Evaluasi Kondisi Geologi Teknik dan Kestabilan Lereng Kuari Batugamping Bukit Tajarang PT. Semen Padang, Kota Padang, Provinsi Sumatera Barat

FAIZ MUHAMMAD HUSAIN, Dr. Eng. Ir. Wawan Budianta, S.T., M.Sc., IPM.

2024 | Skripsi | TEKNIK GEOLOGI

Produksi semen oleh PT. Semen Padang menggunakan bahan baku berupa batugamping yang diperoleh dari aktivitas penambangan di Bukit Tajarang, Kelurahan Batu Gadang, Kecamatan Lubuk Kilangan, Kota Padang. Sistem penambangan yang diterapkan yaitu sistem tambang terbuka (open pit mining) sehingga menghasilkan lereng yang berpotensi menyebabkan longsor. Tercatat pada tahun 2023 terjadi longsor pada salah satu lereng di area pertambangan sehingga menutupi seluruh badan jalan di bawahnya. Berdasarkan permasalahan tersebut dilakukan penelitian yang bertujuan untuk mengevaluasi kondisi geologi teknik dan kestabilan lereng menggunakan metode Slope Mass Rating (SMR) dan metode kesetimbangan batas (LEM) pada kuari batugamping PT. Semen Padang sehingga diharapkan dapat mengurangi resiko terkait keselamatan operasional tambang. Metode yang digunakan dalam penelitian ini meliputi pemetaan aspek geologi teknik berupa geomorfologi, batuan dan tanah, struktur geologi, serta kondisi air tanah dengan skala 1:4.000, yang kemudian dilanjutkan dengan analisis kestabilan lereng menggunakan metode Slope Mass Rating (SMR) dan metode kesetimbangan batas (LEM). Selain itu, dilakukan juga analisis laboratorium yang mencakup petrografi, sifat indeks, dan sifat keteknikan pada sampel batuan dan tanah. Hasil penelitian menunjukkan daerah penelitian terdiri dari morfologi dataran bergelombang hasil proses penambangan, lereng miring - agak curam hasil proses penambangan, kawasan disposal berlereng miring hasil proses penambangan, lereng curam - sangat curam hasil proses penambangan, dan perbukitan berlereng sangat curam hasil proses denudasional. Litologi berupa batugamping kristalin dan granit. Karakteristik geologi teknik berdasarkan kualitas massa batuan GSI pada daerah penelitian terdiri dari satuan batugamping kristalin kualitas sedang, satuan batugamping kristalin kualitas buruk, dan satuan granit berkualitas sangat buruk. Struktur geologi yang hadir berupa sesar geser sinistral berarah gaya NE-SW dan kekar gerus dengan arah gaya utama NNW-SSE. Kualitas batuan pada lereng berdasarkan RMR terbagi menjadi RMR kelas II dan RMR kelas III sedangkan Tingkat kestabilan lereng berdasarkan SMR terbagi menjadi Lereng Stabil (II), Lereng sebagian stabil (III), Lereng tidak stabil (IV), dan Lereng sangat tidak stabil (V). Analisis kestabilan lereng menggunakan metode LEM pada tiga section lereng aktual menunjukkan kondisi yang stabil dan tidak stabil. Redesain geometri lereng kemudian dilakukan dengan alasan optimalisasi produksi dan peningkatan angka keamanan lereng dengan modifikasi berupa bench angle: 90°, bench width: 5 m, bench height: 10 m pada lereng batugamping dan bench angle: 50°, bench width: 6 m, bench height: 7 m pada lereng tanah residu granit, sehingga menghasilkan angka faktor keamanan lebih besar dari 1.3 pada kondisi statis dan 1.1 pada kondisi dinamis.

Cement production by PT. Semen Padang utilizes raw materials in the form of limestone sourced from mining activities in Bukit Tajarang, Batu Gadang Village, Lubuk Kilangan District, Padang City. The mining system employed is an open-pit mining system, which creates slopes that have the potential to cause landslides. In 2023, a landslide was recorded on one of the slopes in the mining area, covering the entire roadway below. This study was conducted to evaluate the engineering geological conditions and slope stability using the Slope Mass Rating (SMR) and Limit Equilibrium Method (LEM) at the limestone quarry of PT. Semen Padang, with the aim of reducing risks related to operational safety in the mine. The methods used in this research include mapping the engineering geological aspects such as geomorphology, rock and soil characteristics, geological structures, and groundwater conditions at a scale of 1:4,000. This was followed by slope stability analysis using the Slope Mass Rating (SMR) and Limit Equilibrium Method (LEM). Additionally, laboratory analyses were conducted, which included petrography, index properties, and engineering properties of rock and soil samples. The results of the study indicate that the research area consists of undulating plains formed by mining activities, moderately steep slopes to steep slopes from mining processes, sloped disposal areas from mining, steep to very steep slopes resulting from mining, and very steep hill slopes formed by denudational processes. The lithology consists of crystalline limestone and granite. The engineering geological characteristics, based on the Geological Strength Index (GSI), classify the rock mass in the study area into crystalline limestone of fair quality, crystalline limestone of poor quality, and granite of very poor quality. The geological structures present include a sinistral strike-slip fault trending NE-SW and shear fractures trending NNW-SSE. The rock slope quality, based on the Rock Mass Rating (RMR), is classified into RMR Class II and RMR Class III, while based on SMR, it is classified into SMR Class II, SMR Class III, SMR Class IV, and SMR Class V. Slope stability analysis using the LEM method on three actual slope sections showed both stable and unstable conditions. A redesign of the slope geometry was then carried out for production optimization and to improve slope safety factors. The modifications include a bench angle of 90°, bench width of 5 m, and bench height of 10 m for limestone slopes, and a bench angle of 50°, bench width of 6 m, and bench height of 7 m for residual granite soil slopes. These modifications resulted in a safety factor greater than 1.3 under static conditions and greater than 1.1 under dynamic conditions.

Kata Kunci : PT. Semen Padang, geologi teknik, kestabilan lereng, longsor, GSI, RMR, SMR, LEM