A Procedure to Estimate the Installation Torque of Multi-helix Piles in Clayey Sand Using SPT Data

The present study aimed to establish a simple and rapid procedure to calculate the installation torque of multi-helix piles, widely used as guy-wire anchors and foundations for transmission line towers in Brazil, based on the standard penetration test (SPT) results. For this purpose, an equation was developed considering a theoretical relationship between the pile uplift capacity and installation torque and soil disturbance effect on the torque resisted by the pile shaft and helices during installation. The proposed expression was fitted to the data obtained from 571 six-helix piles installed in clayey sand sites (457 piles were used as the training dataset and 114 were used as the validation dataset). Among other findings, this study indicated that (i) the proposed equation results in a measured-to-predicted ratio of 1.00 with a coefficient of variation (COV) of 10.2% for the training dataset and of 1.01 with a COV of 11% for the validation dataset; (ii) the accuracy of the method presented was improved using a correction factor based on the average N-value of the soil around the first three tapered helices; (iii) although slightly conservative, the torque obtained by the current method was also consistent with the prediction of the installation torque of additional four-helix pile data; and (iv) a parametric analysis indicates that the installation torque depends strongly on the area of the helices. Finally, the method described in this note can successfully predict the installation torque of multi-helix piles in conditions similar to those of the database, which are typical in Brazil.