ABSTRACT
In 1989 the Mexican authorities realized that deterioration of the Cathedral was unacceptable and that an intervention was inevitable if the monument was to be preserved. In order to record and assess the effect of the regional subsidence in the development of differential settlements, several precision topographic levelling surveys were carried out at the Cathedral and the Sagrario during the stage of preliminary studies. The levellings were performed at the plane of the plinth of the columns supporting the Cathedral, therefore allowing continuity in the levellings of this surface that have been carried out since 1907. A forecast of long-term settlements was carried out using traditional procedures and methods used in soil mechanics. This prediction was made assuming that the churches would be left as they were in 1989, i.e. without intervention. Future settlements of the Cathedral and of the Sagrario would have been governed by the evolution of the pore-water pressures in the clay deposits.
