Strength and Compressibility Characteristics of Fiber-Reinforced Subgrade and their Effects on Response of Granular Fill-Subgrade System

In the present study, strength as well as compressibility characteristics of fiber-reinforced clay are studied. Synthetic fibers with different lengths (3, 6 and 12 mm) and different quantities (0.25, 0.5, 1 and 1.5 % by weight) are added with clay. Strength characteristics are studied by conducting unconfined compressive strength test and compressibility characteristics are studied by conducting oedometer tests on unreinforced as well as fiber-reinforced clay. All the samples are prepared at optimum moisture content to achieve maximum dry density. Based on the obtained experimental results, the effect of fiber reinforcement on settlement response of granular fill-fiber-reinforced subgrade system has been studied. The granular fill (representing subbase or base) is modelled by Pasternak Shear layer. The improved and unimproved sub-grade is modeled as nonlinear springs with different stiffness (modulus of subgrade reaction). The governing differential equations of the model are solved by finite difference scheme in an iterative manner and the results are presented in non dimensional form. It is observed that unconfined compressive strength of the clay increases due to addition of fiber. In terms of strength, 0.5–1 % fiber is observed as optimum quantity for the selected soil. It is also observed that compression index of the clay decreases due to the addition of fiber which indicates the reduction of consolidation settlement of the soil. The coefficient of consolidation of the soil increases due to the addition of fiber which indicates that rate of consolidation of the soil increase due to the addition of fiber. It is further observed that settlement of the granular fill-subgrade system decreases at a decreasing rate as the thickness of the fiber-reinforced zone increases. Significant improvement of the settlement due to the increase in thickness of the fiber-reinforced subgrade zone is not observed during the initial stage of consolidation. However, as the degree of consolidation (or time) increases more settlement improvement is observed due to the increase in the thickness of the fiber-improved subgrade zone i.e. the effectiveness of the use of fiber reinforcement increases as the degree of consolidation increases. Thus, to get significant amount of settlement improvement due to increase in thickness of the fiber-reinforced clay, sufficient amount of consolidation settlement should be occurred.