A kinematic analysis of open excavation slopes in discontinuous rock masses: a case study of Gataka building stone quarries in Ololua-Nairobi area, Kenya

Abstract

This project work presents the results of a kinematic analysis of the open excavation slopes at the building stone quany sites in OIoIua - Nairobi area. The study was aimed at establishing modes of failure mechanisms associated with the quarry excavation slopes in discontinuous rock masses found in OIoIua- Nairobi region. Three major zones habouring randomly situated quarry sites.were studied and tbeic respective disoontiwJities analyzed in detail to depict their geometrical and mechanical characteristics. Discontinuity data that comprise intersection distance, orientation, semi-trace length.and termination of discontinuity structures were obtained from various quarry faces by physical measurement. The shear strength parameter of various fracture planes and samples was tested in the laboratory as a mechanical characteristic. The geometrical and mechanical data obtained from the study area were statistically analyzed using stereograpbic projection, through which various modes of failure mechanisms that can affect the excavated slopes and the rock quality designation (RQD) of the excavated rock-masses of the quarry sites in the Ololua - Nairobi area have been established. The results of this work indicate that the quarry slopes in Ololua region mainly experience wedge type failure mechanism Toppling and Planar failure mechanisms are present only on minor scale. The Rock Quality Designation (RQD) and discontinuity Termination Index were established and they indicate that the overall discontinuity -frequency in the region is well over the threshold value of O.lm. Most of the discontinuity traces terminate in intact rock rather than in other discontinuities, The seepage of surface water into these cracks contribute to block failure mainly through changes in pore water pressure which decrease plane cohesive friction. The tremors associated with high seismic distribution in the area also contribute to slope yields. The average dip angles of discontinuity planes is well over 85°while the mean angle of friction is 37°. A combination of these findings supports the possibility of excavations in this region to posses slope angles as high as 60°, designed as benches that may remain stable without artificial reinforcement for a considerable period of time.