PHYSICAL AND MECHANICAL CHARACTERISTICS OF SOIL AS A CONTROLLING FACTOR IN LANDSLIDES IN CURUG PANJANG VILLAGE, CIHUNI HAMLET, LEBAK, BANTEN: ANALYSIS OF DISTRIBUTION, NORMALITY, AND INTER-PARAMETER RELATIONSHIPS

<ns5:p> Background Landslides are heavily influenced by the geotechnical properties of the subsurface material. This study aims to evaluate the physical and mechanical properties of residual soil as a controlling factor in landslides in Cihuni Village, Lebak Regency. Methods A total of 30 residual soil samples were collected using the grid sampling method at depths of 0–1 meter. Laboratory testing included analysis of natural moisture content (w), plasticity index (PI) using the Atterberg limits, and Unconsolidated Undrained (UU) Triaxial tests to determine the cohesion (c) and internal friction angle (φ). The characteristics of the data distribution were assessed using a normality test via histograms and Q-Q plots, while inter-parameter relationshios were analyzed using scatter plots and the coefficient of determination (R <ns5:sup>2</ns5:sup> ). Results Statistical tests confirm that the parameters w, c and φ satisfy the assumptions of a normal distribution. Correlation analysis reveals a very strong negative linear relationship in the soil material. An increase in PI (10–41) linearly reduces the value of φ until it reaches a minimum of 5.5° (R <ns5:sup>2</ns5:sup> = 0.987). Degradation occurs as a result of water hydration, whereby an increase in water content weakens the electrochemical bonds in clay soil and causes a sharp drop in the cohesion value from 0.53 kg/cm <ns5:sup>2</ns5:sup> to 0.09 kg/cm <ns5:sup>2</ns5:sup> (R <ns5:sup>2</ns5:sup> = 0.995). The characteristics of the residual clay soil at the study site are highly sensitive to water. Conclusions Rainwater infiltration into the loose soil pore structure triggers saturation, increases pore water pressure, and causes an extreme deterioration in the shear strength parameters (c and φ). This significant reduction in total shear strength meant that the soil was unable to withstand the gravitational thrust of the slopes (8° – 35°), thereby confirming the internal mechanism that triggered the landslide in the study area. </ns5:p>