Use of SPT Blow Counts to Estimate Shear Strength Properties of Soils: Energy Balance Approach

In foundation designs, standard penetration test (SPT) blow counts are typically used to estimate shear strength properties of soils. Few correlations are widely in use to make such estimations. However, the selection of these correlation equations are not often justified or explained. This manuscript describes a new approach to estimate the shear strength properties based on the SPT blow counts. The proposed method treats SPT analogous to driving a miniature open-ended pipe pile. During SPT, part of the energy is transferred into the soil. This energy is dissipated at the soil-sampler interface to overcome skin and point resistance to penetrate a sampler into the soil. Energy balance was used to correlate the SPT blow count to the shear strength properties of the soil at the depth of testing. Two separate equations were derived: one to estimate the friction angle $\left({\varphi }^{\prime }\right)$ of sand and the other to estimate the undrained shear strength $\left(c_u\right)$ of clay. SPT results from two sites were used to calibrate the proposed equations, and then two other sets of data were used to verify them. With a low average standard deviation in the calibration process, the proposed $N_{60}-{\varphi }^{\prime }$ equation demonstrated a strong correlation. The proposed $N_{60}-c_u$ equation did not provide as strong a correlation as the $N_{60}-{\varphi }^{\prime }$ equation. However, a statistical analysis revealed that for the data used in this research, both equations could estimate shear strength properties better than the commonly used, other existing correlations. The proposed equations may not work in very stiff clay or very dense sand and should not be used to analyze SPT results with poor recovery.