Analisis Pengaruh Kadar Air Terhadap Penurunan Nilai Kohesi (C) dan Sudut Geser (Φ) pada Tanah Longsoran Sembalun

Abstract

Abstract: Landslides are one of the most frequent natural disasters in Sembalun Subdistrict, East Lombok Regency, due to steep topography, volcanic material, and high rainfall, which increases soil moisture content. Increased moisture content causes a decrease in effective stress, thereby reducing the cohesion (c) and angle of internal friction (φ), which leads to a decrease in soil shear strength and an increased risk of landslides. This study aims to analyze the characteristics of landslide soil in Sembalun Bumbung Village and to determine the effect of variations in moisture content on changes in the cohesion (c) and angle of internal friction (φ) values. The study employed laboratory methods on landslide soil samples from Sembalun Bumbung Village. The tests included soil physical properties, the Standard Proctor Test to determine the optimum moisture content, and the Direct Shear Test at various moisture contents to determine the cohesion and angle of internal friction. The data were analyzed descriptively to determine the effect of changes in moisture content on the soil’s shear strength parameters. The results showed that the soil had a moisture content of 14.27%, a specific gravity of 2.68, a wet bulk density of 1.41 g/cm³, and was classified as SC (Clayey Sand) under the USCS system and A-2-6 under the AASHTO system. An optimum moisture content of 17.53% with a maximum dry bulk density of 1.608 g/cm³ resulted in the highest cohesion of 0.142 kg/cm² and a angle of repose of 33.258°. A gradual increase in moisture content triggers mechanical degradation, from the original condition to 19.294% (cohesion decreased by 18.68%, angle of repose decreased by 0.08%) to 21.048% (cohesion decreased by 9.46%, angle of repose decreased by 6.05%) and up to an extreme moisture content of 43.33% (cohesion decreased by a total of 70.53%, angle of repose decreased by a total of 10.32%). The increase in moisture content was shown to eliminate the binding capacity of the particles and trigger slope instability. These results are expected to serve as a reference for landslide disaster mitigation.

Abstrak: Tanah longsor merupakan salah satu bencana yang sering terjadi di Kecamatan Sembalun, Kabupaten Lombok Timur, akibat kondisi topografi yang curam, material vulkanik, dan tingginya curah hujan yang meningkatkan kadar air tanah. Peningkatan kadar air menyebabkan penurunan tegangan efektif sehingga nilai kohesi (c) dan sudut geser (φ) berkurang, yang berdampak pada menurunnya kuat geser tanah dan meningkatnya potensi longsor. Penelitian ini bertujuan untuk menganalisis karakteristik tanah longsoran di Desa Sembalun Bumbung serta mengetahui pengaruh variasi kadar air terhadap perubahan nilai kohesi (c) dan sudut geser (φ). Penelitian menggunakan metode laboratorium terhadap sampel tanah longsoran dari Desa Sembalun Bumbung. Pengujian meliputi sifat fisis tanah, Standard Proctor Test untuk memperoleh kadar air optimum, dan Direct Shear Test pada beberapa variasi kadar air guna menentukan nilai kohesi dan sudut geser. Data dianalisis secara deskriptif untuk mengetahui pengaruh perubahan kadar air terhadap parameter kuat geser tanah. Hasil penelitian menunjukkan tanah memiliki kadar air 14,27%, berat jenis 2,68, berat isi basah 1,41 g/cm³, serta tergolong SC (Clayey Sand) USCS dan A-2-6 AASHTO. Kadar air optimum 17,53% dengan berat volume kering maksimum 1,608 g/cm³ menghasilkan kohesi tertinggi 0,142 kg/cm² dan sudut geser 33,258°. Peningkatan kadar air secara bertahap memicu degradasi mekanis, dari kondisi asli ke 19,294% (kohesi turun 18,68%, sudut geser turun 0,08%) ke 21,048% (kohesi turun 9,46%, sudut geser 6,05%) hingga kadar air ekstrem 43,33% (kohesi turun total 70,53%, sudut geser turun total 10,32%). Peningkatan kadar air terbukti menghilangkan kapasitas ikat butiran dan memicu ketidakstabilan lereng. Hasil ini diharapkan menjadi acuan mitigasi bencana longsor.