The role of crosshole seismic tomography for site characterization and grout injection evaluation on Carmo convent foundations

Abstract

The Carmo convent (XIV century) is founded on a silty-sand formation subjected to several negative impacts that have weakened the stiffness and strength characteristics of the local soils. A previous finite element analysis was carried out to assess the stability conditions for constructing a new underground railroad passing 20 meters below the convent foundations. The results indicated that upper soils might be, in some areas, highly uncompressed and very close to failure or in a plastic state. To confirm these and to try mapping the higher uncompressed zones, various instrumental measurements and in situ tests were carried out, including an extensive crosshole seismic tomography survey on 26 crosshole sections to obtain their P-wave velocity field. In general, the seismic tomographies obtained validated the finite element results. A jet grout treatment test was carried out at a zone under the convent foundations where the seismic tomography had revealed very low P-wave velocity. Afterwards, the crosshole seismic test was repeated and the corresponding tomography showed that upper soil velocities had increased to values near the velocities for deeper (and less disturbed) soils. Additionally, three crosshole seismic tomographies were performed at a control site with a geological setting similar to the convent foundations, but not subjected to the same negative impacts. The P-wave velocities obtained for these undisturbed soils are about the same as the velocities for the convent foundations after the grout treatment, which confirms the test injection efficiency in improving soil characteristics below the Carmo convent foundations.