ABSTRACT
The project for hardening selectively the clay strata under the Cathedral was based on a comprehensive review of geotechnical conditions at the site including a reassessment of stratigraphical details and of the hydraulic conditions within the clay mass. The grouting experiment began in December 1997 at the Cathedral’s west atrium Grout percentages varied from 2 to 7% at the Cathedral and from 1 to 5% at the Sagrario. The effects of the grouting for improving the treated soil were established as reference targets for evaluation purposes, together with the topographic surveys and the levelling of deep benchmarks. The mortar grout injected to reduce the compressibility of the subsoil under the Cathedral is constituted of cement, bentonite, pumiceous sand and additives. Soils hardened by the grout are less deformable than the natural clay. Pneumatic and electric drilling rigs were used after being adapted to operate within the very narrow aisles of the crypts.
