The inclusion of gypsum within the soil poses challenges for the structural foundations constructed upon this particular soil type, particularly shallow foundation , where the rapid solubility of gypsum soils in water causes large gaps as a result of melting and drifting with water and this causes a subsidence in the foundations built on this type of soil and the landing may be irregular, which increases the structural problem complexity . This study focused on studying the structural properties of a shallow foundation based on two layers of soil (sandy and gypsum soil). Gypsum soil was prepared and brought from the city of Samarra in Salah al-Din Governorate with a gypsum rate of (63%). To know the properties fully and in detail, the study included forty experiments to load a square foundation with dimensions of (100 * 100) mm based on two layers of soil . The soil was compacted inside an iron box manufactured locally for this purpose and dimensions (900 * 900 * 500). To clarify further .The practical experiments were categorised into two groups. The first group consisted of four experiments conducted on a single layer of soil , specifically gypsum soil. These experiments encompassed various scenarios, including field density with immersion .The variables of interest include field density in the absence of immersion, maximum density when immersed, and maximum density without immersion. The obtained values for the field and maximum density values are (18.8,14.8) KN/m3. These four experiments were selected and used as a reference for comparison of values before and after replacement with a sandy soil layer. While the second group included conducting (36) experiments to replace a layer of gypsum soil with a sandy one, and the two layers were loaded until failure was reached. The used sand had a density ratio of (35%, 65% , 85%) and depth of the sand layer was measured at three different points:, B, and B/2 , 3/2 6 Abstract B. A correlation was established between the pressure and the resulting decrease. The findings The soil that contained gypsum demonstrated a notable degree of resistance, achieving a compaction value of 467 KN/m2 when subjected to field density, and this resistance reached (1069 KN/m2) when it was compacted with the maximum density, so the replacement process did not improve the Capacity of bearing of the soil. With immersion, the gypsum soil suffered a significant loss in its capacity of bearing , When the soil was subjected to compaction at field density, the act of removing it and subsequently replacing it with a layer of sandy soil resulted in an enhanced capacity of the soil to bear loads. The ratio of densities of layer type sand It was observed that the value reached 85% , when measured at B, B/2, and 3/2 B depths, respectively. This improvement did not appear when the soil was compacted to the maximum density . Finally, from the results, it is also clear that the process of soaking the gypsum soil under the sand with water clearly affects the Capacity of bearing of the measured sand, and thus leads to its decrease, and this effect decreases ,as the thickness of the sand layer increases, there is an inverse relationship between the two variables.
