Evaluation Of Improvement Techniques For Strip Foundation On Expansive Clay Soils In Gezira

Abstract

Most buildings and irrigation structures founded on Gezira soil, especially in agricultural areas e.g., Gezira Scheme, were subjected to damages due to up-heave forces caused by expansive soils. The owners of these buildings have paid much money in repairing and rebuilding these structures. The first aim of this study is to identify and classify the Gezira soil in order to facilitate studies and researches on it. The second aim is to evaluate the effect of some intercepting medias used between the expansive soil and brick strip foundation, through laboratory and field tests, to reach the adequate one to be used in Gezira soil. Disturbed and undisturbed soil samples were collected from depths up to 6 m at 13 locations in Gezira Scheme area, selected to represent the whole Gezira area. The samples were tested in the laboratories of the BRRI, University of Khartoum. The results showed that the soils of the whole investigated area, especially that taken from the upper 3 m, are clay of high plasticity and highly expansive. The results also showed that the clay depth in Gezira decreases from south to north and from near the Blue Nile to far from it. This supports the theory of Tothill (1946,1948) and Jewitt (1955), that the clay of Gezira was transported by the Blue Nile from Ethiopian high lands. To study and evaluate the effect of intercepting medias on expansive soil, some medias were tested in the laboratory by putting them between the compacted expansive soil, brought from Barakat, the headquarter of Gezira Scheme, and weights which simulate the weights of brick strip foundations on site. The used medias were cohesive non-expansive soil (CNS) of 2.5, 5.0, and 7.5 cm thickness, sand, expansive soil stabilized with 6% hydrated lime, gravel, and 10 mm steel plate with 20% perforations. The result of this test showed that the CNS was the best among the tested medias in resisting the vertical heave of the soil and the thickness has an important role. A field test was carried out at Barakat, where the soil samples were taken for the laboratory test, to study on site, the effect of the intercepting medias used in the laboratory test. Typical brick walls were built in the test area with the intercepting medias between them and the expansive soil. Two cycles of wetting and drying were carried out for a total period of 537 days. The results also showed that the CNS was the best in resisting vertical heave through the first two months of the first cycle but it lost this position in the second cycle. Another laboratory test was carried out to compare the developed swelling pressure between the CNS and sand of different thicknesses. The results showed that when the thickness of intercepting layer was small, less than half of expansive soil thickness, the sand was better in resisting the swelling pressure than the CNS and vice versa. From the above, it is clear that the CNS is effective in resisting the heave and swelling pressure of expansive soil but more investigations are required to insure the durability of this option.