Influence of soil heterogeneity on load redistribution and settlement of a hyperstatic three-support frame

Spatial and geometrical variability of mechanical soil properties can induce differential settlements and load redistribution in hyperstatic structures. Therefore damage prevention requires specific attention to be paid to the global mechanisms of soil–structure interaction. A reduced-scale model of a hyperstatic three-support frame (scale 1:100) is installed on the CEA-CESTA centrifuge, up to 100g. Various configurations are studied, with different loadings, different structural stiffnesses, and different geometries of the soil layer. Strain gauges are fixed at various points so as to enable the retro-analysis of all components of forces at free ends. Displacements are recorded at several points on the structure and on the free surface. A numerical model of the frame (based on beam theory with elastic supports) is calibrated, first to determine an equivalent support stiffness, and second to quantify the effect of variations of the structural stiffness/soil modulus ratio on the structural response. A probabilistic approach based on Monte Carlo simulation is thus developed. It mixes the soil description, based on random field theory and enabling the description of spatial correlation of properties and the frame model identified above. The influence of (a) scattering in soil properties and (b) spatial correlation of soil properties—through correlation length or scale of fluctuation—is quantified in terms of displacements and bending moments in the frame. The approach reveals the important weight/influence of soil variability on differential settlement and load redistribution, particularly when the scale of fluctuation of soil and length of span have the same magnitude. The analysis makes it possible to evaluate the risk that the bending moment in any critical section may be larger than predicted by the usual engineer's model. Some conclusions are drawn to better estimate ‘characteristic bending moments’ in the frame, accounting for both the soil variability and the soil–frame interaction.