Landslide initiation in saprolite and colluvium in southern Brazil: Field and laboratory observations

The weathering of granitic and gneissic rocks in tropical regions can reach depths of more than 100 m. In southeast Brazil there are situations where landslide initiation depends on the fluctuation of the groundwater level, on the impact of falling rocks and on intense rainfall, causing superficial slides. The fluctuation of groundwater induces cyclical variations of the pore water pressure, and consequently of the effective stresses. This variation causes cyclic expansion and contraction of the structure of the saprolitic soil, weakening the imbrication of grains and loss of the cementation that may exist. This could be called a “fatigue” phenomenon. The practical effect is the lowering of the Mohr shear strength envelope, and a sudden rupture of the soil at a lower groundwater level than that which would be compatible with the intact soil strength properties, initiating a landslide.Another situation arises during intense rains, when a rock slab or a rock block detaches from the uppermost parts of a slope. This occurs where steep rock outcrops exhibit relief joints or where residual blocks of granite roll down the slope, impacting the compressible, saturated colluvial soil overlying the saprolitic soil. The sudden increase of pore pressure can liquefy the soil. Finally, another mechanism is that of the advance of a saturation front in a steep slope of unsaturated saprolitic soil, reaching a depth below the root zone. The loss of the cohesion due to suction, without the beneficial contribution of the roots to the shearing strength, causes a sudden slide. During extreme rain episodes literally hundreds of such superficial slides, reaching 1 to 3 m in depth, occur in a given basin. The concentrated runoff that flows along the surface of the thalweg of the basin carries this soil in a muddy state, and a debris flow ensues.