The effects of fibers or bentonite mixed with cement on the behavior of sandy soil

The main purpose of this study is to improve the engineering properties of sandy soil by mixing it with different materials such as cement, bentonite and fiber. The tests were performed at Istanbul Technical University, Geotechnical Laboratories. First, the soil was classified by performing sieve analysis and then its specific gravity and permeability were determined after that standard Proctor compaction test was performed to obtain the optimum water content and maximum dry unit weight for the soil. To evaluate the strength properties of the soil, unconfined compression tests and shear box tests were performed for the prepared samples. For the first stage samples were prepared to be used in unconfined compression test by using mini compaction device by mixing sand with (3%, 5% and 7% cement by weight) and randomly distributed 19 mm fiber with different ratios of (0%, 0.1%, 0.3% and 0.5% by weight) and optimum water content of the sand. The samples preserved in a moisture tank for 7 days or 28 days as curing time. The results for this stage showed that cemented samples with no fiber behaved in brittle way while by adding fiber the behavior changed from brittle to ductile behavior and it showed that the addition of fiber increased the unconfined compressive strength but the improvement percentage was dependent on cement percentage and fiber ratio. For smaller cement percentages like 3% cement, the effect of fiber was higher and more noticeable and it reached 80% improvement for 0.5% fiber content compared to the cemented samples without fiber and 50% improvement for 0.3% fiber while for 7% cement the effect of fiber was less where the percentage of improvement was between (20% to 30%) for (0.3% to 0.5%) fiber compared to the cemented samples without fiber. The unconfined compressive strength for the samples increased after curing the samples for 28 days compared to the 7 days cured samples. At the second stage, samples were prepared for unconfined compression test by using mini compaction device. The way of preparation was by mixing sand with (0%, 3%, 5% and 7% cement by weight) and (0%, 5%, 10% and 15% bentonite by weight) and after sufficient mixing, optimum water content was added to the mixture to ensure maximum dry unit weight. The samples were preserved in a moisture tank for 7 and 28 days curing time. The obtained results from the second stage showed that cemented sand samples and cemented samples mixed with bentonite had a brittle behavior while the sand-bentonite samples had a ductile behavior. The results showed that increasing the bentonite content for (0%, 3% and 5%) cement increased the unconfined compressive strength radically while for 7% cement; the addition of bentonite increased the UCS for 7 days in a small percentage and decreased UCS after 28 days. Curing the samples for 28 days increased the unconfined compressive strength compared to the 7 days cured samples. After preforming unconfined compression test, direct shear test was performed to gain a better knowledge about the effect of fiber reinforcement and bentonite on the shear strength and friction angel properties for the sandy soil. First fiber was mixed with sand in different ratios (0% 0.1%, 0.3% and 0.5% by weight), later optimum water content was added to the samples, and after one day, the samples were sheared by using direct shear machine. Adding fiber to the sandy soil increased its shear strength and friction angle for the tested samples. For this second stage (0%, 5%, 10% and 15% by dry weight of sand) bentonite were mixed with sand and later optimum water content was added to the mixture and after one day the samples were molded and sheared by using direct shear machine. The addition of bentonite in general decreased the shear strength of the samples and the friction angle.