Analysis of Stress- Strain and Deflection of Flexible Pavements Using Finite Element Method Case study On Bako-Nekemte Road

Abstract

In Ethiopia the number and type of traffic increases from day to day. This enforces the construction of road infrastructures that needs economical and safe design of roads. Most roads found in Ethiopia are flexible pavements. Nowadays, the failure of surface of flexible pavement roads are common before the expected design period. For the example BakoNekemte road/ has become a critical issue in our country. The most common parameters that cause stress, strain and deflection of the roads are loads and pressures that come from vehicles. Moreover, modulus of elasticity, Poisson’s ratio and thickness of each layer needs to be characterized. Further, the load magnitude, contact pressure (or load radius) and location are defined for each load (wheel) considered. Finite element method (FEM) is a numerical analysis technique to obtain the stress-strain and deflection of each pavement layers. Analytical method usually uses layers thickness, loads, elastic modulus and Poisson’s ratio of the pavement materials as design parameters. The objective of this research was to study the sensitivity of the road parameters in analyzing the major causes of failure in asphalt pavement layers fatigue cracking and rutting deformation which came due to the critical tensile strains at the bottom of the asphalt layer and the critical compressive strains on the top of subgrade using the finite element method by relating the standard specification of ERA and laboratory test result. This thesis studied the analysis of stress-strain and deflection of flexible pavements using Everstress finite element method. The Ever stress program will take into account any stress dependent stiffness characteristics. This thesis dealt with ways to reduce deflections by varying the design configuration, such as increasing the HMA modulus, the base modulus, sub base modulus, the subgrade modulus and increasing thickness of each layers. Based on type of materials to use the value of elastic modulus and poison’s ratio are various in each layers, in layer 1 is varied from 1500 to 3500 MPa, in layer 2 is varied from 200 to 1000MPa, in layer 3 is varied from 100 to 250 MPa and in layer 4 is varied from 20MPa to 150MPa. As observed throughout study analysis of laboratory test result and standard specification result, the vertical deflection reduces as the modulus increases at all values of Elastic modulus. Therefore at the maximum elastic modulus horizontal strain in layer 1 is 0 microstrain and the average vertical strain in the layer 1 is 24.5 microstrain. The average vertical strain in the layer 2 is -11 microstrain while the average vertical strain in the layer 3 is -171.7 microstrain. Vertical strain at depth 0mm in layer 4 is -253.1 microstrain and at 150mm in layer 4 is -214.4 microstrain. The minimum elastic modulus horizontal strain in layer 1 is 7.2 microstrain and the average vertical strain in the layer 1 is 97.6 micro strain. The average vertical strain in the layer 2 is -28.3 microstrain while the average vertical strain in the layer 3 is -429.4 microstrain. Vertical strain at depth 0mm in layer 4 is -1237.7 microstrains and at 150mm in layer 4 is -1000.5 microstrain. In general as the elastic modulus of each layers and layers thickness increases stress-strain and deflection in each layers decreases.