Geotechnical and Mineralogical Properties of Granite Regolith Related to Nucleation Mechanisms of Debris Flows in Tropical Areas

抄録

Debris flows are a singular type of mass movement with high impact and great destructive power. They occur in small catchments with high relief. Debris flows are generated by landslide nucleation in steep initiation zones. The understanding of geological and geotechnical behavior of the regolith and colluvium in these areas is very important to understand debris flows generation and runoff. The present study analyzes a 2.5 m thick regolith profile in a granite saprolite, located in a large scar slip (4.3 km² area) in the Gigante's Creek catchment in the Serra do Mar range, southern Brazil. Six samples were taken, corresponding to (1) Fresh rock ; (2) Slightly weathered rock ; (3) Soft weathered rock II ; (4) Soft weathered rock I ; (5) Transition zone material and (6) Homogeneous saprolite. Quartz is the most common mineral occurring throughout the profile, while biotite only occurs in deeper horizons. Illite, halloysite, gibbsite, and kaolinite are the secondary minerals at the intermediary zones of the profile, while kaolinite and gibbsite dominate the top of the profile. The proportion of texturally sandy material increases from top to bottom of the profile. The transition zone material, sampled at intermediate portions, have the same textural behavior as the catchment's colluvial materials. The physical indexes show that specific weight of solids (γ s) increase with depth, with the less dense newly formed phase in the upper portions of the regolith. The differences in the voids ratio ( e ) and soil moisture ( W ) show variations within the profile, and that coincides with the kaolin minerals formation and major landslide disruption zones observed in the field. The understanding of these phenomena will help in defining the initiation zones of landslides with the potential to set off debris flows generated by translational landslides.