Analisis Tipe Longsor dan Kestabilan Lereng Berdasarkan Orientasi Struktur di Dinding Utara Tambang Batu Hijau, Sumbawa Barat

FARIDHA APRILIA, I Gde Budi Indrawan, S.T., M.Eng., Ph.D

2014 | Skripsi | TEKNIK GEOLOGI

Batu Hijau merupakan salah satu tambang emas dan tembaga terbesar di Indonesia yang dioperasikan oleh PT. Newmont Nusa Tenggara dengan metode penambangan terbuka. Selama pengoperasian tambang terjadi beberapa kasus longsoran akibat struktur geologi yang intensif serta kekuatan massa batuan yang lemah. Tujuan penelitian ini untuk mengetahui tipe longsoran dan kondisi kestabilan lereng utara Tambang Batu Hijau dalam desain Phase 6 dan untuk mengetahui potensi longsoran pada desain Phase 7 yang telah direncanakan berdasarkan data struktur geologi yang telah teridentifikasi saat ini. Penelitian ini meliputi studi pustaka, observasi lapangan awal, pengambilan dan analisis data. Data yang digunakan antara lain data hasil pemetaan struktur sepanjang lereng (line mapping), data pemboran geoteknik, data sifat keteknikan batuan yang didapatkan dari uji laboratorium dan back analysis longsoran yang pernah terjadi. Analisis data meliputi analisis kinematika yang dilanjutkan dengan analisis kesetimbangan batas untuk mengetahui kondisi kestabilan lereng tambang. Analisis kinematika dilakukan menggunakan Schmidt net dalam Software Dips pada setiap blok analisis yang dibagi berdasarkan orientasi dan besar sudut kemiringan lereng. Dinding utara desain Phase 6 dibagi menjadi tujuh blok. Blok TL-1, TL-2 dan TL-3 terletak di bagian timur laut, blok U-1, U-2 dan U-3 terletak di bagian utara, sedangkan blok BL-1 terletak di bagian baratlaut. Dinding utara desain Phase 7 juga dibagi menjadi tujuh blok, antara lain blok NE-1, NE-2 dan NE-3 di bagian timurlaut, blok N-1, N-2 dan N-3 di bagian utara, dan blok NW-1 di bagian baratlaut. Analisis kesetimbangan batas dilakukan pada daerah yang tidak stabil secara kinematika dengan memanfaatkan software Slide menggunakan metode General Limit Equilibrium (GLE) dan perhitungan secara manual menggunakan prinsip Mohr-Couloumb dan Hukum Newton untuk plane failure serta menggunakan Hoek-Brown Wedge Stability Chart untuk analisis wedge failure. Hasil analisis menunjukkan bahwa longsoran yang mendominasi lereng tambang Phase 6 adalah tipe wedge, planar dan kombinasi keduanya. Analisis kinematika dan kesetimbangan batas pada desain Phase 6 menunjukkan bahwa blok yang paling kritis adalah TL-1 dan BL-1. Untuk Phase 7 potensi longsoran terdapat pada blok NE-1, N-2 dan NW-1.

Batu Hijau is one of the largest gold and copper open pit mine in Indonesia which is operated by PT. Newmont Nusa Tenggara. A number of rock slope failures occured since operation of this mine that were mainly caused by intensive geological structure and weak rock mass strength. The main objective of this research is to determine the types of rock slope failures and the stability of the north sector of Phase 6 design of Batu Hijau Mine. In addition, the potential unstability of a newly planed Phase 7 design based on the current geological structure conditions is also determined. This research consisted of preliminary study, early field observation, and field data collection and analyses. Data used in this research included data of structural mapping along the slope face (line mapping), geotechnical coring, and engineering properties of rock obtained from laboratory testing and back analyses of slope failures that had been occured in the study area. Data analyses included kinematic analyses followed by limit equilibrium analyses to determine rock slope stability in the study area. Kinematic analyses used Schmidt net and were performed using Dips software for each block of slopes that has been determined based on the slope orientation and angle. The north sector of the Phase 6 design was divided into seven blocks of analyses. TL-1, TL-2 and TL-3 blocks were located in the northeast, U-1, U-2 and U-3 blocks were located in the north, while BL-1 block was located in the west. The north sector of the Phase 7 design was also divided into seven blocks of analyses, namely NE-1, NE-2 and NE-3 blocks located in the northeast, N-1, N-2 and N-3 blocks in the north, and NW-1 block located in the west. Limit equilibrium analyses were applied to kinematically unstable slopes using Slide software based on the General Limit Equilibrium (GLE) method. In addition, limit equilibrium analyses were also performed manually using Mohr-Couloumb Criterion and Newton Law concept for plane failure and Hoek-Brown Wedge Stability Chart for wedge failure. Results of this research indicated that types of slope failure in the Phase 6 design were dominated by a wedge failure, plane failure and combination of those failures. Kinematic and limit equilibrium analyses showed that TL-1 and BL-1 blocks were the most potential unstable and critical slopes. Potential unstable slopes in the Phase 7 design were NE-1, N-2 and NW-1 blocks.

Kata Kunci : Kestabilan lereng, Tambang terbuka