Perubahan Parameter Kuat Geser Tanah Lempung Organik Tak Jenuh pada Variasi Kepadatan Kering Alami Tanah

Abstract

Longsor yang dipicu hujan di wilayah tropis kerap bermula dari melemahnya kohesi semu saat tanah tak jenuh berangsur menuju jenuh. Penelitian ini menelusuri perubahan kohesi semu (c') dan sudut geser dalam (f') lempung organik tak jenuh seiring derajat kejenuhan (Sr) pada tiga kondisi kepadatan kering (?d) yang dikalibrasi dari lapangan, menggunakan sampel dari tiga titik longsor di Desa Petir, Dramaga, Bogor. Benda uji remolded dipadatkan ke tiga nilai ?d acuan (0,984; 0,870; dan 0,810 g/cm³), lalu dibasahi bertahap hingga tujuh tingkat Sr menyerupai infiltrasi hujan; sebanyak 231 benda uji geser langsung dan 42 benda uji kertas saring (Whatman No. 42) diuji untuk memperoleh parameter geser dan kurva karakteristik tanah-air (SWCC). Hasil menunjukkan nilai tekanan masuk udara (AEV) yang rendah dan hampir seragam (1,52; 2,09; dan 1,54 kPa), sehingga sumbangan matric suction cepat hilang begitu air meresap. Kohesi semu tidak menurun monoton, melainkan memuncak pada kejenuhan menengah (Sr 44–56%) dengan nilai berjenjang menurut kepadatan (10,60; 6,30; dan 4,69 kPa), sedangkan sudut geser menyusut konsisten (Pearson r = -0,953 hingga -0,806). Keunggulan kohesi pada tanah lebih padat hanya nyata saat lembap dan menghilang ketika mendekati jenuh, saat ketiga kepadatan menurun ke nilai rendah yang hampir sama. Hubungan ini dirangkum dalam model c' = 16,162·?d - 0,089·Sr - 5,076 (R² = 0,598; RMSE = 1,576 kPa). Temuan ini menegaskan bahwa kepadatan awal yang tinggi tidak menjamin kestabilan lereng ketika hujan deras mendorong tanah menuju jenuh, sehingga pengendalian infiltrasi sama pentingnya dengan pemadatan dalam mitigasi longsor.

Rainfall-triggered landslides in tropical regions often begin with the loss of apparent cohesion as unsaturated soil gradually approaches saturation. This study examines how the apparent cohesion (c') and internal friction angle (f') of unsaturated organic clay change with the degree of saturation (Sr) under three field-calibrated dry density (?d) conditions, using samples from three landslide sites in Petir Village, Dramaga, Bogor. Remolded specimens were compacted to three reference ?d values (0.984, 0.870, and 0.810 g/cm³) and progressively wetted to seven Sr levels mimicking rainfall infiltration; 231 direct shear specimens and 42 filter paper specimens (Whatman No. 42) were tested to obtain the shear strength parameters and the soil-water characteristic curve (SWCC). The soils exhibited low and nearly uniform air-entry values (1.52, 2.09, and 1.54 kPa), so the suction contribution dissipated quickly once infiltration began. Apparent cohesion did not decline monotonically but peaked at intermediate saturation (Sr 44–56%), with values tiered by density (10.60, 6.30, and 4.69 kPa), whereas the friction angle decreased consistently (Pearson r = -0.953 to -0.806). The cohesion advantage of denser soil was evident only under moist conditions and disappeared near saturation, where the three densities converged to a similarly low value. These relationships are summarized by the model c' = 16.162·?d - 0.089·Sr - 5.076 (R² = 0.598; RMSE = 1.576 kPa). The findings indicate that high initial density alone does not ensure slope stability when heavy rainfall drives the soil toward saturation, making infiltration control as important as compaction in landslide mitigation.