Use of DCP and CBR Tests to Characterize Subgrade Shear Strength The Case of Volcanic Ash/Pyroclastic Deposit of Bulbula - Alage Section

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Addis Ababa University


The existing manuals adopted in Ethiopia for road construction and management purpose have made CBR and/or DCP tests mandatory in order to characterize strength of unbounded materials. Executing CBR and DCP tests is obligatory that, the data is available easily in every road stretch throughout the country. Contrary to this, the shear strength parameters used in slope stability analyses and bearing capacity computations are not available in sufficient number. For this reason, this research is initiated with the objective of obtaining values of shear strength parameters of pyroclastic/volcanic ash deposits using CBR and/or DCP test results on a pilot study area Bulbula – Alage road section. Different test results of Atterberg limit, sieve analysis, proctor, California bearing ratio, direct shear, dynamic cone penetration, field density, collapsibility and dispersity tests have been used. According to the test results, the plasticity of subgrade is low to none; the sand fraction is significant; the maximum dry density of the deposit is mainly below 1.5 g/cm3; the deposit has high CBR and low CBR swell values; DCP penetration rate decreases with depth in the majority of the stretch. The level of collapsibility potential is moderate with dispersity potential ranging from none to moderate. For this non-plastic to low plastic, low dry density, high CBR and moderate collapsible potential pyroclastic deposit a correlation is developed between CBR and/or DCP test results with that of angle of internal friction value obtained from direct shear test. The correlating equation has been developed using Excel and SPSS statistical analysis software.



DCP, CBR Tests, Subgrade Shear Strength, Bulbula - Alage Section