During the excavation for the construction of a large building in Coimbra, it was found a wide variability of the excavation resistance of the sandstones of the Grés de Silves formation. In order to quantify and to understand the variability in the excavation conditions the seismic velocity of the P waves was measured. As the outcrop surfaces that allowed the better measurement conditions were the vertical ones, resulting from the excavation for the foundation and walls of the building, it became necessary to use an unusual technique, with the geophones specific for the determination of the S waves, staked to the ground in the horizontal position, allowing to measure the P waves. The sandstone shows great variations in cement type (siliceous or ferruginous) and quantity, confering to the ground a behavior from a soil to a rock that required the use of explosives to blast the excavation. The analysis of the seismic velocities showed that only one of the tested places could be considered marginal, according with the rippability chart, while the rest of places could be excavated by mechanical equipment.
