Research on the influence of soil base stiffness reduction on the stress-strain state of the “base-structure” system

Abstract

This article investigates the complex influence of rheological processes, specifically consolidation and viscous creep of the soil skeleton, on the evolution of the stress-strain state of the spatial "soil base - foundation - structure" system over time. The relevance of the work is driven by the need for accurate forecasting of post-construction settlements to ensure the operational reliability of buildings, especially under complicated geological conditions. Using the "LIRA-SAPR" software package and the "MONTAZ+" subsystem, a methodology for the staged modeling of changing equivalent soil stiffness was implemented. The theoretical foundation of the numerical model is the classical linear hereditary creep theory of Boltzmann-Volterra, utilizing the exponential relaxation kernel of Maslov-Arutyunyan. This justified the application of a transitional stiffness coefficient K, which non-linearly degrades from 1.0 (instantaneous elastic state at the moment of external load application) to differentiated values of K_∞ (ranging from 0.85 for sands to 0.65 for weak clays depending on their physical nature) after the completion of hydrodynamic consolidation processes. The research encompassed several representative computational scenarios: 1) load on a foundation slab without accounting for superstructure spatial stiffness; 2) joint operation of a three-story reinforced concrete frame with a homogeneous soil base; 3) joint spatial operation of the building with heterogeneous geology, complicated by the presence of lenses of weak soil (using various schemes for differentiating rheological properties). The modeling results conclusively prove that ignoring the spatial stiffness of the load-bearing frame leads to an overestimation of maximum absolute settlements by more than 28%. At the same time, incorporating the frame into joint operation with a time-degrading base generates a load-relieving effect in the central zone of the slab (the "hanging" effect), with a subsequent redistribution of contact pressure to the periphery. It was established that using a traditional averaged creep coefficient for all layers artificially overestimates stabilized settlements. The correct application of differentiated coefficients, which takes into account the ability of low-compressibility sands to maintain long-term stiffness, demonstrates a significantly softer redistribution of forces in load-bearing elements. The obtained regularities expand the understanding of the mechanics of interaction between structures and visco-elastic bases and prove the absolute necessity of considering the time factor during the design stage to prevent progressive collapse.