Long Term Structural Deficiencies of a Mat Foundation on Clay Soil

A three storey building with a mat foundation consisting of a slab on a grid of grade beams performed poorly on clay soil. Cracking of the slab became progressively worse due to incorrect design and to fluctuations in groundwater pressure under the foundation. The cyclic presence and absence of water rusted steel reinforcement, and the sulfates in the clay soil caused formation of ettringite in the concrete. Plastic hinges formed in the slab, and settlements occurred, causing damage to the beams. The situation is still in progress and may lead the structure to collapse under normal service conditions. The geotechnical and structural investigations performed to survey and assess the aforementioned problems are described in this paper. The causes and the consequences of the detected anomalies are analyzed, and a prediction of potential future structural problems is provided. A strengthening procedure is proposed, and an estimate of the cost to realize it is provided. Recommendations are offered to help avoid the problem in new constructions. Mat foundation; concrete slab; progressive collapse; differential settlements; soil water