Stability of cut slopes in coastal Tanzania

Lucas, R. O

Stability of cut slopes in coastal Tanzania

Date

1979

Authors

Lucas, R. O

Publisher

University of Dar es Salaam

Abstract

It appears that no standard tangible guidance for the design of cut slopes is available at the moment to cater for the soils of the coastal area of Tanzania. Thus a slope stability research was under taken, in which the instability problem was studied, assessed and design guidelines proposed. The research was conducted by a systematic empirical approach. Long term literature confirms that the instability of cut slopes is much more marked in the case of high (H> 5.0 m) slopes than with low (H< 5.0 m) slopes. Thus the instability problem first became appreciable when it was necessary to cut high slopes. This dates back to the early 19th century. during the pioneer railway construction works in the United Kingdom. Initially the problem was solved by trial and error. However, since the introduction of the science of Soil Mechanics some 40 years ago engineers have developed rigorous quantitative analyses of the stability of cut slopes. These methods are based on the characteristic slip surface shapes namely, the plane surface for homogeneous non-plastic granular soils and the logarithmic curve (generally approximated as circular for computations) surface for homogeneous cohesive soils. The empirical approach is based on the critical relationships between slope angles and their corresponding slope heights for each major soil type as observed from the field performances of the existing out slopes. In the tropics, such as the coastal area of Tanzania the soils are in general partially saturated. They are wetted during the rainy seasons, by infiltrating water, only to a limited depth. The depth of wetting in the research area extends to less than 3.0 m into the ground surface. This situation results in shallow failure zones. The analytical methods for slope stability were developed for deep failure zones. Thus these methods are reliable in the cases of shallow zones. On the other hand the empirical adoptable to this condition. Previous researches elsewhere in the tropics have that under these conditions the empirical is superior to the analytical methods. The research area lies coastal belt of mainland Tanzania between 5°15'S 7°50'S; and between longitudes 38°00'E and 39°30'E. It extends as far north as the Pangani river and as far as the Rufiji river. The area is mostly by well developed ne Neogene to recent while the north west area is covered by a crystalline basement. There are three major types in the research area namely the red laterite soil, the mottled clay/clayey sand and the grey clayey sand. These soils represent the different soil of the various stages of the laterization process. Thus the soils were by the Extended Unified Soil Classification System (EUSCS) for leterite soils as CI, SCI and SCI respectively. A total of 49 selected slopes were systematically studied. The field observations showed that slope heights and slope angles had profound effects on the stability of slopes. Out of 26 low slopes studied only 2 had failed by shear and out of 23 slopes, 11 had shear failures and 9 had erosion failures. Of the 22 flat slopes (6<50°) studied shear and one was eroded; and of the 29 slopes (6>50°), 10 failed by the shear and 9 by erosion. For each type slope heights were plotted against their corresponding slope angles. From these plots critical curves were obtained for stable slopes, and guides determined. The mode of application of these design guides are fully explained in the main text. Of the three major soil types the laterite soil was found to be the most resistant to both shear and erosion failures. It is followed in this respect by the grey clayey sand and lastly by the mottled clay/clayey sand. The difference field performances of the various soils were adequately explained by each of the following laboratory comperative tests: suction pressure test, water absorption capacity and the triaxial tests.

Description

Available in print form

Keywords

Slopes (mechanics)

Citation

Lucas, R. O (1979) Stability of cut slopes in coastal Tanzania, Masters dissertation, University of Dar es Salaam. Available at (http://41.86.178.3/internetserver3.1.2/detail.aspx?parentpriref= )