Optimisation and enhancement of uplift performance of steel grillage foundations for OHLs

Abstract

This thesis investigates the uplift behaviour of steel grillage foundations as an alternative concrete-free foundation solution for overhead lines (OHLs). Steel grillages are well-suited to remote and challenging terrain where traditional concrete foundations, such as pad-and-column foundations, pose significant logistical and environmental challenges. This research addresses key knowledge gaps in the uplift performance of grillage foundations in granular soils, focusing on the influences of grille spacing-to-grille width ratio (s/w), embedment depth-to-width ratio (H/B), soil relative density (Dr), and potential enhancement techniques on the load-displacement response.

A combined programme of plane-strain numerical simulations and reduced-scale (1:10) 1g physical tests provided initial insights into the failure mechanisms and the influence of the s/w ratio. The primary investigation focused on a 1:28 scale centrifuge testing programme, which examined the influence of s/w and H/B ratios under realistic stress conditions representative of the prototype scale. The load-displacement data across multiple s/w ratios, H/B ratios and relative densities enabled the development of normalised design tools to predict both capacity and mobilised displacement.

The results showed that grillages can achieve a peak uplift resistance (Qpk) comparable to a solid plate for s/w ≤ 1 in medium-dense sand (Dr = 42%) and for s/w ≤ 2 in dense sand (Dr = 82%), far exceeding the current technical guidance (s/w ≤ ⅓). The normalised load-displacement data facilitated the development of two bilinear correction factors to predict Qpk for a given s/w ratio, capturing the transition from the optimal s/w ratio to reduced capacity at wider grille spacings. To aid in the serviceability assessment and evaluate the 25 mm vertical displacement criterion, a hyperbolic model fitted to a 95% exceedance limit was proposed to estimate the mobilised displacement (Δ) for a given design load (Q/Qpk).

To mitigate the loss in resistance and stiffness observed at wider grille spacings, three enhancement methods were investigated, including: geogrid reinforcement (Method 1), a dense crushed rock analogue (CRA) backfill (Method 2), and a combination of the two (Method 3). The geogrid reinforcement increased the optimal s/w ratio and reduced the loss of resistance at the widest s/w ratios, while the CRA backfill enhanced the stiffness across all s/w ratios. The combined approach (Method 3) offered an improved response in both resistance and stiffness, and acted as a semi-rigid mattress, effectively suppressing soil flow-through behaviour post-peak. However, these gains in stiffness dissipated at higher loads, limiting their effectiveness at higher design loads (Q/Qpk).

This research demonstrates the potential of optimised grillage design to reduce material usage, transportation requirements, and embodied carbon, while also eliminating haul road construction and the 28-day curing time associated with concrete foundations. The proposed design tools provide a practical design approach for the uplift design of steel grillages for OHL foundations and support their adoption in remote and challenging terrain.

Date of Award	2026
Original language	English
Awarding Institution	University of Dundee
Sponsors	Scottish and Southern Electricity Networks
Supervisor	Michael Brown (Supervisor), Matteo Ciantia (Supervisor) & Christopher Beckett (Supervisor)