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Fine-grained soils, especially those consist of high content clay minerals, generally have high strength in dry state, but they loss their strength when subjected to absorb water as well as they may be swell. These phenomenon may lead to damage the structures located on them and it is required to stabilize them with different additive materials. Common additives for stabilization of problematic soils are cement, lime, fly ash, etc., which are almost costly and have environmental consequences. Nowadays, non-traditional material such as polymeric materials were added to the soils in order to their stabilization.
In this research, the effect of polyester polymer resin on the physical and mechanical properties of fine-grained bentonite soil, with liquid limit and plasticity index of 226% and 179% respectively, was studied by performing Atterberg limits and unconfined strength tests. For this purpose, a polyester polymer resin were added to bentonite soil with various amount of 1.0, 2.5, 5.0, and 7.5 percent in dry weight. Soil mixtures were cured about 7 days and Atterberg tests were performed on them. Moreover, cylindrical specimens with 50 mm in diameter and 100 mm in height were prepared and cured during 7, 14 and 28 days. Soil specimens compacted in a splitted steel mold at four equal thickness layers. Then, the unconfined compression tests were carried out on these specimens with loading rate of 0.5 mm/min.
The results of Atterberg tests indicated that the addition of polyester resin reduces liquid limit and increases plastic limit of bentonite and consequently it is led to decrease in plasticity index of the soil.  Maximum decrements in liquid limit and plasticity index observed in treated soil with 7.5 percent polyester, which are about 41.0 and 65.0 percent respectively. In addition, the polyester resin improves the unconfined compressive strength of the soil and the rate of increment is high when the polyester amount rises from 0 to 7.5%. It was revealed that the polyester resin influences the unconfined strength considerably in a short time, and the rate of improvement gradually decreases with passing time. For example, adding 5.0 percent polyester polymer resin to bentonite soil improves the strength up to 2.05, 2.11 and 2.58 times respectively after curing times of 7, 14 and 28 days. It also means that after 7 day curing time the improvement effect of stabilizer is considerable and by passing time its effect diminishes. Moreover, adding polyester polymer resin to the soil decreases deformability of soil; it means that this additive cause the treated soil exhibits more brittle behavior rather than the pure soil. The photos of specimen after failure explain that failure surface of pure bentonite is inclined in relation with horizontal, while it tends to vertical direction in treated specimens. Analysis of SEM images, results of XRD analysis and FTIR spectroscopy suggest that polyester stabilizer penetrates into the layers of soil particles and, by inducing effective interaction makes the layers closer or sticks them together. These phenomena decrease water absorbing tendency in soil minerals and improve the plastic characteristics and shear strength of the soil.

Keywords: Bentonite stabilization polyester resin unconfined strength SEM images XRD analysis FT-IR spectroscopy

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