It is common practice to analyze the basemat using soil springs to represent the supporting soil media. A uniformly loaded mat foundation analyzed for a uniform spring constant will indicate uniform deflection. However, in reality, the behavior of the basemat is influenced by the flexibility of the supporting soil media. The settlement profiles of the basemat demonstrate smaller displacements at the edge and larger displacements toward the center. The spring constants used in the analysis should also consider stiffness effect of superstructure and the effect due to loading pattern.

For APR1400 Nuclear Power Plant, the mat foundation of the Reactor Containment Building and the Auxiliary Building is a common reinforced concrete mat covering an area approximately 12,210 square meter (110 m × 111 m) with a thickness of 3.05 m in the Auxiliary Building area and various thicknesses from 3.35 m to 10.06 m in the Reactor Containment Building area.

In this study, the soil layers are directly modeled into the structural analysis model using finite/infinite solid elements to represent the soil properties and then the analysis results are compared with the analysis using spring constants by iterative method, and uniform spring constant by conventional method. The results of analysis using the Direct Soil Model are similar to the results calculated by the Iterative Soil Spring Model, but have wide variance with the results calculated by the Uniform Soil Spring Model.

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