A numerical modelling approach for designing a capping layer on weak subgrades with geogrids

• Validated FE design for ultra-weak subgrades (CBR ≤ 5%). • Practical design curves for capping with and without geogrid. • Geogrid benefit diminishes as capping thickness and CBR increase. • Guidance to reach working-platform stiffness targets on soft clays. . Ultra-weak subgrades with California Bearing Ratio (CBR) values below 5% pose significant challenges in working-platform design, as thick granular capping layers are often required and can be uneconomical. Geogrid reinforcement offers a potential solution, but its effectiveness depends strongly on subgrade strength and capping thickness. This study develops and validates two-dimensional finite element (FE) models against large-scale model-box tests at CBR = 1.5% and 2.5%. The validated models are then extended to simulate a wider range of subgrades (CBR = 0.5 – 5.0%) and capping thicknesses of 100 – 500 mm, with and without geogrid reinforcement at the capping–subgrade interface. Platform performance is assessed in terms of ultimate bearing capacity (UBC) at 25 mm settlement and stiffness modulus EV 2 derived from Plate Load Test protocols. The FE models reproduced laboratory responses with good accuracy (UBC 300kPa lab vs 320kPa model at CBR 2.5%; EV 2 10.81MPa lab vs 11.25MPa model). Geogrid reinforcement reduced settlement by up to 26.6% at CBR 1.0% with 200 mm capping and 23% at CBR 3.5%. EV₂ improvements were most pronounced for thin caps (100–200 mm), typically 1.2 – 1.8 times but dropped to 1.04 times for thick caps (400–500 mm). For CBR < 1.0%, geogrid with ≥ 200 mm capping was generally required to reach stiffness thresholds of 40 MPa. The validated design graphs provide practical guidance on capping selection with or without geogrid, highlighting that reinforcement is most beneficial for thin caps over very weak subgrades.