Abstract:
Determination of subgrade soil strength is a critical step in preliminary investigation of every
project. Californian bearing ratio (CBR) and Unconfined compressive strength (UCS) are the
widely used subgrade strength parameters used for determination of strength of foundation soil
both at in-situ and in laboratory. However, these tests involves sampling, transporting,
preparing, compacting, soaking, and penetrating with a plunger of CBR and UCS machine
which is difficult and needs much time to have the end result. To avoid such problems, this
research introduces the use of Dynamic Cone Penetration (DCP) which is a simple test device
that is inexpensive, portable, and easy to operate and understand.
The objective of the study to correlate Dynamic Cone Penetration (DCP) with Californian
bearing ratio (CBR), and Unconfined compressive strength to derive bearing capacity of typical
Jimma clay soil. In this study, field tests and laboratory tests were conducted. Laboratory tests
needed to classify the soil and the parameters that affect the result were conducted and the test
results are analyzed by statistical software (SPSS) and Microsoft excel to find their correlation
functions using regression analysis. The data has been categorized in to two categories,
prediction of CBR and UCS with dynamic cone penetration index had been developed.
These research work revealed that Californian bearing ratio (CBR) estimated by the developed
equation logCBR = 1.9976 – 1.0192*ln(DCPI) and logCBR = -1.0078LogDCPI+2.1001 while
Unconfined compressive strength estimated by UCS = 557.06 – 120.8*ln(DCPI) for black clay
soils and UCS=630.82-133.2*ln(DCPI) for red clay soils of Jimma town. The corresponding
developed bearing capacity equation of this study based on the bearing capacity theory were
qult = 1431.644 – 310.456*ln(DCPI)+γh for black clay soils of Jimma town while qult =
1621.207 – 342.32*ln(DCPI)+γh for red clay soils of Jimma town.
The results are expected to have wide application in the construction sector.
