The effects of fiber size on the mechanical properties of fly ash stabilized high plastic clay soil
This study describes the laboratory evaluation of the effects of the size and type of randomly oriented fibers on the strength and failure strain of fly ash stabilized high plasticity soil. Three different lengths of 12 mm, 25 mm, and 50 mm of two types of fibers, one with low and the other with high tensile strength were mixed (1%) with high plasticity clay soil and Class C fly ash (10%). The mixtures were prepared with water content 4% higher than the optimum (to achieve maximum strength) and compacted to prepare specimens with the same dry unit weight for unconfined compression test and split tensile test. After curing all specimens for seven days, the specimens were subjected to laboratory testing. The fiber inclusions increased the peak compressive strength of fly ash stabilized soils as much as 30% for polypropylene (black) fibers and 50% for polyethylene (white) fibers. Similarly, the strength at 15% strain increased from 0 to 34 psi for Black Fiber and 26 psi for White fiber. The strength of soil increases as the fiber length was increased from 0 to 25 mm, and then remains the same for Black fibers and decreases slightly for White fibers as the fiber length was increased from 25 to 50 mm. The 25 mm fiber specimens, which has an aspect ratio between the fiber lengths and the minimum dimension of the specimen of 0.44, showed the maximum enhancement of strength for both compressive and tensile strength. The slight decrease in compressive strength with 50 mm fiber (aspect ratio of 0.89) specimens might be due to the boundary effect and lower number (50% of 25 mm fiber and 25% of 12 mm fiber) of fibers along the failure plane. The 12 mm fiber specimen, which has an aspect ratio between the fiber length and the minimum dimension of the specimen of 0.22, showed minimum increase in failure strain and the failure occurred due to pullout of the fibers, whereas the 25 mm fiber (aspect ratio of 0.44) and 50 mm fiber (aspect ratio of 0.89) specimens showed the maximum increase in failure strain, and failure did not occur even at 15% strain. The length of fiber has a relatively lower impact on gaining strength, but has pronounced effect on failure strength or the ductility of the soil, which is usually severely compromised due to fly ash stabilization of high plasticity soil. The failure strain increased at least 132% when the 1 inch fiber was used. The fiber increased the tensile strength of fly ash stabilized soils as much as 70% for Black fibers and 80% for White fibers. Similar to the compressive strength, the tensile strength of soil increased as the fiber length was increased from 0 to 25 mm, and then remains the same for Black fibers and decreases slightly for White fibers at a fiber length of 50 mm. The 25 mm fiber specimens, which have an aspect ratio between the fiber lengths and the minimum dimension of the specimen of 0.36, showed the maximum enhancement of tensile strength for both types of fibers. The slight decrease in tensile strength with 50 mm fiber (aspect ratio of 0.72) specimens might be due to boundary effect and lower number (50% of 25 mm fiber and 25% of 12 mm fiber) of fibers along the failure plane. The tensile strength and the elastic modulus are much higher for White fibers compare to those of the Black fibers. However, the weight of White fibers is approximately seven times higher than Black fiber, and the 1% fiber content yields seven times more Black fibers than White fibers. Although the pullout resistance of White fibers is approximately 3 to 4 times higher than the Black fibers, but the Black fibers outnumbered the White fibers approximately seven times and this ultimately compensates for the lower pullout resistance compared to the White fibers where pullout is the primary reason of failure such as in split tensile test of soil stabilized with fly ash and 13 mm fiber and performed almost similarly to White fibers. The White fiber showed slightly higher compressive and tensile strengths where tensile strength and elastic modulus of the fiber is more important to resist failure, such as for soils stabilized with 25 mm and 50 mm fibers because of its higher tensile strength and elastic modulus.