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
