Evaluation of relative density and friction angle based on SPT-N values

This study is an attempt to evaluate relative density and friction angle of sands on the basis of SPT-N values. In order to develop the relationships among relative density ( D r ), friction angle ( ϕ ) and SPT-N value, field and laboratory test results from sixty boreholes executed in sandy deposit were used. The field tests include the SPTs conducted in the boreholes and the determination of in-situ density at various depths in the boreholes using the pitcher sampler whereas the laboratory tests include routine classification, direct shear box, maximum and minimum density and specific gravity tests. The SPT-N values were observed to vary between 4 and 100 and the in-situ bulk density of undisturbed samples as recovered through pitcher sampler are in the range of 13.24 ~ 18.44 kN/m 3 . The soil samples are classified as poorly graded sand (SP), poorly graded sand with silt (SP-SM) and silty sand (SM) on the basis of Unified Soil Classification System. The values of minimum dry unit weight fall in the range of 12.22~14.95 kN/m 3 and maximum dry unit weight varies in the range of 14.64~19.17 kN/m 3 as obtained through vibratory table. Based on the test results, correlation analysis was performed to identify the parameters that affect relative density. The parameters include SPT-N value corrected for field procedures ( N 60 ), relative density ( D r ), void ratio range ( e max – e min ), effective overburden stresses (σ v ’ ) and in-situ dry density ( γ df ). Based on the results of correlation analyses, it was observed that SPT- N values alone cannot be related to D r . Therefore, multiple regression analysis was carried out using Statistical Package for the Social Sciences (SPSS) software and relation between relative density, corrected SPT- N value and effective overburden stress is being proposed. The variation between experimental and predicted values falls within ± 10% at 95% confidence interval. Validation of the proposed correlation was also performed by using an independent data set which indicated that the prediction by using the proposed correlation also falls within ± 10%. Further, comparison of the proposed correlation with other similar relationships already available in the literature was also performed. In addition to the above, correlation between ϕ and SPT- N 60 has also been proposed. It has been observed that the experimental and predicted values of friction angle fall within ± 10% with 95% confidence interval. The proposed correlations may be very useful in the field of geotechnical engineering during feasibility/preliminary design stage for rapid estimation of relative density or friction angle based on the field SPT- N 60 values.