Slope Stability Analysis Using Tectostructural and Geotechnical Parameters: a Case Study of Tshondo Area, South-kivu Region (DRC)

Abstract

The study area is located in the Democratic Republic of the Congo, along the western branch of the East African Rift. The problem within this highly strained zone, is the lack of knowledge on slope stability and the diversity of interpretations emanating from several researchers on the geology and the tectonic evolution of Tshondo area. The study aims at broadly assessing the stability of slopes using tectonic, structural and geotechnical input parameters. In order to achieve this objective, a multidisciplinary approach has been adopted in this research. The structural survey has provided a comprehensive understanding of the tectonic evolution of Tshondo area and how different fractures influence on slope stability. Two phases of deformation were acknowledged by the PT and the Right Dieder methods: The extensional and the compressional deformation phases, highlighted by fractures generally trending NW-SE. The stress tensors responsible for these phases are made of principal axes defined such as σ1 ≥ σ2 ≥ σ3 and σ3 ≥ σ1 ≥ σ2. The corresponding stress ratios were, respectively R= 0.46 and 2.5. On a Mohr diagram, the failure analysis provided some slip tendency values (T’s) ranging from 1 to 0. The fault planes were above the critical stress (0.8 < T’s < 1), the joints, tensile and conjugate fractures were close to the critical stress (0.4 < T’s < 0.8), and the bedding planes were far below the main frictional line (0 < T’s < 0.4). The geotechnical field and laboratory investigations have revealed the presence of cohesionless soils on the upper horizons of the slope profiles. The underlying rocks presented poor strengths. Their undrained cohesion (Cu) values were 20.5 Kpa for completely weathered black shale, 19.5 Kpa for moderately weathered conglomerate, 32 Kpa and 27 Kpa for highly weathered sandstone and pelite. The thin section microscopy has revealed the presence of high proportions of clay minerals and oxides in most of these rock samples. The computation of the safety factors provided some values < 1: Respectively, 0.74 < FoS < 1 for slope 1, and 0.48 < FoS < 1 for slope 2. After applying the seismic coefficient of 0.21 in the pseudostatic analysis, these safety factors further reduced to 0.55 < FoS < 1 for slope 1, and 0.36 < FoS < 1 for slope 2.