Abstract: The Underground Mining in PT Nusa Halmahera Minerals site Toguraci uses a combination method, that is Under Hand Cut and Fill (UHCF) and Open Stope (Sub Level – Blind Stope). The ore mining has large amount of volume. The condition always causes big chances of fall pr obability to the Tunnel and the Stope. The present sequence is on the elevation ±130 meter under ground surface with the Lava Andesites formation. The Evaluation of this research is done to the ore body (vein) of Damar with 23 Stope in 15m – 20m Stope lengt6h (span). From the result of classification of rock mass with Q-system modify (Q’) and Stability Number (N’), the Hanging wall part of the ore body (vein) of Damar is classified as Fair Rock Category, and Poor Rock for the Foot Wall. Mathews Stability Graph is a approach method that used in Open Stope design and planning, which provides Equivalent Linear Overbreak/Slough (ELOS) value that can be rating to the stability. Rock mass quality which is classified as Poor Rock at the Foot Wall part of the Damar ore body (vein) causes to Overbreak that can be happened in a actual condition. According to the calculation of ELOS (1m – 2m), it is different with the Hanging Wall that categorized as Stable (ELOS <1m). According to the result of this research, this condition is assumed less snycronized geometry design of Open Stope (Hydraulic Radius) to the competence of rock mass (Stability Number N’). To minimalize the overbreak possibility or big ELOS, there is some given recommendations; (a) At the Hanging Wall, the maximum Hydraulic Radius (HR) is [3m in Stability Number (N’) value < 1 or stope (span) length is not longer than 15m; and (b) At Foot Wall, the recommended maximum Hydraulic Radius (HR) is <2,5m in Stability Number (N’) <1 or stope (span) length is not longer than 15m.Keywords: Underground Mining Stability, Rock Mass Quality, Open Stope, Mathews Stability Graph, ELOS Abstrak: : Penambangan bawah tanah di PT Nusa Halmahera Minerals site Toguraci menggunakan kombinasi metode yaitu Under Hand Cut and Filll (UHCF) dan Open Stope (Sub Level – Blind Stope) yang mana pengambilan Ore dilakukan dengan jumlah volume yang besar. Kondisi ini selalu menimbulkan potensi runtuh yang besar pada terowongan baik Tunnel maupun Stope. Penggalian saat ini berada pada elevasi ±130 meter di bawah permukaan tanah dengan jenis batuan lava andesit. Evaluasi dilakukan di badan bijih (vein) Damar sebanyak 23 Stope dengan panjang Stope (span) 15m – 20m. Dari hasil pengklasifikasian massa batuan dengan Q-system Modify (Q’) dan Stability Number (N’), bagian Hanging Wall dari badan bijih (vein) Damar termasuk dalam kategori Fair Rock, dan Poor Rock pada bagian Foot Wall. Mathews Stability Graph adalah salah satu metode pendekatan yang digunakan dalam desain dan perancangan bukaan Stope, dimana metode ini menyediakan nilai Equivalent Linear Overbreak / Slough (ELOS) yang dapat dijadikan penilaian terhadap kestabilannya. Kualitas massa batuan yang dinilai Poor Rock pada bagian Foot Wall dari badan bijih (vein) Damar mempengaruhi besarnya Overbreak yang terjadi pada kondisi aktual. Berdasarkan perhitungan ELOS (1m – 2m) pada bagian Foot Wall yang berbeda dengan bagian Hanging Wall yang tergolong stabil (ELOS < 1m). Dari hasil penelitian, kondisi ini dinilai kurang sinkronnya desain geometri bukaan stope (Hydraulic Radius) terhadap kompetensi massa batuan (Stability Number N’). Untuk meminimalisir potensi overbreak atau ELOS yang besar, adapun rekomendasi yang diberikan; (a) Pada bagian Hanging Wall, Hydraulic Radius (HR) maksimum adalah [3m pada nilai Stability Number (N’) < 1 atau panjang stope (span) tidak lebih dari 15m; dan (b) pada bagian Foot Wall, Hydraulic Radius (HR) maksimum yang direkomendasi adalah < 2,5m pada nilai Stability Number (N’) < 1 atau panjang stope (span) tidak lebih dari 15m. Kata Kunci: Kestabilan Tambang Bawah Tanah, Kualitas Massa Batuan, Open Stope, Mathews Stability Graph, ELOS
Abstract: The Underground Mining in PT Nusa Halmahera Minerals site Toguraci uses a combination method, that is Under Hand Cut and Fill (UHCF) and Open Stope (Sub Level – Blind Stope). The ore mining has large amount of volume. The condition always causes big chances of fall pr obability to the Tunnel and the Stope. The present sequence is on the elevation ±130 meter under ground surface with the Lava Andesites formation. The Evaluation of this research is done to the ore body (vein) of Damar with 23 Stope in 15m – 20m Stope lengt6h (span). From the result of classification of rock mass with Q-system modify (Q’) and Stability Number (N’), the Hanging wall part of the ore body (vein) of Damar is classified as Fair Rock Category, and Poor Rock for the Foot Wall. Mathews Stability Graph is a approach method that used in Open Stope design and planning, which provides Equivalent Linear Overbreak/Slough (ELOS) value that can be rating to the stability. Rock mass quality which is classified as Poor Rock at the Foot Wall part of the Damar ore body (vein) causes to Overbreak that can be happened in a actual condition. According to the calculation of ELOS (1m – 2m), it is different with the Hanging Wall that categorized as Stable (ELOS <1m). According to the result of this research, this condition is assumed less snycronized geometry design of Open Stope (Hydraulic Radius) to the competence of rock mass (Stability Number N’). To minimalize the overbreak possibility or big ELOS, there is some given recommendations; (a) At the Hanging Wall, the maximum Hydraulic Radius (HR) is [3m in Stability Number (N’) value < 1 or stope (span) length is not longer than 15m; and (b) At Foot Wall, the recommended maximum Hydraulic Radius (HR) is <2,5m in Stability Number (N’) <1 or stope (span) length is not longer than 15m.