Shallow foundations are usually built above the groundwater table. In such cases, this soil is considered dry in analysis and design, while it is actually an unsaturated soil. This unsaturated soil ...can lead to many geotechnical issues, such as swelling when wet and collapsing when dry. It can also change the soil’s shear strength, which in turn affects the bearing capacity of the soil. The classical principles of saturated soil are often not suitable for explaining these phenomena. Limited studies dealt with a large scale model close to the field conditions for such soil and therefore, there is much more room for improvement. The study reveals that compacting the expansive soil on the wet side of optimum is capable of reducing swelling pressure and the expansion index for both the bentonite–sand mixture and kaolin soil. The bearing capacity of a footing resting on both swelling and non-swelling soils decreases noticeably with the increase in the soil degree of saturation due to the effect of matric suction which increases the soil stiffness. The results show that the measured bearing capacity is (1.1–3.3) higher than that estimated by Terzaghi equations.
Limited studies dealt with the expansive unsaturated soils in the case of large-scale model close to the field conditions and therefore, there is much more room for improvement. In this study, ...expansive （bentonite–sand （B–S） mixture） and non-expansive （kaolin） soils were tested in different water contents and dry unit weights chosen from the compaction curve to examine the effect of water content change on soil properties （swelling pressure, expansion indices, shear strength （soil cohesion） and soil suction） for the small soil samples. Large-scale model was also used to show the effect of water content change on different relations （swelling and suction with elapsed time）. The study reveals that the initial soil conditions （water content and dry unit weight） affect the soil cohesion, suction and swelling, where all these parameters slightly decrease with the increase in soil water content especially on the wet side of optimum water content. The settlement of each soil at failure increases with the increase in soil degrees of saturation since the matric suction reduces the soil ability to deform. The settlement observed in B–S mixture is higher than that in kaolin due to the effect of higher swelling observed in B–S mixture and the huge amount of water absorbed which transformed the soil to highly compressible soil. The matric suction seems to decrease with elapsed time from top to bottom of tensiometers due to the effect of water flowing from top of the specimen. The tensiometer reading at first of the saturation process is lower than that at later period of saturation （for soil sample B–S3, the tensiometer #1 took 3 d to drop from 93 kPa to 80 kPa at early stage, while the same tensiometer took 2 d to drop from 60 kPa to 20 kPa）.
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