Study of Influence of Subbase Compaction on Strength of Concrete Mat Foundations by Method of Critical Stochastic Inputs

Abstract eng:
Structural reliability of large mat foundations depends critically on the uniformity of subbase preparation. Although base compaction is universally used for foundation settlements reduction, there is currently no methodology to calculate the base stiffness in relation to the achieved percentage of Standard Proctor maximum dry density (SPMDD). The present study introduces a new consolidation model for the compacted granular subbase, combined with a “finite grid” superelement interpretation for the concrete mat and the elastic half-space model for the native soil. The study utilizes statistical data on subbase compaction collected from four typical shop and warehouse projects in Central and Northern Alberta. Since this is an uncertain data source, the method of critical stochastic inputs is used for its interpretation. The mat is divided into “domains” of varying size, depending on the inferred probability of local under- or over-compaction. The areas of the maximum moment amplitudes in the mat are assigned either very low (under 96%) or very high (over 102% SPMDD) degree of compaction, whereas the areas of the mat with low moments (“mediocre” areas) have an average degree of compaction (98% SPMDD). The possible combinations of “soft” and “hard” areas are varied following a factored plan, striving for the most critical combination. The results of the study (Fig. 1) show that even for moderate magnitude of loads, undercompaction in the central part of the mat can cause bending stresses in the concrete exceeding its modulus of rupture. The study underscores the importance of proper base quality control in large mat foundation construction.

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