Effects of screw pile installation on installation requirements and in-service performance using the discrete element method

Abstract

Existing guidance on the installation of screw piles suggest that they should be installed in a pitch-matched manner to avoid disturbance to the soil which may have a detrimental effect on the in-service performance of the pile. Recent insights from centrifuge modelling have shown that installing screw piles in this way requires large vertical compressive (or crowd) forces, which is inconsistent with the common assumption that screw piles pull themselves into the ground requiring minimal vertical compressive force. In this paper, through the use of the Discrete Element Method (DEM), the effects of advancement ratio, i.e. the ratio between the vertical displacement per rotation to the geometric pitch of the helix of the screw pile helix, on the installation resistance and in-service capacity of a screw pile is investigated. The findings are further used to assess the applicability of empirical torque capacity correlation factors for large diameter screw piles. The results of the investigation show that it is possible to reduce the required vertical compressive installation force by 96% by reducing the advancement ratio and that although over-flighting a screw pile can decrease the subsequent compressive capacity, it appears to increase the tensile capacity significantly.

Assessing single-helix screw pile geometry on offshore installation and axial capacity

“…Viscous damping was not used in either of the contact models and critical damping was set to 0 as the simulation occurred under quasi-static conditions. The maximum vertical velocity used within the DEM simulations was 0.5 m/s at the full installation depth and was calculated using the equations outlined in Sharif et al (2020b) ensuring that the simulation remained quasi static. The centrifuge tests of Davidson et al (2020) were conducted at 3rpm and 21 mm/min, such that both the centrifuge and DEM tests are conducted under drained soil conditions.…”

“…This phenomenon is currently only known to occur in sand, with the effects of AR not fully investigated in other soil types. These effects have been observed though both the Discrete Element Method (DEM) (Sharif et al 2020b) and scale model centrifuge testing (Cerfontaine et al, 2021). One possible way to reduce the installation torque and potentially further optimise the installation force is to change the geometric features of the screw piles, such as the base geometry, shaft diameter and helix pitch.…”

“…It was also found that AR influences the in-service or post-installation performance of the screw pile, with AR < 0.8 (over-flighting) improving the tensile performance but decreasing the compressive performance and AR > 0.8 having the reverse effect (Sharif et al 2020b). The increase in tensile capacity for screw pile installed at AR < 0.8 is due to the particles being transported through the helix and compressed above it during the installation process, resulting in a denser soil and a high stress region being present above the helix post installation.…”

“…This suggests there is a need to investigate how installation requirements can be reduced to further develop their use offshore. Sharif et al (2020b) have shown that by reducing the advancement ratio (AR), the ratio between the vertical displacement for one rotation (Δz h ) over the helix pitch (P h ), below 1 (where 1 represents pitch-matched installation; Equation 1), it was possible to reduce the compressive installation forces of screw piles significantly and even install a screw pile under its own "self-weight", although this had little effect on the installation torque which remained substantial.…”

“…However, the data set used by Perko (2009) contains considerable scatter using a limited number of shaft diameters, which were far below those which may be used for offshore deployment (Davidson et al 2020). Lutenegger (2013) suggests that it may be incorrect to assume that a single parameter model is effective for all screw pile geometries as is suggested by Equation 2&3, as it does not consider the effect of any geometric property other than the shaft diameter, nor any influence of pile depth (H/D h ) or soil properties (Davidson et al 2020, Sharif et al 2020b. Sakr (2015) further developed the values of K t for screw piles with larger shaft and helix diameters.…”