MODELLING OF FIBRE REINFORCED FLEXIBLE PAVEMENTS

Abstract

One of the major challenges in pavement design is to investigate the innovative methods of improving the mechanical properties of subgrade soils. Reinforcing the soil with short fibres is one such method, which can give technically as well as economically a superior solution for improving the engineering performance of subgrade soils. Since the pavement layer thicknesses are dependent on the strength of subgrade soil, reinforcement of subgrade with short fibres help in reducing the requirement of layer thickness. This results in saving of construction time and natural resources. Three types of soils were selected in the present study. The index properties of these soils were determined and the soils were classified as Clay soil (A-6), Silty clay (A- 2-4) and Fine sand (A-3) as per AASHTO and CL, ML and SM as per US systems respectively. The polyproplene fibres having a diameter of 0.3mm were cut into pieces having lengths of 15 mm, 25 mm, and 30 mm giving an aspect ratio of 50, 84 and 100, respectively. In the first part of the present study, optimum quantity of fibre content was decided based on California Bearing Ratio (CBR) and unconfined compressive strength. These tests were conducted on soils with four different fibre contents and three different aspect ratios. Also, static triaxial compression strength tests were conducted on unreinforced and reinforced soils at three confining pressures of 40, 70, and 120 kPa to study the effect of confining pressure on static response of unreinforced and reinforced soils. Results of these tests suggest that the values of cohesion and modulus of elasticity increase substantially and angle of internal friction increases only marginally due to presence of reinforcement. n Cyclic triaxial tests were conducted with the intension of comparing the resilient behavior of unreinforced and reinforced soils at optimum fibre content. Cyclic loads producing the vertical stress (oO of 194 kPa and 264 kPa were applied at the rate of 70 cycles per minute. For each loading cycle, two different confining pressures (a3) of 40 and 70 kPa were selected to produce four deviator stress components (oi-a3). The results showed that both resilient strain and the permanent strain decrease with confining pressure but increase with number of load cycles and the deviatoric stress for both unreinforced and reinforced specimens. The resilient modulus has inverse relation with resilient strain and hence it decreases with number of load cycles and the deviator stress. Also, the resilient modulus of reinforced specimens at any number of load cycles and deviator stress are larger than that for unreinforced specimens. Constitutive laws of soils define the mechanical response and mathematical behaviour of soils to different loading. These are of prime importance for analyzing almost all applied non-linear problems ofsoils. In the present study, data obtained from static triaxial tests for different unreinforced and reinforced soils have been analyzed by using Duncan and Chang (1970) model and the variation in values of material parameters due to fibre reinforcement was studied. In the analytical part ofthe present study, it was decided to employ multilinear isotropic elasto-plastic model (MISO) available in the software ANSYS for prediction of the behaviour of pavement sections resting on unreinforced and reinforced subgrade soils. The results thus obtained have been compared with those obtained by using a twodimensional nonlinear elastic (2dnln) finite element program developed by Noorzaei (1991) and also with the data obtained from semi-field tests. Finite element program in (2dnln) uses the Duncan and Chang (1970) model for estimation of tangent modulus corresponding to different state of stress and strain. The results show that elasto-plastic analysis of pavement using ANSYS gives a better agreement with the semi-field test results and hence it was used for further development of design charts for reinforced and unreinforced flexible pavements. The behaviour of flexible pavements resting on unreinforced and reinforced subgrade soils has been studied through FEM, Also a methodology has been proposed for the design of fibre reinforced flexible pavements. It is a mechanistic design approach considering benefits in terms of extension of service life of the pavement for the same layer composition or reduction in pavement layer thicknesses for equivalent service life. The properties of different layers required for carrying out the FE analysis are the initial tangent modulus, £,-value, the Poisson's ratio (v) and the stress-strain data. These parameters were evaluated through triaxial tests on different layer materials. The structural failure in a flexible pavement is of two types, viz, surface cracking and rutting. Since the scope of the present study is limited to reinforcing the subgrade soil only, rutting has been considered as a failure criterion. The Indian standard code, IRC 37-2001 considers a rut depth of 20 mmas a failure criterion for flexible pavements and the same has been used in the present study also. Two design alternatives have been considered in the present study: Alternative I: Same pavement sections for unreinforced and reinforced subgrade. It would result in extra life of the pavement due to fibre reinforcement. It has been expressed in terms of Traffic Benefit Ratio (TBR) as TBR =^- (1) N„ IV where, N is the number of traffic passes required to produce a pavement surface deformation (rutting) upto the allowable rut depth. R and U denote reinforced and unreinforced pavement sections. Alternative II: Same service life for the reinforced and unreinforced pavement sections. It would lead to reduction in sub-base, base or bituminous layer thicknesses. It has been expressed in terms of LayerThickness Reduction (LTR) as LTR A/ TOO (2) Du and DR are the layer thicknesses of unreinforced and reinforced pavement sections respectively. Vertical compressive strain developed at the top of unreinforced and reinforced subgrades was captured for different thicknesses of sub-base, base and DBM and benefits of reinforcing the subgrade in terms of TBR and LTR were evaluated using Eqns. 1 and 2. The pavement sections resting on unreinforced and reinforced subgrade have been analyzed for different tyre pressures. It was observed that the vertical compressive strain developed at the top of subgrade increases with tyre pressure but at any particular pressure, the strain in reinforced section is less than that in the unreinforced section. This behaviour has been used to recommend reinforced pavements at locations where the overloading is rampant. Economic analysis is an integral part ofdesign of flexible pavements. Hence, it has been undertaken for all the two alternatives to decide most economical combination of layer thicknesses of flexible pavements resting on reinforced subgrade soils. It was observed that reduction in DBMthickness is more beneficial.