The area of Leiria, Central Portugal, is included in the Lusitanian basin belonging to the epi-hercynian cover, composed by MesoCenozoic sediments and with important magmatic and diapiric events.
The city of Leiria is crossed by the river Liz and part of the urban structures were developed over the alluvial plain. As the city continues to grow, any intervention in this zone requires great attention, being fundamental a detailed geotechnical characterization of the underlying ground. Several important engineering projects as underground parking, bridges, roads, football stadium or the market of Santana required the execution of drilling works. The results were analyzed and interpreted, with relevance to the SPT tests, allowing defining the litostratigraphy and the geologic structure of the alluvial materials, as well as the underlying materials.
The alluvium was fully intersected in all drillings, showing thickness between 0.5m and 13.5m, with an average of 5.6m, 50% showed a thickness below 4.5m and 75% below 9.0m. From the 440m of alluvium crossed, 47% are sands, 31% are clays, 17% are gravels and cobbles and 6% are silts.
The Eurocode 7 suggests a number of corrections to the SPT test (Nspt). The corrections used in the present work were: the energy delivered to the drive rods, the energy losses due to the length of the rods and the effect of overburden pressure in sands. Some correlations were obtained allowing characterizing the strength of the intercepted materials, as well as its classification with relationship to its compactness, consistency and propagation of seismic waves. The number of SPT tests considered was 237. The results showed that the Nspt doesn't vary significantly with depth, even when they are corrected, staying usually below 20.
The values of Nspt above 40 are only 19% of the tests, and correspond to the presence of coarser materials. Using the formula of Otha & Goto for granular soils, the computed velocities of the shear waves (Vs) vary between 95 and 213m/s, considering Nspt between 4 and 30 and depths up to 12m. For the gravely layers with an Nspt of 20, the computed Vs velocities are between 238 and 252m/s.
Based on the obtained results it can be stated that the alluviums of the river Liz in the studied area don't offer good characteristics as foundation material. The finer materials present a medium to hard consistency and are overlying coarser materials of low compactness and higher velocity of the seismic waves. In most of the situations the foundation levels needed to be deepened till the bedrock below the alluviums, corresponding to Jurassic terrains, constituted by clays and marls, sometimes with gypsum.
As the rehabilitation of ancient buildings in the centre of the city is required, a good knowledge of the geological and geotechnical conditions of the foundations will allow the identification of the problems and a better definition of the constructive techniques to use, contributing to the prevention of accidents.
