Research On Stability Of Bangui-beloko Road Surface Under Load Of Heavy Vehicle

The state and the service life of roads is an important concern in the region of sub-Saharan Africa especially in Central African Republic(CAR) due to two main factors: economic and transportation. The economic factors include the fact that the roads are built in a develop ing nation with almost a total lack of maintenance service for the roads due to inadequate resources for such services. On the other hand, the transportation facto rs include the fact that the regulated weight limits of the axle loads are not respected. The vehicles are always overloaded and consequently, the weight of the axle load is greater than the maximum allowable weight. Therefore on the one hand, these conditions lead to an increase of environmental impacts on the pavement structures; and on the other hand, they facilitate and anticipate the phenomenon of fatigue in the pavement structures. Thus, these conditions are at the basis of the orientation of this stu dy, which is focused on the performance of the flexible pavement structure and the stability of road surface under heavy vehicle load.In this study, the computer program BISAR(Bitumen Structures Analysis in Roads) was used to compute the stresses, the strains, and displacements in flexible pavement structure subjected to uniform loads, acting uniformly over pavement surface in order to determine relationship between axle load level and repetitions and pavement performance; to quantify the loss in performance(functional and structural); and to evaluate effects of materials and pressures of tire. For this, firstly the climate variations analysis including rainfall and temperature variation analysis and, traffic loads analysis including contact pressure, whe el load, moving loads, loads repetition and axle configuration are conducted in order to analyze the relationship between axle load and pavement performance under the change of the environment. Then the flexible pavement structure analysis, including environmental and loads impacts on the stability of road surface, is conducted in order to estimate the effects of materials and the tire pressures. Furthermore, this study work is only done on the flexible pavement structure for the five climate regions defined in CAR.From this study, a relationship was found between flexible pavement performance and axle load in relation to each climate region for the flexible pavement structure network, giving due regard to the local range of materials and construction techniques. Also in this study, the generalized fourth power law was used to calculate the Load Equivalency Factors(LEF). These factors relate various axle load combinations to the standard 80 k N(18,000 Ibs) single axle load and to the standard 151.2 k N(34,0 00 Ibs) tandem axle load for flexible pavement structure. Through a summary of traffic load statistics, the Equivalent Single Axle Load(ESAL) was also estimated.Finally, basing on the range of locally available materials and construction techniques on the one hand, and on the environmental and traffic loads impacts on the other hand, the materials and calibration performance for each climate region was established in this study.From the analysis of vertical load effect on flexible pavement structure, it was observed that the vertical stresses and strains were more sensitive to vertical load. In the thicker surfacing layers, a 24% increase in vertical load resulted in a corresponding increase of about 6% and 14% for the vertical stress and strain respectively, whereas, a 13% increase in tire pressure caused an increase of about 7% in vertical stress and 10% for the strain. This phenomenon is related to the tire-pavement surface contact area, and clearly shows that the tire pressure has a substantial effect on the vertical stress and strain. The highest stress-strain values(Figure 4.1), and the lowest number of fatigue load cycles are shown for 120 k N-1000 k Pa traffic loading compared to 80 k N-700 k Pa. The relative number of fatigue load cycles to failure versu s modular ratio for the four different traffic loading and for similar material constants are evaluated. The least number of fatigue load cycles was obtained under the high traffic loading, 120 k N-1000 k Pa. This is an indication of more damage compared to the other traffic loading. At modular ratio of 10, the 80 k N-700 k Pa traffic loading had a relative fatigue life of about 5x106 load cycles whilst 1200 k N-1000 k Pa had only 0.2x106. This represents a reduction of about 87% in fatigue life, clearly indicating the potential damage of high traffic loading.Based on BISAR 3.0 pavement simulations of a flexible pavement structure, it was concluded that severe pavement damage often occurs in areas of vehicles tires, was caused by an excessive vertical stress induced by the effect of traffic loads for normal and overload load conditions. However, we can note that even without the influence of the traffic loads, the pavement structures can be degraded with environment conditions such as rain, important thermal variations and rising water.