Guidelines for unpaved road materials and structural design

Standard specifications used for road works in Tanzania have limited information on unpaved roads with regard to their structural design and material specifications. The main objective of this research was therefore, to suggest guidelines for unpaved road materials and structural design. The-state-of-the-art of materials specifications and structural design were reviewed. From the review, critical factors for the performance of unpaved roads were identified. Guideline values for further investigation were also identified. Field studies to evaluate the effect of the critical factors on the performance of unpaved roads were conducted. From the field studies the critical factors were adjusted and guidelines for unpaved road materials and structural design proposed.Gravel samples were collected from two geological zones and subjected to laboratory tests. The laboratory tests conducted included:- gradation, Atterberg limits, Proctor compaction and California Bearing Ratio. The results obtained from the tests were compared with the suggested guidelines to asses the impact of the guidelines on the availability of unpaved road materials in the sampled areas. It is noted that unpaved road materials are not as abundant as one may be led to think and for good performance care has to be exercised in selecting unpaved road materials. If inferior materials have to be used for whatever reasons likely consequences on performance must be predicted and maintenance schedule planned. A structural design method has also been proposed. It is recommended that the proposed guidelines be tried in the on-going IRP projects so that they may be refined with an ultimate aim of formulating Tanzanian specifications for unpaved road materials and structural design.