Studi Geoteknik untuk Redesign Geometri Lereng Low-Wall pada Tambang Batubara di PT Antang Gunung Meratus, Kabupaten Tapin, Provinsi Kalimantan Selatan

Abstract

Abstract. PT Antang Gunung Meratus (PT AGM) is one of the coal mine open corporations in Tapin Regency, South Kalimantan Province. The landslides of low-wall slopes frequently happen in the mining site of PT AGM. The initial hypothesis assumed that the landslides happened because of the slopes geometry that was not in accordance with the initial design in the panel of 13 to 17. The data collection techniques of this geotechnical study upon the coal mine at PT AGM included geotechnical mapping, observation of the landslide model that happened in the site, and boulder sampling for point load test in the laboratory. The data resulted from the parameter input were obtained from the result of the laboratory test by PT AGM. The geotechnical study that was done was the landslide analysis in the panel of 13 to 17, the fault effect analysis in the panel area of 22 to 32, the thin layer of IB L6/L7 analysis, as well as the waste dump slope analysis. The analysis method applied to the landslide analysis in the panel of 13 to 17 as well as the waste dump slope analysis was back analysis method. This analysis was applied to the slopes with landslides since the rock mass value would be different. This back analysis method was also applied to get new rock mass parameter values; they were cohesion value (c) and friction angle (Ø). The values of c and Ø obtained were assumed to represent the actual rock mass values in the site. The landslide back analysis was counted based on the formula developed from the limit equilibrium method (LEM) with the help of Slide version 6.0 software by entering the rock mass parameter until it was got FK ~ 1. Next, the actual and design slopes were reanalyzed by using the new parameters so that it was got a new FK value. The slopes condition was made into the saturated condition since there were groundwater seepage in panel 13 at the level of ± 21 masl so that it was assumed at the worst condition; that was saturated condition. The results of the modeling and analysis showed that the causes of the landslides were because of the slopes’ steep inclination, the rain water/groundwater, and the existence of thin interburden layers in some actual slopes which were not in accordance with the design. The landslide back analysis in the panel of 13 to 17 presented new rock mass parameters: clay stone lithology claystone (c = 224,4 kPa, Ø = 23,96^o), sandstone (c = 176,5 kPa, Ø = 22,19^o), and coal (c = 246,9 kPa, Ø = 24,88^o). Furthermore, the back analysis for the waste dump slope landslide in panel 23 showed the design changes: overall slope of 13^o for the total waste dump height of 45 m or 50 m and overall slope of 11^o for the waste dump slope height of 55 m as it was planned.

Keywords: Low-Wall, Landslide, Slope Geometry, Back Analysis, LEM, Cohesion, Friction Angle

Abstrak. PT Antang Gunung Meratus (PT AGM) adalah salah satu perusahaan tambang batubara terbuka di Kabupaten Tapin, Provinsi Kalimantan Selatan. Di lokasi penambangan PT AGM sering terjadi kelongsoran lereng low wall. Hipotesa awal diasumsikan kelongsoran terjadi karena geometri lereng yang tidak sesuai dengan desain awal pada Panel 13 s.d 17. Pengumpulan data studi geoteknik pada tambang batubara terbuka PT AGM ini meliputi pemetaan geoteknik, pengamatan model kelongsoran yang terjadi, dan sampling boulder untuk uji point load di laboratorium. Data hasil input parameter diperoleh dari hasil pengujian lab oleh PT AGM. Studi geoteknik yang dilakukan adalah analisis kelongsoran panel 13 s.d 17, analisis pengaruh patahan di area panel 22 s.d 32, analisis lapisan tipis IB L6/L7, dan analisis waste dump slope. Metode analisis yang digunakan dalam analisis kelongsoran panel 13 s.d 17 dan analisis waste dump slope adalah metode analisis balik. Analisis balik dilakukan pada lereng yang mengalami longsor, karena akan berbeda nilai massa batuannya. Metode analisis balik ini dilakukan untuk mendapatkan nilai parameter massa batuan yang baru, yaitu nilai kohesi (c) dan sudut gesek dalam (Ø). Nilai c dan Ø yang didapat, diasumsikan mewakili nilai massa batuan yang sebenarnya di lapangan. Analisis balik longsoran dihitung berdasarkan rumus yang dikembangkan dari metode kesetimbangan batas (LEM) dengan bantuan software Slide versi 6.0, yaitu dengan cara memasukkan parameter massa batuan hingga mendapatkan FK ~ 1. Selanjutnya menganalisis kembali lereng desain dan aktual dengan menggunakan parameter baru sehingga didapat nilai FK yang baru. Kondisi lereng dibuat dalam kondisi jenuh, karena terdapat rembesan air tanah di panel 13 pada level ± 21 mdpl, sehingga diasumsikan ke dalam kondisi terburuk, yaitu kondisi jenuh. Dari hasil pemodelan dan analisis yang dilakukan dapat disimpulkan bahwa, penyebab terjadinya kelongsoran diantaranya karena kemiringan lereng yang curam, pengaruh air hujan / air tanah, adanya lapisan interburden yang tipis dan terdapat beberapa lereng aktual yang tidak sesuai dengan desain. Hasil analisis balik kelongsoran panel 13 s.d 17 didapat parameter massa batuan baru, yaitu untuk litologi claystone (c = 224,4 kPa, Ø = 23,96^o), sandstone (c = 176,5 kPa, Ø = 22,19^o), dan coal (c = 246,9 kPa, Ø = 24,88^o). Kemudian, untuk kelongsoran lereng waste dump pada panel 23 dilakukan analisis balik dengan diperoleh perubahan desain yaitu overall slope 13^o untuk tinggi total waste dump 45 m atau 50 m, dan overall slope 11^o untuk tinggi waste dump slope 55 m sesuai rencana.

Kata Kunci: Low Wall, Kelongsoran, Geometri Lereng, Analisis Balik, LEM, Kohesi, Sudut Gesek Dalam