CPT-Based Design Procedure for Installation Torque Prediction for Screw Piles Installed in Sand

Abstract

Screw piles (helical piles) have been used widely as foundations for onshore projects due to their ability to provide high compressive and tensile resistance as well as reduced noise/vibration during installation. These types of piles have been proposed as a potential innovative foundation for offshore wind turbines in deeper water. In order to adopt the screw pile technique as an offshore foundation, the geometry of the piles would need to be scaled up so they can provide the high capacities required for this application. Such a change in size and geometry will lead to uncertainties in predicting the required torque for installation in different soil types and stress histories. Without the ability to accurately predict installation torque it is difficult to design screw piles for offshore use or develop appropriate installation plant with the required torque capabilities in different soils. This paper presents centrifuge test results of screw piles and CPT tests undertaken in dense sand. The installation torque (T) has been correlated to the cone resistance qc to establish a proposed CPT-based design method to predict the required installation torque for modified screw pile geometries.

Cone Penetration Testing IV

“…The proposed equations differ to those suggested by Al-Baghdadi et al (2017) in several ways. Firstly, the rotation force reduction factors required for the shaft and base components (f 1 = 0.75 and f 2 = 0.7, respectively) for force prediction during rotary installation, after Deeks and White (2008), have been omitted from equations 4 and 5.…”

“…This paper presents modifications to the CPTtorque prediction method for screw pile installation proposed by Al-Baghdadi et al (2017) based on centrifuge tests of single and multi-helix screw piles in dense, dry sand. The changes include: removal of the force rotation reduction factors, which are not deemed appropriate for torque calculations; for the base of the core, a missing tan(δ crit ) term is added and the full q c value is used to reflect the full flow conditions during installation; the stress drop index used to modify the cone resistance to a radial stress is related to the interface friction angle and CPT friction ratio as well as being incorporated into the helix leading edge calculation.…”

“…Previous publications Spagnoli, 2016;Al-Baghdadi et al, 2017) regarding CPT-torque correlations have restricted their predictions to single helix piles only.…”

“…Therefore, during full flow installation, it would be more appropriate for q b to equal q c (Randolph and Gourvenec, 2011). Thirdly, an erroneously absent tan(δ crit ) term in Al-Baghdadi et al (2017) has been correctly added to equation 5. Fourthly, equation 8 addresses an integration error and uses the radial stress instead of the full cone resistance value.…”

“…Consequently, several authors have suggested correlations which relate CPT data to the installation torque of screw piles , Spagnoli, 2016, Al-Baghdadi et al, 2017. This paper aims to provide an improvement on the method proposed by Al-Baghdadi et al (2017), based on further centrifuge modelling of screw piles in dense sand. The method is also validated against multi-helix screw piles to address the lack of available CPT-torque prediction methods for such piles.…”