lereng bagian Tenggara dengan tinggi = 95 m dan kemiringan = 20° memiliki FK = 0,914 dan PK = 99,5%. Lereng Barat Laut dikatakan stabil karena memiliki nilai FK >1,1 dan PK <5%, sementara lereng Utara, Timur, dan Tenggara dikatakan tidak stabil (berpotensi longsor) karena nilai FK <1,1 dan PK >5%. Stabilisasi lereng timbunan adalah dengan penirisan air tanah guna mencegah lereng berada dalam kondisi muka air tanah yang jenuh, dilakukan dengan cara pembuatan Horizontal Drain Hole (HDH). Berdasarkan simulasi penurunan muka air tanah dari keadaan jenuhnya maka menghasilkan, pada lereng bagian Utara diperlukan penurunan air tanah sedalam 25 meter sehingga tercapai kestabilan pada FK = 1,144 dan PK = 0,0%. Lereng Timur diperlukan penurunan air tanah sedalam 10 meter sehingga diperoleh FK = 1,106 dan PK = 1,1%. Serta lereng Tenggara diperlukan penurunan air tanah sedalam 15 meter sehingga diperoleh FK = 1,149 dan PK = 0,0%.

Kata Kunci: Timbunan, Stabilitas Lereng, Faktor Keamanan, Muka Air Tanah.