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 compressibility in 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.

The Barigui River is a tributary of the Iguaçu River, located at Parana State, Brazil. This river is the main waterstream of the Metropolitan Area of Curitiba, Southern Brazil. In the flood plain of the Barigui River final stretch,nearby its delta at the Iguaçu River, very soft organic clay deposits are found. The region suffered until someyears ago predatory mining activities of both clay and sand, for construction material purposes. Consequently,a degradation process took place resulting in lack of vegetation, excavation open pits and random materialsdump areas. Simultaneously several engineering activities have been installed along the river banks such as thePetrobras Oil Refinery (located at the right bank), the Santa Rita Rural Subdivision, the CIC Sewage TreatmentPlant of the State owned company Sanepar, the Bolívia-Brazil Gas Pipeline (Gasbol) and the Rio Bonito HousingDevelopment (all located along the left bank). The flood plain area considered herein is located on the river leftbank, encompassing approximately an area of 500 m wide by 1000 m long. This paper describes the geologicaland geotechnical characteristics of the flood plain area, presenting results of several soil tests. A discussion aboutsome practical experiences, including engineering difficulties and solutions implemented during the constructionof the facilities above described, are shown. This paper concludes that even though the local organic clay depositsare very soft and compressible, this natural material of the Barigui River flood plain area presents uniqueengineering behavior, much more complex than well known problematic soils.

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