In-situ geotechnical characterisation of the Brasilia porous clay

The Brasilia metro encompasses a tunnel, 6.8 km long, excavated in a layer of porous clay with collapsible characteristics, using the NATM. There is a study, in progress, with the purpose of better understanding the particular behaviour of the Brasilia porous clay for further numerical simulations of the tunnel construction, using the FEM. The geotechnical characterisation of the soil affected by the tunnel excavation was made by in situ and laboratory tests. In this paper, the results obtained from in situ tests are presented, which include SPT-T, CPT and DMT tests. The porous clay is unsaturated, and for this reason, field works were divided in two stages in order to evaluate the effect of the moisture content changes along the year in the soil behaviour. It was also tried to evaluate the effect of the tunnel excavation on the behaviour of the surrounding soil. Thus, in both stages, the same in situ tests were developed in two locations. One of those was defined in the lateral of the tunnel and the other is about 75 in away from the tunnel axis. Comparisons of the behaviour observed in each one of the testing locations and in the two moisture content conditions are presented. The soil anisotropy effect was also analysed, accomplishing for such, dilatometer tests, in that the steel plate introduced into the soil at different orientations.