Engineering geological mapping procedures in data-scarce hillsides for shallow landslide assessments: Applications in southeastern Brazil

Abstract

This paper shows the results obtained from a group of simple and basic procedures used to assess slope instability in terms of shallow landslides on data-scarce hillsides at the slope unit scale. The procedures are based on engineering geological mapping to obtain geological and geotechnical data and on an adapted Hovland 3D safety factor (SF3D) associated with a van Genuchten infiltration model to simulate the rainfall percolation. The procedures were applied on a data-scarce hillside in the Campos do Jordão municipality located in the Mantiqueira Mountains, São Paulo state, Brazil. This area was chosen because it is affected by shallow landslides due to a loss of shear strength when suction decreases or dissipates due to rainfall infiltration. The combined procedures permitted the total SF3Dto be obtained from the partial SF of each grid cell (considering 4 different rainfall events and quasi-saturation conditions) and the failure probability (SF3D < 1) and marginal probability (1 <SF3D < 1.5) to be calculated for 2 potential rupture surfaces (PRS1 and PRS2) on each slope. The total SF3Dresults under quasi-saturation conditions, which are generated primarily by rainfall events of long duration, are less than 1 for part of the slopes but less than 1.5 for all the slopes, which reflects the favorable conditions for shallow landslides to occur. The failure probability values (SF < 1) for all slopes vary from 0.01 to 0.05 for PRS1 and from 0.02 to 0.12 for PRS2, and the reliability index values range from 1.64 to 2.35 and from 1.22 to 2.08 for PRS1 and PRS2, respectively.All of these results verify that all the slopes and selected potential rupture surfaces are compatible with the occurrence of shallow landslides on December 31st, 1999/January 1st, 2000.Large-scale engineering geological mapping proved to be fundamental because it provided detailed mapping of the spatial arrangement and characterizations of the geological material, thereby permitting adequate definitions of the potential rupture surfaces and the adoption of geotechnical and hydraulic parameters to calculate the SF3D and the failure probability (SF < 1).