The study looks at the impact of poor soil properties and improper design on highway pavement failure on critical locations along Sylhet-Sunamganj road, Bangladesh. The entire study was conducted in the month of June to July, 2011 which was usually rainy season in Bangladesh. Pavement structural condition was evaluated by Dynamic Cone Penetrometer (DCP) test and sub-grade soil properties were obtained by laboratory soil tests. The California bearing ratio (CBR) values and thickness of the pavement layers were found as 66, 66 and 47% and 90, 90, and 110 mm for surface, base and sub-base layers respectively. According to Roads and Highway Department, the respective values needed were 100, 140 and 180 mm respectively with CBR values of 90, 55 and 16% respectively. It was clear from such comparison that requisite depth of construction were not maintained in various sections which might assist the pavement to its distressed condition. Sub-grade soil properties were evaluated as sandy soil of category A-3 (According to American Association of State Highway and Transport Officials) having moisture content range from 11.11 to 14.94%, specific gravity ranges from 2.21 to 2.49, optimum moisture content range from 9 to 16% were not appropriate for pavement construction. Clayey soil sample that was found from SUST gate arena having moisture content of 16.51%, specific gravity of 2.3, liquid limit of 40.80%, plastic limit of 17.85%, plasticity index of 22.95% and dry density of 2060 kgm-3 was totally unsuitable for such an important pavement construction. Specially, maintaining of liquid limit < 40 and plasticity index < 15 of soil is a must for such important pavement construction.
Keywords: Dynamic cone penetrometer test, California bearing ratio, penetration rate, sub-base, sub-grade soil.
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