Performance Improvement of Weak Sub-Grade Soil by Nylon Synthetic Fiber

Abstract

Improvement in subgrade has always been an area of concern to the highway and geotechnical engineers. In the case of a highway, a weak subgrade results in a greater thickness of pavement layer, which increases the cost of pavement construction. The important factor for the design and construction of pavement is the behavior of the underlying sub-grade. Large deformations in the sub-grade will lead to a continuous deterioration of the paved surface. As the subgrade soil is too weak to bear the load, engineering solutions such as replacement, chemical stabilizations, geo reinforcement using geosynthetics which are earlier used in sub-grade improvement, and such experimental study is carried by previous authors. In this study, nylon synthetic fiber (NSF) was used to improve the weak sub-grade. Synthetic fibers mixed randomly with subgrade soil have a significant impact on the improvement of subgrade characteristics over the last decade. Keeping this in view an experimental study was conducted on locally available weak sub-grade soils by reinforcing with nylon synthetic fiber (NSF). The soil samples were prepared with and without fiber reinforcement. The percentages of fibers by dry weight of soil were 0.5%, 1%, 1.5%, and 2.5% and the aspect ratio (fiber length of 10 mm and 20 mm, and the diameters were 0.3 mm and 0.4 mm). Field density, unconfined compression strength, natural moisture content, particle size distribution, Atterberg’s limit, specific gravity, Proctor test, and soaked California bearing ratio (CBR) tests were performed on natural soil whereas Proctor test and soaked California bearing ratio (CBR) tests were performed on nylon synthetic fiber (NSF) reinforced with weak subgrade soil to investigate its suitability as subgrade material. Soil samples for California bearing ratio (CBR) tests were prepared at its maximum dry density corresponding to its optimum moisture content in the CBR mold without and with nylon synthetic fiber (NSF). The effects of nylon synthetic fiber (NSF) on CBR and swells of the soil investigated, by varying its contents and aspect ratios (diameter and lengths) of the fiber. Results showed that there was a decrease in the value of swell with an increase in fiber content. From CBR test results, the CBR value of soil increases with the increase in nylon synthetic fiber (NSF) content and aspect ratio. It was also observed that increasing the diameter of NSF further increases the CBR value of reinforced soil, and this increase is substantial at a fiber content of 1.5% for an aspect ratio of 50 (length = 20 mm, diameter = 0.4 mm). The maximum enhancement in values of CBR is 4.42 times of the natural soil with 1.5% fiber reinforcement of 20 mm length and 0.4 mm diameter, under soaked conditions. Swelling of the soil decreases as increasing the percentage of fibers in the soil. It also observed that the optimum moisture content almost remains constant with the increase in nylon synthetic fiber (NSF) content and the maximum dry density increases with the increase in NSF content