A MATHEMATICAL MODEL FOR ROAD PAVEMENT FAILURE SUSCEPTIBILITY INDEX: A CASE STUDY OF SOME ROADS IN SOUTHWESTERN NIGERIA

Abstract

This research investigated the contribution of geotechnical indices, traffic load, cross-sectional area of drains and road cross-section elements in road pavement failure of Akure-Ilesa (68km), Ilesa-Osogbo (32km), Osogbo-Iwo (45km), Akure-Ado-Ekiti (45km) and Akure-Ikare-Akoko (85km) roads. The condition survey of the roads was carried out and the traffic volume counts along each of the roads were taken for seven consecutive days. Thirty-Nine (39) monitoring wells were installed at failed and stable segments along the roads measuring their ground water levels for twenty-two (22) months. The drainage study at the failed and stable segments of the studied roads was carried out, the geotechnical study and the laboratory tests of soil samples taken were carried out. The road cross-section measurement and analysis of the failed and stable segments of the roads were also carried out. The failure susceptibility index (value) of the roads using Traffic load(T), Depth to watertable(D), Cross-sectional area of drains(C), Soaked CBR(R), Cambering(A), Maximum dry density of soil(M), Index[group] of soil(I) and Asphalt thickness(S) [TDCRAMIS] which is a modification of Traffic load(T), Depth to watertable(D), Soaked CBR(R), Cambering(A), Maximum dry density of soil(M) and Asphalt thickness(S)[TDRAMS] was evaluated, a model using TDCRAMIS rating system and weights was calibrated and the model was validated using field data. The result of the condition survey showed the Total Physical Distress and Cambering Condition Score (TPDCCS) at the failed segments as 220, 165, 180, and 185. The TPDCCS at the stable segment is 115 for Akure-Ilesa road. The traffic census revealed that heavy vehicles do more damage to the roads than the lighter vehicles. The water table measurements along the roads revealed that most of the failed segments have the water table within 1.0m of the pavement structure. The result of drainage and cross-section Measurements and analysis along the roads showed inadequate drain cross-sectional areas and cambering that do not conform with the specifications. The result of geotechnical tests using American Association of State Highway and Transportation Officials (AASHTO) method carried out on soil samples taken along the roads showed about 80% of soils classified as poor to fair and about 20% classified as fair to good. The TDCRAMIS rating system and weights produced a calibrated mathematical model that gave impressive results when validated with field data. The result revealed that 73% of the data of all the roads used for validation has error less than 5% while 15% has the error less than or equal to 10% and 12% has the error greater than 10%. Comparing the road condition survey and the Total TDCRAMIS Index which is the calibrated failure evaluation model the results show the TPDCCS and the T.TDCRAMIS.I as well correlated (R=0.80). This therefore confirms that the TDCRAMIS model could be used to prioritize road maintenance activities because the road condition could sufficiently be predicted by TDCRAMIS model