Engineering properties of tropical transported soils: case study from the Republic of the Congo

Tropical residual soils are often reported to exhibit highly variable geotechnical characteristics, due to formation by rock weathering under hot, humid conditions. Less is known about the engineering properties of tropical reworked (i.e. transported) soils. The latter soil type is also encountered in tropical regions, but can exhibit sorting and include weak structures formed during transport and deposition, causing instability when exposed in road/rail cuttings. We report a case study of geochemical and engineering properties of tropical reworked soils from the slopes of Mt Lekoumou (Republic of the Congo), a Banded Iron Formation (BIF) ridgeline overlain by loose, unconsolidated clayey gravel. Aside from the upper-most slopes, the area is overlain by colluvium of varying thickness, the target of testing, here. Goethite, kaolinite and hematite are dominant minerals, and the colluvium displays moderately high liquid limits, but comparatively low plasticity index values. Effective cohesion (c') values were 0 and 31 kPa and effective friction angles (ϕ') were 26°-39°. Considerable variation in matric suction pressures for the same sample sites, at the same moisture contents, occurs. Thus, as with tropical residual soils, determining test values that are ‘representative’ of site conditions in tropical transported soils can be problematic, but remains important.