Mechanical response of randomly disposed and deep layered fill-soft soil composite foundation investigated via field loading tests

The construction method utilizing only drainage plates combined with direct surcharge preloading remains widely adopted in the construction and improvement of raw material storage yards on soft foundation soils along rivers and waterways. In such projects, the shallow fill layers are often designed as soft clayey strata. Through a large-scale field surcharge model test, this study investigates the influence of layered fill on composite foundation behavior—composed of randomly disposed, deep layered fill and soft clay—under stepwise surcharge loading of bulk mineral materials, with particular focus on the distribution and transfer mechanisms of additional stress. Analysis of in-situ stress and strain monitoring data reveals that the fill layer within 3.0–4.5 m below the surface acts as a stress-diffusing zone similar to an “artificial hard crust”, which develops dynamically and adaptively during loading; this study terms it the Self-Adaptive Hard Crust. In contrast, the fill below 4.5 m shows no evident stress-diffusion effect. An additional stress diffusion angle of 27° is recommended for such foundation conditions. Based on the two-layer foundation theory, the Boussinesq’s and the Flamant’s solutions, and the theoretical distribution in the Schmertmann’s method, an improved correction model for the additional stress coefficient is proposed. This model offers a practical and simplified method for accurately predicting the magnitude, distribution, and transfer of additional stress along the central axis of bulk storage yards during the design phase. The application of the proposed correction model to the design prediction for this project yielded a root mean square error (RMSE) of 27.3 kPa compared to the measured additional stress values. This represents a reduction of 58.1 kPa in RMSE relative to the discrepancy between the Boussinesq solution and measured data, demonstrating a marked improvement in prediction accuracy over the conventional method. Furthermore, a comparative study conducted on the Ma Steel blast furnace raw material stockyard engineering case confirmed the strong practical applicability and engineering value of the proposed model. The findings provide valuable reference for the construction, renovation, and related engineering practices of bulk storage yards along rivers and waterways worldwide.