Estimation of SPT N Values from the Results of DCPT Counts After Elimination of the Soil Friction Effect

The dynamic cone penetration test (DCPT) measures the thickness and in situ strength of soil layers at shallow depths. The DCPT offers advantages such as low cost, portability, speed of operation and the ability to provide a continuous profile. Much research has been conducted by correlating the DCPT results with those of the standard penetration test (SPT). However, the diversity of the test tools used in such research has increased the complexity of choosing the proper equation for a given site. Skin friction can affect the test accuracy as well. The current research addresses both issues and develops a correlation between DCPT and SPT for fine cohesionless deposits. Several tests initially were performed using different cones and energy classes to obtain the optimal cone geometry. Next, the effect of soil failure around the test tools was evaluated by comparing the cone results and measuring the torque. After eliminating the test friction effect, the DCPT results were correlated with SPT N values at the same locations. The existing correlation formulas were transformed to the NSPT-qd form, where qd denotes the soil dynamic resistance and is independent of the level of energy applied. The proposed correlation was compared to existing correlations and showed better estimation of the SPT values, primarily because of its ability to select the proper cone geometry and eliminate the friction effect. Most correlations used less data than are required for the currently used equation, which is obtained from 76 data points in fine cohesionless silty and sandy soils.