Stability of earth slopes with detailed consideration of sliding failures along TANZAM railway
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Date
1982
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University of Dar es Salaam
Abstract
Failures of cut and earth fill embankment slopes in the Mlimba-Makambako Section of the Great Uhuru Railway (Tanzam Railway) that occurred in the rainy season (March-April) 1979 were so extensive that the line had to be closed for 30 days while initial remedial works were being undertaken. There being no consistent and comprehensive information available in TAZARA as regards soils classifications, strength parameters and changes in pore water pressures for the main soils in this section has impeded progress of the rectification works. Most of the repairs done on the rail road have been mostly on adhoc basis and on the basis of the expertise of the Chinese Engineers who gathered quite some practical experiences with these soils during the initial construction of the Railway. It has therefore been necessary for the management of TAZARA to hire the services of soils engineering consultancy to carry out a comprehensive study of this particular section and propose definite specifications for the design and construction of remedial works for the failed slopes and stabilization for the unstable ones. To this end M/S Coffey and Partners of Australia are currently engaged in this study. This thesis however is not part of this formal undertaking commissioned by TAZARA as afore-mentioned. It is an independent effort by the author geared to the study of the conditions that generally led to instability of slopes in this area and particularly on the two major slides that occurred at both ends of Kitete station. The thesis covers a general review of the climate conditions of the area, the general review on the state of the art in slope stability analysis beginning with the historical development of various methods of analysis to the present stage of computerised systems of analysis. The route location in this area is studied in relation to the topography and geology with emphasis on the soil formation and pertinent geological features. A review of the design specification of the slopes and subsequent construction is undertaken. The location and types of failures are generally studied and classified in general terms as shallow and medium slides, deep seated side slope slips, base failures, erosion on cuttings, and underground erosion and collapse of grain structure. The post failure investigations entailing boring and logging, measuring of penetration resistance, soil sampling, measurement of ground water level (phreatic surface) in cased boreholes, and laboratory classification and strength tests for the two slides at Kitete station are recorded. The main soil comprises of a three layer system of residual soils, rising out of the weathering of gneisses and granites, that begins at the top with a reddish - brown silty sandy clay followed by a pinkish - purple clayed silty sand which overlies the grayish clayed sand which in turn is next to the weathered rocks. Sixteen samples selected from assumed slip zones are classified and 9 of them are tested on a triaxial cell to determine the strength parameters namely cohesion and internal friction under consolidated undrained conditions. The parameters are found to be related to the types of soils mentioned above with the reddish-brown sandy clay having angles of internal friction ranging from 23.8° to 29.5° and effective cohesion of 52.9 KN/M2 and 39.2 KN/M2 respectively for the naturally formed soils and Ø' = 18.5° with C' =58.9 KN/M2 for the remolded (fill material) sample from same type of soils. The pinkish-purple layers have angles of friction ranging from 25° to 31° and C' = 19.6 KN/M2 and 0 - 9.8 KN/M2 respectively. The greyish soil has a lower angle of friction than the pinkish purple soil. The pore water pressures were also recorded. These parameters are used in the analysis of the stability of the slopes that existed prior to slides No. I and II. The variation of pore pressures and the assumption of various shear cases involving the reduction of effective cohesion to as much as 50% are considered in the stability analyses using a special computer programme and results indicate the conditions of failure of each slide. Proposals are made for the general rectification options of each of these slides.
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Available in print form
Keywords
Landslides, Slopes (Soil mechanics), Tanzania
Citation
Salema, L. M (1982) Stability of earth slopes with detailed consideration of sliding failures along TANZAM railway, Masters dissertation, University of Dar es Salaam. Available at (http://41.86.178.3/internetserver3.1.2/detail.aspx)