An analysis of hydrological and geotechnical parameters of rotational landslides in pegmatite lithology in North Western Rwanda

An analysis of hydrological and geotechnical parameters of rotational landslides in pegmatite lithology in North Western Rwanda

Walraven, Bas (TU Delft Civil Engineering and Geosciences)

Bogaard, Thom (mentor)

Delft University of Technology

Water Management | Hydrology

The effect of land use change on the hydrogeotechnical processes triggering landslides in North Western Rwanda

2018

Worldwide landslides cause a great amount of damage, as is also the case in Rwanda. Here more and more slopes fail as anthropogenic activities such as building, farming, or deforestation, are moved to marginal lands such as hillslopes. To investigate the hydrological response of typical hillslopes in North Western Rwanda five landslides are chosen from a previously set up landslide inventory of the region. These five landslides form the basis of a regional assessment for which geotechnical parameters like soil texture, cohesion, and angle of internal friction, are analysed. For one of the five also hydrological data is gathered. This data consists of soil moisture content, groundwater level, hydraulic conductivity, and infiltration for both the moved and the stable parts of the slope. With all this data a back analysis is performed to gather why the slope failed.
The soil texture results show that most of the soil layers investigated are sandy soil, with a slight fraction of clay. This is supported by the values for hydraulic conductivity and infiltration, and by the results of the back analysis, which is coherent with literature values. The direct shear results, however, yield quite high cohesion values, typical for clay, and high angle of internal friction values (even too high for sand sometimes). Thus, the soils can be classified as sand, but the influence of the fines is significant. The slope failure can be a result of a very thin weak soil layer, or anomaly in the soil skeleton, but this is difficult to represent in the tests carried out, with such small samples. Another reason for slope failure does not have to be internal but can be external, such as anthropogenic activities, or toe erosion by a river. It is therefore important to analyse the surroundings of the failed slope carefully.
It is wishful to extend hydrological measurements to more landslides and also wait longer to be able to gain more insight into the relation between precipitation, infiltration and groundwater levels, and the hillslope’s hydrological response.

Rwanda
Landslides
Hydrology
Geotechnical

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© 2018 Bas Walraven