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Clay soils, due to their abundance and easy accessibility in nature, are significant materials implemented in the civil projects such as earthfill dams, canal embankments for water conveyance, and road subgrades. However, some types of these soils have low resistance parameters, which constitute major weakness and lead to numerous issues in engineering projects. Due to limitations and the scarcity of borrowed materials in certain engineering projects, soils from agricultural lands are used as embankments although they may contain high chemical salts due to excessive chemical fertilizers This study investigates the influence of four chemical substances—KH2PO4, NH4NO3, CaSO4, and NaCl—primarily added as chemical fertilizers to agricultural lands, and their potential impact on soil resistance parameters due to alterations in soil cations and anions content. To achieve this, natural soil was mixed with the mentioned substances in ratios of 10, 7, 5, and 3 wt percent of dry soil. The soil was then subjected to coarse structure tests on prepared specimens, including the Atterberg limits test, density, uniaxial compressive strength, elastic modulus, and direct shear tests. The results indicate that an increase in the percentage of these four substances leads to a decrease in the Atterberg limits., Additionally, it leads to an increase in soil volume unit weight and a reduction in optimum moisture content for specimens containing NaCl. For specimens containing CaSO4, NH4NO3, and KH2PO4, it causes a decrease in Atterberg limits, dry unit weight of soil volume, and an increase in optimum soil moisture content. Moreover, an increase in compressive strength was observed over the 180-day curing period for specimens containing 3 and 10 percent NaCl and CaSO4, demonstrating increments of 21.22–23.7% and 145–155%, respectively. Conversely, a reduction in compressive strength during this period was evident for specimens containing 3 and 10 percent NH4NO3 and KH2PO4, with reductions ranging from 70% to 125% and 43.9–70%, respectively. Furthermore, the interaction between these substances and soil particles was discussed and investigated to validate laboratory test results through microstructural analysis using X-ray diffraction and scanning electron microscopy. Electron microscopy images effectively illustrate the formation of calcium aluminate hydrate (CAH) and calcium silicate hydrate (CSH) phases, which can alter soil structure and consequently affect soil mechanical behaviors. |
