Reliability of Standard Penetration Test to Determine Bearing Capacity of Fine Grained Soil: a Case Study in Addis Ababa Soils

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Addis Ababa University


Standard penetration test (SPT) is widely practiced in Ethiopia to define fine grained soils’ strength parameter for bearing capacity analysis in foundation design. However, there is no published correlation between the in-situ measured SPT data and soil design parameters to define the reliability of the method. In this research, it is attempted to measure the reliability of bearing capacity estimation from SPT data of fine grained soil using linear regression analysis. SPT data were collected from newly measured SPT data, 22 boreholes in Addis Ababa, with a record of N-value, hammer and anvil type, penetration interval, hammer blow rate, drill rod type, rod length, and drill rig type of to define the bearing capacity of fine grained soil. Besides, corresponding samples were also obtained to determine undrained shear strength and other physical and engineering properties of soil in the laboratory. Initial efforts were made to establish a correction factor of SPT blow N-values to different N-values; N55 N60 and N70. Besides, further correlation and comparison were made between SPT N-values and undrained compressive strength (Cu) considering other parameters such as natural water content (Wn), liquid limit (LL) and plasticity index (PI). Results have shown that SPT N-values are strongly correlated to the undrained compressive strength within the range of limited estimation error. It is also noted that there is mild correlation between two parameters based on the coefficient of regression. The correlation factors are also compared with the previously established coefficients for fine grained soil to show their relevance. As a result, empirical correlation between soil test results and bearing capacity design values are established. Further, correlation values were established between Cu and the in-situ measurement data to define the degree of uncertainty.



Standard Penetration Test, Bearing Capacity, Undrained Shear Strength, N-Value