SPT Interpretation for Engineering Properties of Residual Silt in Vietnam

The strength and stiffness of residual silt, which is extensively dispersed in Vietnam’s mountainous regions, are the most crucial design factors for wind power project foundations. Standard penetration test (SPT) is one of the most used in-situ testing techniques in geotechnical investigation and a practical way to assess soil parameters in the absence of soil strength and stiffness data from laboratory test results. To establish SPT correlation for residual silt in Vietnam, this paper carried out laboratory tests, in-situ SPT, and downhole seismic tests based on the Ia Pet wind power project, in Gia Lai province, Vietnam. The correlation between SPT-N values and undrained shear strength su of residual silt was established based on the results of the direct consolidated quick shear test (DSq) of 31 boreholes. The empirical coefficient ‘a’ for strength interpretation of residual silt in Vietnam ranges from 5.25 to 10.72, with an average value of 7.31. For SPT interpretation, it is advised to take a conservative approach and use the standard value of 6.158 for the first residual silt layer. The wave velocity and SPT-N values of the residual silt are found to have typical power functions when combined with the downhole seismic test results at the indicated three holes. The established SPT correlations for undrained shear strength su and shear wave velocity Vs are in good agreement with the measured results, while the interpreted pressure wave velocity Vp has a relatively bad prediction. The undrained shear strength and shear wave velocity of residual silt in Vietnam can be determined using the existing SPT relationships.