The Prediction Of The Performance Of Road Sub-Bases Under Construction Traffic By Shear-Box Tests

A sub-base must, inter alia, provide a working platform on which the materials for upper pavement layers can be transported, laid and compacted. there have been many cases in the UK where this has not been achieved because of the inadequate shear strengths of some granular sub-bases, especially in wet weather conditions. Substantial benefits would arise if a reliable means of identifying adequate local materials could be standardized. The thesis begins with a review of the literature and completed research. A programme of trials and testing is then described, and Tt is shown that a shear-box test can be used to characterize granular sub-bases in terms of their ability to resist deformation resulting from the passage of the tires of heavy lorries. From the findings of these studies it is recommended that a sub-base haying a thickness of 150 mm and underlain by a capping layer should be able to withstand 1000 standard axles of lorry traffic without accumulating more than 30 mm of surface deformation measured from original level. To meet this requirement the sub-base should have a level of shear strength (expressed as Peak Shear Stress Ratio) exceeding 2.4. Some materials with PSSR values in the range 1.9 to 2.4 can give satisfactory performance, but this should be checked with a preliminary trafficking trial. Information is given on the precision of the shear-box test. A preliminary assessment has also been made of other measures of sub-base stability: the CBR test, the Clegg Impactometer, the Dynamic Cone Penetrometer, the Plate Bearing Test and the Benkelman Beam.