Measured Settlements of Mat Foundations on Soft Clay with and without Reinforcement from Sand Columns

This paper shows the behavior of two mat foundations built on soft clay, representing full size structures for low income houses. The lack of habitation in Brazil is tremendous right now and any contribution to a better geotechnical solution for this problem is more than welcome. Very often, areas used for housing developments have clay of high compressibilityin the subsoil profile with settlement, becoming a major concern. Two full size mat slabs (4 x 4 m each) were built directly on the problematic soils, the Belém River Valley in Curitiba, Southern Brazil. Geologically, the subsoil profile is defined by about 4 meters of soft organic clay, basically OH (USCS) deposited by the river over more competent silty clay material, from the local Guabirotuba Formation. One of the slabs was partly supported by short sand columns attempting to give a better idea about how different the behavior of the foundation would be over time, considering a reasonable surcharge representing the real house loads. A careful topographic settlement monitoring program was then carried out for a few months, allowing for some conclusions to be drawn and later applied to other similar projects. The main finding was the substantially smaller vertical movements of the slab on sand columns compared to the no sand columns, with a 1:2 approximate ratio, based on settlement. Only small settlement readings were obtained (about a few millimeter). Examples of some applications of what was learned from the research are also mentioned, showing the importance of using experimental data in engineering practice.