Mekanisme gerakan longsoran tanah lempung akibat input curah hujan di pegunungan Kendeng Kabupaten Grobogan Jawa Tengah
HUDA, Miftahul, Prof.Dr. H. Sutikno
2001 | Tesis | S2 GeografiPenelitian ini bertuj uan menganalisis mekanisme gerakan longsoran tanah lempung alubat input curah hujan di Pegunungan Kendeng Kabupaten Grobogan Jawa Tengah. Penekanan pada perilaku tanah lempung khususnya sifat geotekmk yang mempenganh stabilitas lereng dan kejaQan longsoran alubat perubahan kondisi ludrologi lereng. Cara penelitian dengan metode survei pada lokasi longsoran Q lapangan dan pengujian di laboratorium. Data meliputi kondisi curah hujan, kondisi geologi, geomorfologi, hidrologi, penggunaan lahan dan tanah. Berdasarkan peta satuan medan dllakukan analisis pengaruh bentuklahan, batuan, kemiringan lereng dan macam tanah terhadap agihan longsoran. Perilaku dan sifat geotekmk tanah lempung dianalisis berdasarkan uji triaksial terkonsolidasi tanpa drainase dan uji geser langsung. Uji simulasi untuk mengetahui pengaruh kandungan lengas terhadap perilaku mengembang dan mengkerut, perubahan tahanan geser, kohesi dan sudut gesek dalam, serta proses terjadinya longsoran. Simulasi longsoran di laboratorium berdasarkan pola curah hujan, keminngan lereng material tanah daa bidang gelincir untuk mendapatkan model-model persamaan matematis variabel debit airta.nah, debit sedimen airtanah, debit limpasan, debit sedimen permukaan dan kejenuhan tanah yang mempengaruhi proses longsoran. Uji regresi ganda dengan metode backward ezimination digunakan dalam pembuatan model persamaan matematis prediksi. Hasil penelitian menunjukkan bahwa di daerah penelitian terdapat 3 tipe longsoran yaitu rayapan, nendatan dan longsor, yang agihannya terpenganih kondisi variabel penyusun satuan medan. Secara alami faktor keamanai lereng medan tidak stabil (Fscl). Perilaku tanah alami menunjukkan perubahan ketika terjadi pergeseran tanpa drainas, seperti retak, pelebaran clan pemendekan dimensi sampel. Pen& kadar air terlihat dari sifat mengembang dan mengkerut material tanah longsorm deDgm bentuk grafik hubungan COLE dan kadar air yang berbeda. Material simulasi mempunyai kondisi kandungan lengas optimum unkk dapat Qanalisis sifat tahanan geser, kohesi dan sudut gesek dalamnya. Perubahan sifat geotelcnik tanah dapat diamati pada proses ketika dilakukan uji simulasi dengan menggunakan curah hujan. Simulasi bequmlah 16 model uji dengan sebuah model dltetagkan sebagai model umum, berdasarkan seluruh uji yang dilakukan pada 15 model lainnya. Hasil simulasi menunjukkan bahwa proses terjadinya longsoran Qawali dengan penjenuhan dan konsolidasi material secara alami oleh curah hujan, erosi per& yang berkembang menjab ervsi alur dan parit, pembentukan retakan dan gawk longsoran, material tanah bergerak dalam bentuk rayapan yang berkembang menjadl nendatan dan longsor, material menjadi aliran dan selanjutnya terdeposisi dan berkembang ke arah pembentukan meandering alur. Kondisi uji simulasi hampir sama seperti yang terjadi di lapangan. Model-model -prsamaan yang dikembangkan dari hasil simulasi mempunyai determinan- tinggi (R2>0,7), kecuali untuk model persamaan kejenuhan tanah. Perbedaan kemiringan material dan bidang gelincir yang semahn besar menjadikan material tebal dan pada kemiringan >30°, tanpa dorongan curah hujan material *elah bergerak karena gravitasinya Lajn infilb-asI yang terpengaruh curah hujan besarnya sangat ditenhkan oleh proses geomorfologi selama simulasi. Model-model persamaan hasil simulasi yang &pat +gunakan dengan kriteria rnendekati kondisi lapangan serta rnempunyai nilai detenninanb> 0,7.
The objective of the study is to analyse the movement mechanism of the landsliding of clay soils in Kendeng Ridge, Grobogan District, Central Java. The study focusses on the behavior of clay soil, especially, its geotechnical characteristic affecting the slope stability and landsliding which are caused by the changes of hillslope hydrology. Two methods, i.e. surveys on landslidmg location and laboratory testing, were carried out in study. The data collected in this study included the condition of rainfall and geology, geomorphology, hydrology, land use and soil. Based on terrain unit map, the analysis of effects of the landforms, rock types, slope steepness and sub group of soil to landsliding distribution was conducted. The behavior and geotechnical characteristics of the clay soil was then analysed by using triaxial consolidation undrained and direct shear test. After triaxial consolidation undratned and direct shear test: simulation test was conducted to find the effect of the water content to swelling and shnnkage behavior, the shear strength changes, cohesion, and angle of the internal friction, and the landsliding processes. Landsliding simulation in laboratory which was based on the rainfall pattern, the slope of the material, and slip surface was conducted to construct/formulate mathematics formula models of the variable groundwater dwharge, sediment discharge in groundwater, overlandflow discharge, sediment of the overlandflow and water content influencing landsliding. Multiple regression by backward elimination method was used as the instrument to construct the predictive mathematics formula models. The result of the study show that there are three types of the landsliding in research area., i.e. creep, slump and slide, which their distributed is affected by terrain unit variables. Naturally, the safety factor of the terrain is unstable (Fs-4). Natural behavior of the clay soil indicates that the changes happen when the undrained shearing like cracks and the changes in the Sample dimension occur. The effect of the water content can be seen from the swelling and shrinkage characteristic in graphical form of COLE and water content in different conditions. The chcteristics of the shear strength, cohesion and angle of internal friction can be amlysed when the simulation material has optimum water cofiteut. The changes of geotechnical characteristics can be observed through the prccess of simulation done with the rainfall input. There were 16 models in the simulation with one modelwhich serve as as general model, decided on the basis to the other 15 models. The result of the simulation show process of the landsliding started from natural saturation and consolidation materials leading to splash erosion whch then turned into rill and gully erosion. This creates cracks and escarpments. The soil material flows in creep, slump, and slide types, solifluction (earth flow) and the deposition material which then developed into meandering rills. The conhtion of the simulation test were closely similar to the conditions on the fields (terrain). The formula models developed have high dterminant value (R2>0,7), except #he water content formula. The differences in the slope steepness and slip surface made the material thck on 30°, without the induced rainfall, the material flows due to gravity force. The infiltration rate affected by the rainfall and geomorphologic processes during the simulation. The criteria of the formula model whch used in the simulation is approximately similar to the field (tenain) conditions. The determinant value of the model >0,7.
Kata Kunci : Geografi Fisik, Gerakan Longsoran, Tanah Lempung, mechanism of landsliding, rainfall input, behavior of clay soil, landsliding simulation, prediction of mathematical formulas
