Effect of Specimens’ Height to Diameter Ratio on Unconfined Compressive Strength of Cohesive Soil in Jimma Town

Abstract

Unconfined compression test is commonly used to detemine the undrained shear strength (Su) and cohesions (Cu) of cohesive soils. The test results are significantly affected by specimen sizes. This leads to overestimate or underestimate for the shear strength of cohesive soils and causes a problem for geotechnical analysis and designs. Therefore, this study assessed the effect of height to diameter ratio on unconfined compressive strength of cohesive soil in Jimma Town by testing a compacted cylindrical specimen at maximum dry density and optimum moisture content with a range of 1 to 3 height to diameter ratio for 38mm, 50mm and 100mm specimen diameters. Disturbed sample specimens were collected from five test pits of different locations in Jimma town. Accordingly, the classification of the collected soil specimens determined from grain size analysis and Atterberge limits tests. The maximum dry unit weight and optimum moisture content of the collected sample specimen also determined from the standard proctor compaction test. Hence, the unconfined compression test for the compacted soil specimens at maximum dry density and optimum moisture content conducted for nine height-to-diameter ratios of the three diameters of cohesive soil specimen. The laboratory test results showed that; the unconfined compressive strength value of cohesive soil drops rapidly with increasing of both height-to-diameter ratios and specimens’ diameter. But the value was stable from 1.75 to 2.25 H/D ratios and the UCS of 2.00 H/D ratio was the closest to mean of all height to diameter ratio for all diameter of specimen. As the specimens’ diameter and H/D ratio increase, the axial strain of peak UCS value was decreased. Similarly, the gap between the axial strains of peak UCS value for the smallest and the largest H/D ratio was decreased with the increasing of specimens’ diameter. The number of failure patterns failed in clear and distinct shear failure planes within the series H/D ratios increased as the diameter of the specimens increased. Therefore, both height to diameter ratio (specimens’ height) and diameter has a significant effect on the unconfined compressive strength of cohesive soils. Then it is recommended to use height to diameter ratio of 2 and larger diameter specimen to reduce the effect of specimens’ size for the test. However, platen end frictions and rate of loading strain may be taken in to consideration for further studies