|Abstract: ||Vast areas of Sudan and South Sudan are covered by alluvial soils derived from the River Nile and its tributaries, across its historic flood plain. These soils are often highly plastic and expansive in nature and cause many problems in the design, construction and operation of geotechnical structures such as highway foundations and embankments involving compacted fills.
The mechanical responses of two natural alluvial (Nile) soils from Khartoum, Sudan, have been investigated experimentally. Disturbed samples of a black cotton clay and red clay, both highly plastic, were used to prepare compacted and reconstituted samples. Undisturbed block samples of the black cotton soil in its unsaturated state were taken to investigate the influence of its natural in-situ fabric.
Both soils have been fully characterised and intact, reconstituted, and compacted samples tested using advanced triaxial and oedometer (conventional and osmotic) apparatus. Soil-Water-Retention-Curves (SWRCs) were generated for the soils using the filter paper technique to establish their unsaturated response during drying and wetting cycles (the influence of sample shape was also investigated). The SWRCs indicate that both soils develop significant hysteresis between first drying and wetting curves but for subsequent cycles little hysteresis occurs. The onset and form of cracking was observed during the SWRC drying. There is no significant effect of sample's sizes on SWRC.
Both clays exhibit a high swelling potential when the moisture content increases from its in-situ value to full saturation. During undrained shearing reconstituted samples had a much more brittle response after K0 consolidation compared with isotropic consolidation while the soils' stiffness response for both K0 and isotropic consolidation is very similar. The strength of natural unsaturated samples was found to increase with confining stress. Tests using osmotic oedometers indicate that, for both the black and red soils, the magnitude of suction has a significant effect on compression and expansion indices, with them decreasing with increasing suction. The elasto-plastic models proposed by Wheeler and Sivakumar (1995) and Alonso et al. (1990) (BBM) were found to not be suitable for predicting the behaviour of the unsaturated compacted black and red soils as the results show only very limited signs of a `yield point', i.e. there was negligible loading collapse behaviour observed at the stress levels used in the tests performed, for both these Sudanese clays.|